An
Introduction
to
CCC
CC CC
CC CC
CC
CC CC
CC CC
CCC
Using the
DDS MICRO-C Compiler
Revised: 20-May-01
Dunfield Development Services
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Intro to DDS MICRO-C
TABLE OF CONTENTS
Page
1. INTRODUCTION 1
2. BACKGROUND INFORMATION 2
2.1 Computer Architecture 2
2.2 Assembly Language 5
2.3 High Level Languages 6
2.4 Interpreters VS Compilers 8
2.5 Object Modules & Linking 9
2.6 Compiler Libraries 10
2.7 Portability 10
3. INTRODUCTION TO 'C' 11
3.1 Functions 12
3.2 Variables 13
3.3 Pointers 16
3.4 A complete 'C' program 16
3.5 'C' memory organization 18
4. EXPRESSIONS 21
4.1 Unary operators 22
4.2 Binary Operators 24
4.3 Other Operators 29
5. CONTROL STATEMENTS 32
5.1 The IF statement 32
5.2 The WHILE Loop 32
5.3 The DO/WHILE Loop 33
5.4 The FOR Loop 34
5.5 Compound Statements 35
5.6 BREAK and CONTINUE 36
5.7 The SWITCH Statement 36
5.8 Labels and GOTO 37
6. RECURSION 38
7. COMMAND LINE ARGUMENTS 41
8. FILE ACCESS 42
8.1 File Pointers 42
Intro to DDS MICRO-C Table of Contents
Page
8.2 File I/O Functions 42
8.3 Standard I/O 43
9. SAMPLE FUNCTIONS 44
9.1 Prime Number Generator 44
9.2 A Simple Sort 45
9.3 Text Display of Value 46
Intro to DDS MICRO-C Page: 1
1. INTRODUCTION
Since releasing my MICRO-C compiler in 1988, I have received many
requests to include information on the 'C' language as part of that
package.
This manual is intended as a companion to the MICRO-C compiler,
and presents an introduction to the 'C' language for those who are
not already familiar with it. The language represented is that
portion of 'C' which is implemented by the MICRO-C compiler. Since
MICRO-C implements a subset of the 'C' programming language as
described by Kernighan and Ritchie (the original developers of the
language), you should have little difficulty using and learning a
full 'C' compiler once you have mastered it.
You should also refer to the MICRO-C technical manual entitled
"MICRO-C a compact 'C' compiler for small systems" for details on the
MICRO-C implementation.
'C' has many inter-relationships between its various constructs
and features. I have attempted to introduce them in a logical and
"building" manner, however it is not always possible to fully
describe each feature before it is mentioned in the description of
some other construct. For this reason, I suggest that you read this
text "lightly from cover to cover" at least once before you try to
fully understand each point.
This is a first draft of this document. In its present form, it is
not very easy reading, but does contain much useful information. I
will be improving and adding to it as I find the time.
Presented herein is a brief summary of the major features of the
'C' language as implemented in MICRO-C.
NOTE: If you are using a version of MICRO-C for an embedded
processor, some of the library functions and features described in
this document may be different or unavailable. For example, the
"file" functions do not exist for any system which does not have an
operating system or disk drives.
Intro to DDS MICRO-C Page: 2
2. BACKGROUND INFORMATION
This section provides some detailed background information for the
novice, and may be skipped if you are already familiar with the
basics of computer architecture and programming languages. This
information is presented here because 'C' is a very low level
language, and an understanding of these basic principals will help
you more easily understand how and why certain constructs work the
way they do.
If you really don't want to be bothered with learning "How
Computers Work", and just want to be able to write simple programs in
'C', you may skip this section and proceed directly with
"Introduction To 'C'".
2.1 Computer Architecture
The basis of any computer system is its Central Processor Unit
(CPU) which controls the operation of all other parts of the
computer, by following a set of "instructions" which make up a
software "program". The program is stored in "memory" and directs
the CPU to read and write data to various "peripheral" devices
(Such as terminals, disks and printers), and to manipulate that
data in a matter which accomplishes the goals set out by the
author of the program.
Although there are a wide variety of CPUs available in modern
computers, they are all very similar, and feature the following
characteristics:
All data accessed by the CPU is represented by circuits which
may be either OFF or ON. This is represented by the digits 0 and
1. Since there are only two states (0 and 1), the computer may be
thought of as operating in a BASE 2 (Binary) number system, and
each individual data element is called a Binary digIT, or BIT.
This BASE 2 number system is used because it is much easier to
build and interface to electrical circuits which have only two
states (OFF and ON) than ones which have many states.
Since single BITs cannot represent much information,
manipulating large amounts of data at the BIT level would be a
very tedious chore. For this reason, modern CPUs access data as
groups of BITs. Usually the smallest group of data which can be
manipulated by a computer consists of 8 BITs, and is called a
BinarY TErm (BYTE).
Intro to DDS MICRO-C Page: 3
Very small computers can often only access data a BYTE (8 bits)
at a time, while larger machines may be able to access and
manipulate data in larger groups called WORDS. The size of the
data group usually manipulated by a CPU is called its WORDSIZE,
and is expressed in a number of bits. This is almost always a
multiple of 8 bits, resulting in an even number of bytes. Thus, if
you hear a CPU or computer called a "16 bit" machine, you know
that it can access and manipulate 16 bits (2 bytes) of data at a
time. A "32 bit" machine would operate on 32 bits (4 bytes) of
data. In general, the larger the WORDSIZE of a CPU, the more data
it manipulates at one time, resulting in faster completion of a
given task.
The CPU has access to external "memory", which consists of many
thousands (and often millions) of WORDS of data. Up to one
complete word of data may be transferred between the CPU and
memory in one memory access.
Often it is known that each data element stored in memory will
not take up an entire word, and is it desirable to access memory
in smaller groups to reduce the number of memory words required to
accomplish a particular task. For this reason, most modern CPUs
can access any single BYTE (8 bits) from a memory word. It should
be understood however, that accessing a single byte causes a full
memory access, and takes just as much time as accessing an entire
word.
In order to provide the programmer with a simple method of
specifying memory locations, each BYTE of memory is assigned an
ADDRESS, which is simply the number of BYTES from the beginning of
memory to the desired byte. Therefore, the first byte in memory
has address 0, the second byte byte in memory has address 1, the
third byte has address 2 etc. Thus, memory from the viewpoint of
the programmer may be considered as a simple array of BYTES,
beginning with an address of 0, and continuing with sequential
addresses up the memory size (in bytes) of the computer.
In addition to the external memory, a CPU has a small amount of
very fast internal memory which is organized into words called
"registers". Registers act as holding places to store the data
words which are to be manipulated. At least some of the registers
are internally connected to an Arithmetic Logic Unit (ALU), which
has the necessary electronics to perform basic operations such as
addition and subtraction on the data in those registers. The
result of the operation is also placed in a register, often one of
the registers which contained the original data.
Intro to DDS MICRO-C Page: 4
One special register called the Program Counter (PC) is used by
the CPU to follow the software program. It contains the address of
the next INSTRUCTION to be executed. At the beginning of an
INSTRUCTION CYCLE, the CPU reads the word of memory at the address
which is contained in the PC, and interprets the value contained
there as an operation to be performed, such as loading a register
from memory, storing a register into memory, or performing an
arithmetic operation on the registers. After performing that
operation, the CPU advances the PC to the next memory word before
beginning another instruction cycle. In this manner, all of the
instructions in a program are read, and the programmed operations
are carried out.
The CPU may also read from and increment the PC during the
execution of an instruction in order to access data bytes which
are OPERANDS to the particular instruction being executed. Such
would be the case if you were instructing the CPU to load a
register with the contents of a particular memory address. The
data bytes following the instruction would contain the memory
address to be used.
There are a few instructions which direct the CPU to store a
new value in the PC rather than advance it. These are called
JUMPS, and are used to make the program begin executing
instructions at a different address. This can be used to create
LOOPS in the program where a sequence of instructions is executed
over and over again.
Some of the "jump" instructions will only store the new value
in the PC if certain conditions are met, such as the last ALU
operation resulted in zero, or did not result in zero. This allows
the program to alter its pattern of execution based on data
values.
For example, here is a program to count to 10 on a very simple
imaginary computer. It shows the use of IMMEDIATE operands to
instructions, which are shown by [PC+], and indicate that during
the execution of the instruction, the CPU reads the next value
from the address in the PC. The PC is advanced so that that value
will not be executed as another instruction when the first
instruction is finished:
Address Interpretation of instruction value
------------------------------------------------------
[0] Load [PC+] data byte into register1
[1] Data: 0
[2] Add [PC+] data byte to register1
[3] Data: 1
[4] Load [PC+] data byte into register2
[5] Data: 10
[6] Subtract register1 from register2
[7] Jump if result not zero to address in [PC+]
[8] Data: 2
[9] Halt CPU
Intro to DDS MICRO-C Page: 5
2.2 Assembly Language
In the preceding section, you have learned how a CPU executes a
program, and how a program may be coded in memory as a series of
instruction and data values. It should be obvious to you that
although you can create programs in this way, it would be a long
and tedious job to write a program of any size using only numeric
values.
Not only is it very hard to remember the hundreds of
instruction values which may be used to perform certain
operations, but managing the memory address which are coded as
operands to the instructions becomes a real headache. This is
particularly true when you change a portion of the program,
causing a change in the number of bytes of memory used by that
portion, and therefore changes all of the memory address of
instructions and data which follow.
To help ease the programming job, each of the CPU manufacturers
have defined an ASSEMBLY LANGUAGE for their CPU, which represents
each of the machine operations with a more meaningful name called
a MNEMONIC. Similar instructions may be grouped under the same
mnemonic with the individual instruction values determined by the
operands. For example, it might be a completely different
instruction value which loads Register1 with a value than that
which loads register2. In assembly language you would use similar
statements such as:
LOAD R1,10
LOAD R2,10
The translation from mnemonics to instruction values is
performed by a program called an ASSEMBLER. In addition to
performing this translation, the assembler also allows LABEL names
to be assigned to addresses. The labels may be referred to from
within other assembly language statements instead of absolute
addresses.
When written in assembly language, our "count to 10" program
would look something line this:
LOAD R1,0
LOOP: ADD R1,1
LOAD R2,10
SUB R2,R1
JMPNZ LOOP
HALT
As you can see, the above program would be much more
understandable than a series of numbers, but it is still not
obvious to someone other that the author what the intent of the
program is until he has followed through the loop, and determined
what is accomplished by each instruction.
Intro to DDS MICRO-C Page: 6
Imagine that you have just started a new job as a computer
programmer, and your manager hands you a listing of several
hundred pages, each of which is full of assembly language lines
looking like the example above, and says "The 'SCAN' command
causes corruption of the database search parameters. This is VERY
important, could you stay and fix it tonight". You would have many
hours (days?) ahead of you trying to determine what is
accomplished by each portion of the program. Now, imagine that the
assembly language looked more like this:
;
; Simple demonstration program to show a counting loop
;
LOAD R1,0 ; Begin with count of zero
; Execute this loop once for each count
COUNT: ADD R1,1 ; Increment count
LOAD R2,10 ; Loop termination value
SUB R2,R1 ; Test R1, (result destroys R2)
JMPNZ COUNT ; Repeat until we reach 10
; We have reached 10 - All done
HALT ; Stop processing
The text statements following the ';' characters in the above
example are called COMMENTS. They are completely ignored by the
assembler, but are very useful to anyone attempting to understand
the program.
2.3 High Level Languages
As you can see in the preceding section, assembly language
programming offers much of an improvement over programming by
direct instruction values, while retaining the capability to
control EXACTLY the individual operations the program will
instruct the CPU to perform. Also, since the assembly language for
a particular CPU is defined by the manufacturer, you can be sure
that using it will allow you to take advantage of EVERY feature
and capability that has been designed into that particular CPU
architecture.
A good assembly language programmer can produce highly
efficient and compact programs because of this power. For this
reason you will often see assembly language used for very time or
size intensive applications.
Intro to DDS MICRO-C Page: 7
It would seem that assembly language would be the ideal method
of doing all your programming. There are however, several
drawbacks to using assembly language:
1) Efficient use of assembly language often requires a "different"
way of looking at a problem and strong "logical" mental
discipline.
** Not everyone is a good assembly language programmer **
2) Assembly language source files are big.
** It takes much codeing to perform even simple operations **
** Significant time is spent entering source text **
** Greater chance of error during design and entry **
3) Poorly documented assembly language is undecipherable.
** It is hard to maintain **
4) Each assembly language is different and incompatible.
** Programs will run on only one type of CPU **
** Programmers have difficulty working on other CPUs **
To help solve these problems, there are a number of "high
level" programming languages available. The main difference
between assembly and high level languages is that assembly
language produces only one CPU instruction for each language
"statement", while high level languages can produce many
instructions for each "statement".
High level languages attempt to provide a method of programming
by expressing ideas, rather than by directing the CPU to perform
each individual operation. When using a high level language, you
are freed from the task of keeping track of register and memory
usage, and can concentrate on expressing the algorithms which
accomplish the goal of the program.
Here are some "high level" versions of our "count to 10"
program:
Basic: 100 FOR I=0 TO 10:NEXT I
Fortran: DO 100 i=0,10
100 CONTINUE
Forth: 11 0 DO LOOP
'C': for(i=0; i < 10; ++i);
Intro to DDS MICRO-C Page: 8
2.4 Interpreters VS Compilers
There are two basic types of high level language
implementations, INTERPRETERS and COMPILERS.
An INTERPRETER is a program which reads your source program,
and performs the actions indicated by its statements. The main
advantages to this approach are:
1) FAST DEVELOPMENT: Interpreters often include complete text
editors, which make it easy to edit and debug your program
without leaving the interpreter. Also, since the program is
interpreted directly, there is no waiting to compile or
assemble it before you can try out a new change.
2) EASY DEBUGGING: Since the interpreter is actually another
program, it will usually allow you to stop your program in the
middle of execution, examine/modify variables, trace program
flow, display callback stacks, etc. This makes for very easy
debugging. Also, a good interpreter will perform very through
checking of your program as it runs, thus finding and reporting
design errors which might otherwise show up only as erratic and
inconsistent program operation.
And of course, there are drawbacks to interpreting:
1) SLOW EXECUTION: The interpreter has to process each statement
in your program and determine what action is to be performed
every time it encounters that statement. Many hundreds or even
thousands of instructions are executed to accomplish this FOR
EACH STATEMENT.
2) USES MEMORY: A good interpreter is a fairly complex program,
and therefore occupies a substantial portion of system memory,
meaning that less is available for your program & variables.
3) DIFFICULTY OF USE: Once you are finished debugging, you would
like to make your program, as easy to use as possible.
Unfortunately, when using an interpreter, you always have to
load and execute the interpreter before loading and executing
your program.
These disadvantages are so severe that interpreters are rarely
used for serious programs which are to be used frequently by a
number of people. They are however, excellent learning tools for
the novice computer user.
Intro to DDS MICRO-C Page: 9
A COMPILER is a program which reads your source program, and
translates its statements into CPU INSTRUCTIONS which perform the
specified function. Instead of actually executing your program, it
converts it to a form which can later be directly executed by the
CPU. Its main advantages are:
1) FAST EXECUTION: Since the program will be executed directly by
the CPU, it will run much faster that the equivalent program
being translated by an interpreter.
2) LESS MEMORY: Although a compiler is a very complex program, and
uses lots of memory when it runs, it only runs once, after
which your program is executed by itself directly by the CPU.
This means that the amount of memory required by the compiler
does not affect the amount of memory which is available for use
by your program when it runs.
3) EASE OF USE: Since your program executes by itself, you can
load and execute it directly from the operating system command
prompt.
The main disadvantages of compilers over interpreters are:
1) LONGER DEVELOPMENT: Many "traditional" compilers require that
you prepare your source program using a separate editor, and
then save it to a disk file, and submit that file to the
compiler. Every time you do this, you have to wait for the
compiler to finish before you can even try your program. NOTE:
some compiler vendors are now providing integrated editors,
eliminating the "save and exit" step, however you may not like
the editor they have chosen for you.
2) MORE DIFFICULT DEBUGGING: Since your program executes by
itself, you have to run a standard system debugger to monitor
its execution. This will usually be somewhat less intuitive
than an interpreters built in debugging features. NOTE: some
compiler vendors provide a "debug" option which includes
debugging information in the program, and a special debugger
which provides debugging facilities equal to or better than
those available from most interpreters.
2.5 Object Modules & Linking
Most assemblers and compilers available today support the use
of a LINKER. The linker is a program which will combine several
previously compiled (or assembled) programs called OBJECT MODULES
into a single larger executable program. This helps speed the
development process by eliminating the need to recompile the
entire program when you have changed only one module.
Intro to DDS MICRO-C Page: 10
2.6 Compiler Libraries
Modern compilers promote the use of STRUCTURED PROGRAMMING
techniques, which make programs easier to debug and maintain. I do
not propose to get into a discussion of structured programming
methods, but the main idea is to divide the program into simple
parts, each of which performs a clearly defined function.
Such functions often perform common algorithms required by many
programs, and hence are made into compiler LIBRARIES. These
libraries are simply collections of small useful programs which
may be used from within your programs without you having to write
them. Most compiler manufacturers provide such a "library" of
functions which they believe to be commonly needed, and the
development tools necessary to link them with your programs.
2.7 Portability
One BIG advantage of high level languages is the fact that once
a program is written and running on one CPU, you can usually get
it running on another completely different CPU with little
difficulty. This is because although the CPUs are different, the
HIGH LEVEL LANGUAGE IS NOT CPU DEPENDENT AND REMAINS THE SAME. All
you have to do is to recompile your program, using a compiler
which produces code for the new CPU.
Actually, it usually takes a bit more effort than that, because
the language or library functions may differ slightly from one
implementation to another.
This concept of PORTABILITY is one of the strong points of the
'C' language, and you will see it mentioned from time to time
throughout this manual. In addition to consistent compiler
language implementation, 'C' benefits from very "standard" library
function definitions which are followed by most vendors.
Intro to DDS MICRO-C Page: 11
3. INTRODUCTION TO 'C'
'C' is a "high level" computer language, which has become very
popular in recent years. It has proven suitable for a large variety
of programming tasks, and unlike most other high level languages, is
quite good for "low level" and "system" type functions. A good
example of this capability is the popular "UNIX" operating system,
which is written almost entirely in 'C'. Before UNIX, it was
generally thought that only assembly language was efficient enough
for writing an operating system.
Programs in 'C' are divided into two main parts, VARIABLES, which
are blocks of memory reserved for storing data, and FUNCTIONS, which
are blocks of memory containing executable CPU instructions. They are
created using a DECLARATION STATEMENT, which is basically a command
to the compiler telling it what type of variable or function you wish
to create, and what values or instructions to place in it.
There are several 'C' KEYWORDS which serve to inform the compiler
of the size and type of a variable or function. This information is
used by the compiler to determine how to interpret the value STORED
in a VARIABLE, or RETURNED by a FUNCTION.
Size: int - 16 bit value (default)
char - 8 bit value
type: unsigned - Positive only (0-2**bits)
+ Default is signed (positive & negative)
Examples: int a; /* 16 bit signed variable */
char b; /* 8 bit signed variable */
unsigned int c; /* 16 bit unsigned variable */
unsigned d; /* Also 16 bit unsigned variable */
unsigned char e; /* 8 bit unsigned variable */
Normally, when you define a function or global variable, its name
is made accessible to all object modules which will be linked with
the program. You may access a name which is declared in another
module by declaring it in this module with with the "extern"
modifier:
extern int a; /* external variable of type int */
extern char b(); /* external function returning char */
If you want to make sure that a function or global variable that
you are declaring is not accessible to another module (To prevent
conflicts with names in other modules etc), you can declare it with
the "static" modifier. This causes the name to be accessible only by
functions within the module containing the "static" declaration:
static int a;
Intro to DDS MICRO-C Page: 12
3.1 Functions
FUNCTIONS in 'C' are collections of C language STATEMENTS,
which are grouped together under a name. Each statement represents
an operation which is to be performed by the CPU. For example, the
statement:
A = A + 1;
directs the CPU to read the variable called 'A', add a value of 1
to it, and to store the result back into the variable 'A' (we'll
discuss variables in the next section). Note the SEMICOLON (';')
at the end of the statement. The 'C' compiler uses ';' to
determine when the statement ends. It does not care about lines or
spaces. For example, the above statement could also be written:
A
=
A
+
1
;
and would still compile without error. Thus, you can have a VERY
long statement in 'C', which spans several lines. You must always
however, be very careful to include the terminating ';'.
Each function within a 'C' program can be "called" by using its
name in any statement, may accept "argument" values which can be
accessed by the statements contained within it, and may return a
value back to the caller. This allows functions in 'C' to be used
as "building blocks", providing extensions to the language, which
may be used from any other function.
Below is a sample 'C' function, which performs an operation
(addition) on two argument values. The text between '/*' and '*/'
is COMMENTS, and is ignored by the compiler.
/* Sample 'C' function to add two numbers */
int add(num1, num2) /* Declaration for function */
int num1, num2; /* Declaration for arguments */
{
return num1+num2; /* Send back sum of num1 and num2 */
}
The names located within the round brackets '()' after the
function name "add" tells the compiler what names you want to use
to refer to the ARGUMENT VALUES. The "return" statement tell the
compiler that you want to terminate the execution of the function,
and return the value of the following expression back to the
caller. (We'll discuss "return" in more detail later).
Intro to DDS MICRO-C Page: 13
Now that you have defined the function "add", you could use it
in any other statement, in any function in your program, simply by
calling it with its name and argument values:
a = add(1, 2);
The above statement would call "add", and pass it the values
'1' and '2' as arguments. "add" evaluates 1 + 2 to be 3, and
returns that value back, which is then stored in the variable 'a'.
Note that 'C' uses the round brackets following a name to
determine that you wish to call the function, therefore, even if a
function has no argument values, you must include '()':
a = function();
Also note, that if a function does not return a value, or you
do not want to use the returned value, you simply code the
function name by itself:
function();
3.2 Variables
VARIABLES in 'C' are reserved areas of memory where the data
manipulated by the program is stored. Each variable is assigned a
name by which it is referenced by other 'C' statements. ALL
VARIABLES IN 'C' MUST BE DECLARED.
Variables in 'C' may be single values as shown earlier, or they
may be declared as an ARRAY, which reserves memory space for a
number of data elements, each with the type declared for the
variable.
int array[4];
The above statement reserves memory for four 16 bit signed
values, under the name "array". It is important to know that 'C'
considers the elements of an array to be numbered from zero (0),
so the four locations in the above array are referenced by using:
array[0]
array[1]
array[2]
array[3]
There are two basic types of variables in 'C', GLOBAL and
LOCAL.
Intro to DDS MICRO-C Page: 14
3.2.1 GLOBAL Variables
GLOBAL variables are set up permanently in memory, and exist
for the duration of the entire programs execution. The names of
global variables may be referenced by any statement, in any
function, at any time. Global variables are declared in 'C' by
placing the declaration statement OUTSIDE of any function. For
example:
int a; /* Declare GLOBAL variable */
inita() /* Function to initialize 'a' with 1 */
{
a = 1;
}
Note that the declaration statement for 'a' is NOT contained
within the definition of "inita".
Since global variables are permanent blocks of memory, it is
possible to INITIALIZE them in the declaration statement. This
causes the variable to be assigned a value at COMPILE time,
which will be loaded into memory at the same time that the
program is loaded to be executed. This means that your program
will not have to explicitly store a value in a.
int a = 1;
Array variables may also be initialized in the declaration
statement, by using the curly brackets '{}' to inform the
compiler that you have multiple values:
int a[4] = { 1, 2, 3, 4 };
In MICRO-C, the initial values for an array are expressed as
a single string of values REGARDLESS of the shape of the array:
int a[2][2] = { 1, 2, 3, 4 };
If an array has only one dimension (set of '[]'s), you do
not have to specify the size of initialized variables. The
compiler will automatically set the size to the number of
initial values given:
int array[] = { 1, 2, 3, 4 };
Intro to DDS MICRO-C Page: 15
3.2.2 LOCAL Variables
Variables which are declared WITHIN a function are
determined by the compiler to be LOCAL. The memory used by
these variables is automatically reserved when the function
begins to execute, and is released when it terminates. Names of
LOCAL variables are only accessible from within the function
where they are defined:
inita() /* Function to initialize 'a' with 1 */
{
int a; /* Declare LOCAL variable */
a = 1;
}
The above function shows the declaration of a local
variable, but is not very useful since the local variable will
cease to exist when the function returns. Once a function
terminates, the content of its local variables is lost. Local
variables are used as temporary locations for holding
intermediate values during a functions execution, which are not
required by any other part of the program.
Each function may have its own local variables, but since
memory is only used by the functions which are actually
executing, the total amount of memory reserved is usually less
that the total size of all local variables in the program.
Since local variables are allocated and released during the
execution of your program, it is not possible to initialize
them at compile time, and therefore MICRO-C does not allow them
to be initialized in the declaration. Some compilers do allow
this, however, the code generated is equivalent to using
assignment statements to initialize them at the beginning of
the function.
The ARGUMENTS to a function (See Functions) are actually
local variables for that function which are created when the
function is called. For this reason, the argument names are
also unavailable outside of the function in which they are
defined.
NOTE: Local variables declared with the "static" modifier
are a special case, and are allocated permanently in memory
like global variables, NOT upon entry and exit from the
function. This has several effects:
- The variable may be initialized at compile time.
- The variable retains its content (and occupies memory)
between calls to the function.
- If the function is called recursively (see later), each
instance of the function will access the same variable.
Intro to DDS MICRO-C Page: 16
3.3 Pointers
A POINTER in 'C' is a memory address, which can be used to
access another data item in memory. All pointers in MICRO-C are 16
bit values, which allows access to a maximum of 65536 bytes of
memory through it.
Any variable or function may be declared as a pointer by
preceding its name with the '*' character in the declaration:
int *a; /* a = 16 bit pointer to int value */
char *b; /* b = 16 bit pointer to char */
extern char *fgets(); /* Returns 16 bit pointer to char */
Later on, I will show you how you can use a special operator
called INDIRECTION to access data items at the address contained
in the pointer.
3.4 A complete 'C' program
With all of the preceding information under your belt, you
should be able to understand most of this simple but complete
program:
#include stdio.h
/* Main program */
main()
{
int a, b, c;
a = 1;
b = 2;
c = add(a, b);
printf("The result of (1+2)=%d\n", c);
}
/* Our old familiar "add" function */
int add(num1, num2)
int num1, num2;
{
return num1+num2;
}
Well... OK, perhaps you weren't quite ready for it. There are a
few new things presented in this program, which I will now
explain.
First of all, you should know that the function name "main" is
a special name which will be called at the very beginning, when
the program is first run. It provides a starting point for your
programmed functions. All 'C' programs have a "main" function.
Intro to DDS MICRO-C Page: 17
You may also be wondering about those "#include" and "printf"
statements. This all comes back to the concept of PORTABILITY, and
has to do with the programs ability to perform INPUT and OUTPUT
(I/O). Methods of performing I/O may differ greatly from one
operating system to another, and hence make it difficult to write
"portable" programs. If you don't know what portability is, go
back and read the "Background Information" section.
In order to insure that 'C' compilers could be easily adapted
to nearly any operating system, the designers of the language
decided not to include ANY I/O capabilities in the compiler
itself. By not implementing it, they didn't have to worry about
it. All such activity is performed by a set of functions in the
'C' STANDARD LIBRARY, which is provided for each operating system.
These library functions are used in nearly all programs, since
a program which can't read or write any data doesn't do much
useful work.
Many of the library functions must be declared as a certain
type, which may be specific to the compiler implementation or
operating system. (For example the "printf" functions must be
declared as "register" in MICRO-C). The file "stdio.h" is provided
with all "standard libraries", and contains any special
declarations required by the library I/O functions.
The "#include" statement causes the compiler to read the
"stdio.h" file, and to process the declaration statements
contained within it. This is equivalent to incorporating the full
text of "stdio.h" at the beginning of your program.
NOTE: If you are using a version of MICRO-C for an embedded
processor, this file is called "<cpu>io.h". For example, in the
8051 developers kit, it is called "8051io.h".
The "printf" statement is actually a call to a STANDARD LIBRARY
FUNCTION. It is available in almost all 'C' implementations, and
in the above example, displays the prompt "The result of (1+2)=",
followed by the decimal value of the passed argument 'c'. For more
information about "printf" and other library functions, refer to
the MICRO-C Technical manual.
At his point you may wish to enter the demonstration program
into a file called "DEMO1.C", and compile it with the MICRO-C
compiler. Remember that 'C' IS CASE SENSITIVE, so be sure to enter
the program EXACTLY as it is shown. Also, make sure that you are
positioned in the MICRO-C directory before you create the file.
After entering and saving the file with your favorite text editor,
compile the program using the command:
CC DEMO1
You can run the resultant "DEMO1.COM" program, by simply typing
"DEMO1", at the DOS command prompt.
Intro to DDS MICRO-C Page: 18
3.5 'C' memory organization
Now that you have seen a complete 'C' program, and know the
basic concepts of functions and variables, you may want to know
how MICRO-C organizes the computer memory when these constructs
are used. Knowing this may help you understand functions and
variables more precisely.
The information in this section is not really necessary for
casual use of the language, if you feel that such detail would
only confuse you, feel free to skip it until later.
The MICRO-C compiler builds your program to occupy a block of
memory. In the case of small 8 bit computers, this block of memory
will usually be the entire free ram in the machine. In the case of
larger machines, it will usually be 64K (65536 bytes), but may be
larger or smaller depending on the implementation.
The exact size of the memory block is unimportant, since it
affects only the maximum size of a MICRO-C program. The methods of
memory allocation remain the same.
3.5.1 Static memory
MICRO-C places all of the executable code from the compiled
functions at the very beginning of the memory block. This
includes all CPU instructions which are generated by the
compiler. MICRO-C places all initialized global and "static"
local variables in this area as well, and also something called
the LITERAL POOL.
The "literal pool" consists of all string data which is used
in statements or initializations in the program. An example of
this is the string used in the preceding demonstration program
("Result of (1+2)=%d\n"), which is a series of data bytes,
which are passed to the "printf" function.
This collection of CPU instructions, Initialized variable
data, and literal pool data is the complete program image which
must be loaded into memory every time the program is executed.
The next section of memory allocated by MICRO-C holds the
global and static local variables which have not been
initialized. Since they have no initial values, they do not
have to be loaded every time the program runs. This also means
that until the program stores a value in a particular memory
location its contents will be some random value which happened
to be at that location in memory before the program was loaded.
All of this memory is called "STATIC" memory, because it is
reserved for code and data at COMPILE time. Once the program is
compiled, the above mentioned items are fixed in memory, and
cannot be moved or changed in size.
Intro to DDS MICRO-C Page: 19
3.5.2 Dynamic memory
When your program begins execution, one of the first things
that happens, is that a STACK is set up at the very top of the
memory block. This stack is always referenced by a STACK
POINTER register which always points to the lowest address of
memory used on the stack. All memory in the block above the
stack pointer is deemed to be in use. This is usually a built
in feature of the CPU.
At the beginning of every function, the code produced by
MICRO-C contains instructions to reduce the value of the stack
pointer by the number of bytes of memory required by the local
variables in that function. When the function "returns" or
terminates, the stack pointer is increased by the same amount.
This allows the function to use the memory immediately above
the new stack pointer for its local variables without fear that
another function will also try to use it. When another function
is called, it will reserve its memory BELOW the memory already
in use by the calling function, and will return the stack
pointer when it is finished. Thus, all local variables may be
accessed as constant offsets from the stack pointer set up at
the beginning of the function.
ARGUMENTS to a function are passed by reserving memory on
the stack, and setting it to the argument values, just PRIOR to
calling the function. When the called function returns, the
stack reserved for its arguments is removed by the function
performing the call. In this way, the arguments are just more
local variables, and may also be accessed as constant offsets
from the stack pointer.
3.5.3 Heap memory
Some programs need temporary memory which will not disappear
when the function terminates, or they may not know the exact
amount of memory they need for a certain operations until some
calculations have been performed.
To resolve these problems, 'C' provides another type of
dynamic memory which is called "HEAP" memory. To make use of
this memory, the program uses the "malloc" function (from the
standard library) which allocates a block of memory, and
returns a pointer value to its address. The program may then
access and manipulate this memory through the pointer.
When the program is finished with the memory, it may then
use the "free" library function to release it, and make it
available for use by other calls to "malloc".
A program may continue allocating memory via "malloc" as
long as there is available free memory to allocate. The library
functions will keep track of which blocks of memory are
allocated, and which blocks are available for allocation.
Intro to DDS MICRO-C Page: 20
A typical memory layout for a 'C' program in the middle of
execution might look something like this:
+----------------------------------------+
| CPU Instructions |
| which make up program | *1
| "code" |
+----------------------------------------+
| Initialized GLOBAL variables |
| and | *1
| Initialized "static" LOCAL variables |
+----------------------------------------+
| LITERAL POOL data | *1
+----------------------------------------+
| Uninitialized GLOBAL variables |
| and |
| Uninitialized "static" LOCAL variables |
+----------------------------------------+
| Memory allocated to the heap |
+----------------------------------------+
| (Heap grows upward) |
| | |
| Free memory, available for growth of |
| the heap and stack. |
| | |
| (Stack grows downward) |
+----------------------------------------+
| Local variables of innermost function | *2
+----------------------------------------+
| Return address of innermost function |
+----------------------------------------+
| Arguments of innermost function |
+----------------------------------------+
| Local variables of middle function | *2
+----------------------------------------+
| Return address of middle function |
+----------------------------------------+
| Arguments of middle function |
+----------------------------------------+
| Local variables of main function | *2
+----------------------------------------+
| Return address of main function |
+----------------------------------------+
*1 = Only these blocks of memory need to be loaded from disk
when the program is started.
*2 = Other than "static" local variables.
For those not familiar with computer architecture, the RETURN
ADDRESS is placed on the stack by a CALL INSTRUCTION, and is the
memory address immediately following that instruction. When a
RETURN INSTRUCTION is later executed, the CPU removes the return
address from the stack, and places it in the PROGRAM COUNTER, thus
causing program execution to resume with the instruction
immediately following the original call instruction.
Intro to DDS MICRO-C Page: 21
4. EXPRESSIONS
An expression in 'C' consists of one or more values, and OPERATORS
which cause those values to be modified in a calculation. Anywhere
that you would use a simple value in 'C', you can also use an
expression. We have already seen that with the "return" statement in
the "add" function in our example program. Knowing that we can use
expressions and values interchangeably, we could shorten the example
"main" function to:
main()
{
int a, b;
a = 1;
b = 2;
printf("The result of (1+2)=%d\n", add(a,b));
}
or even:
main()
{
int a, b;
a = 1;
b = 2;
printf("The result of (1+2)=%d\n", a+b);
}
or even:
main()
{
printf("The result of (1+2)=%d\n", 1 + 2);
}
Or, if we wanted the 'a, b & c' variables set anyway:
/* Note the use or round brackets '()' to incorporate
a SUB-EXPRESSION into the main expression */
main()
{
int a, b, c;
printf("The result of (1+2)=%d\n", c = (a = 1) + (b = 2));
}
You can see that an entire expression, including three ASSIGNMENTS
was included in a place where only a simple value is used. This is
one of the great powers of 'C', and can result in very small fast
efficient code (at the expense of readability).
Intro to DDS MICRO-C Page: 22
4.1 Unary operators
Unary (single operand) operators are those operators which
perform a computation on only a single value. In most cases, the
operator must be placed immediately BEFORE the value to be
operated on.
4.1.1 Negate: -
The NEGATE operator causes the value to its right to be
changed in sign. Thus, a positive value will be converted to a
negative value (of the same magnitude) and vice versa. It is
most often used to enter negative numbers, but it is perfectly
legal to apply this operator to a variable or expression.
eg: a = -5; /* a = negative 5 */
b = 10; /* b = positive 10 */
c = -b; /* c = -10 */
d = -(b+5); /* d = -15 */
e = -b+5; /* e = -5 *
f = -a; /* f = 5 */
4.1.2 Bitwise Complement: ~
The BITWISE COMPLEMENT operator reverses the value (0 or 1)
of each individual BIT in the value to its right. This is very
useful when used with the other BITWISE operators (AND, OR and
XOR), to perform test on combinations of bits in a byte (char)
or word (int).
eg: a = 5; /* a = 00000101 */
b = ~a; /* b = 11111010 */
4.1.3 Logical Complement: !
The LOGICAL COMPLEMENT operator reverse the "logical sense"
(TRUE or FALSE) of the value to its right. In 'C' a value is
considered to be logically TRUE if it contains any value other
than zero. A value of zero is considered to be logically FALSE.
The logical complement gives a value of zero (FALSE) if the
original value was non-zero, and a value of one (TRUE) if the
original value was zero. You will see later that there are
CONDITIONAL statements in 'C', which perform certain operations
only if values are TRUE. This operator provides a simple method
of reversing those conditions to occur when the value is FALSE.
eg: if(a) b = 10; /* Set b to 10 if a is TRUE */
if(!a) b = 10; /* Set b to 10 if a is FALSE */
Intro to DDS MICRO-C Page: 23
4.1.4 Increment: ++
The INCREMENT operator must be used on a value that can be
ASSIGNED (such as a variable or array element). It causes the
value to be INCREASED by one (except for a special case with
POINTERS which are advanced by the size of the type to which
they point).
Unlike most other unary operators, the increment operator
may be placed either BEFORE or AFTER the value, and behaves
slightly differently depending on its position. If placed
BEFORE the value, the variable is incremented, and the new
value is passed on as the result. If placed AFTER the value,
the original value is passed on as the result, and the variable
is then incremented.
eg: a = b = 10; /* Set a & b to 10 */
c = ++a; /* c = 11 (a = 11) */
d = b++; /* d = 10 (b = 11) */
4.1.5 Decrement: --
The DECREMENT operator behaves exactly the same as
increment, except that the value is REDUCED instead of
increased.
eg: a = b = 10; /* Set a & b to 10 */
c = --a; /* c = 9 (a=9) */
d = b--; /* d = 10 (b=9) */
4.1.6 Indirection: *
The INDIRECTION operator may only be applied to POINTERS, or
expressions which result in a POINTER VALUE. It causes the
memory contents at the address contained in the pointer to be
accessed, instead of the pointer value itself.
eg: char *ptr; /* Declare a pointer to character */
ptr = 10; /* Set pointer variable to 10 */
*ptr = 5; /* Set 'char' at address 10 to 5 */
a = ptr; /* a = 10 (pointer value) */
b = *ptr; /* b = 5 (Indirect through address 10) */
4.1.7 Address: &
The ADDRESS operator may only be used on a value which can
be ASSIGNED (such as a variable or array element). It returns
the memory address of the value, instead of the value itself.
eg: a = 10; /* Set variable a to 10 */
ptr = &a; /* Get address of 'a' */
b = *ptr; /* b = 10 (contents of 'a') */
*ptr = 15; /* Store 15 at address of 'a' */
c = a; /* c = 15 */
Intro to DDS MICRO-C Page: 24
4.2 Binary Operators
In addition to the "unary" operators presented above, 'C' has a
large complement of BINARY (two operand) operators. The binary
operators take two values, presented on the left and right side of
the operator, and combine them into some form of computed value.
4.2.1 Addition: +
The ADDITION operator computes the SUM of two values.
eg: a = b + 5; /* a = sum of b and 5 */
4.2.2 Subtraction: -
The SUBTRACTION operator computes the DIFFERENCE of two
values.
eg: a = b - 5; /* a = difference of b and 5 */
4.2.3 Multiplication: *
The MULTIPLICATION operator computes the PRODUCT of two
values.
eg: a = b * 5; /* a = b multiplied by 5 */
4.2.4 Division: /
The DIVISION operator computes the QUOTIENT resulting from
the division of the left value by the right value.
eg: a = b / 5; /* a = b divided by 5 */
4.2.5 Modulus: %
The MODULUS operator computes the REMAINDER resulting from
the division of the left value by the right value.
eg: a = b % 5; /* a = remainder of b divided by 5 */
4.2.6 Bitwise And: &
The BITWISE AND operator performs an AND function on each
pair of bits between the values. Bit positions which have a one
(1) bit in BOTH values will receive a one in the result. All
other bit positions will receive zero (0).
eg a = 5; /* a = 00000101 */
b = 6; /* b = 00000110 */
c = a & b; /* c = 00000100 (4) */
Intro to DDS MICRO-C Page: 25
4.2.7 Bitwise Or: |
The BITWISE OR operator performs an OR function on each pair
of bits between the values. Bit positions which have a one (1)
in EITHER value will receive a one in the result. All other bit
positions will receive zero (0).
eg a = 5; /* a = 00000101 */
b = 6; /* b = 00000110 */
c = a | b; /* c = 00000111 (7) */
4.2.8 Bitwise Exclusive Or: ^
The BITWISE EXCLUSIVE OR operator performs an EXCLUSIVE OR
function on each pair of bits between the values. Bit positions
which have a one (1) in EITHER value, but NOT IN BOTH values
will receive a one in the result. All other bit positions will
receive zero (0).
eg a = 5; /* a = 00000101 */
b = 6; /* b = 00000110 */
c = a ^ b; /* c = 00000011 (3) */
4.2.9 Logical And: &&
The LOGICAL AND operator returns TRUE (non-zero) only if
BOTH values are TRUE. If either value is FALSE, FALSE (zero) is
returned.
MICRO-C accomplishes this by evaluating the left value, and
returning zero (FALSE) if it is equal to zero, otherwise the
right value is evaluated and returned. Some 'C' compilers force
the returned value to be either zero (0) or one (1).
eg: if(a && b)
printf("Both 'a' AND 'b' are TRUE");
4.2.10 Logical Or: ||
The LOGICAL OR operator returns TRUE (non-zero) if EITHER
value is true, if both values are FALSE, FALSE (zero) is
returned.
MICRO-C accomplishes this by evaluating the left value, and
returning its value if it is not zero (TRUE), otherwise the
right value is evaluated and returned. Some 'C' compilers force
the returned value to be either zero (0) or one (1).
eg: if(a || b)
printf("Either 'a' OR 'b' is TRUE");
Intro to DDS MICRO-C Page: 26
4.2.11 Shift Left: <<
The SHIFT LEFT operator returns a value which is equal to
the left value shifted left by a number of bits equal to the
right value.
eg: a = 10; /* a = 00001010 */
b = a << 3; /* b = 01010000 (80) */
4.2.12 Shift Right: >>
The SHIFT RIGHT operator returns a value which is equal to
the left value shifted right by a number of bits equal to the
right value.
eg: a = 80; /* a = 01010000 */
b = a >> 3; /* b = 00001010 (10) */
4.2.13 Equals: ==
The EQUALS operator performs a test of the values, and
returns a one (1) of they are identical. If they do not match,
a zero (0). is returned. NOTE a common difficulty encountered
when learning 'C' is to confuse the EQUALS (==) and ASSIGNMENT
(=) operators.
eg: if(a == 10)
printf("a is equal to 10");
4.2.14 Not Equals: !=
The NOT EQUALS operator perform a test of the two values,
and returns a one (1) if they are NOT identical. If the values
match, a zero (0) is returned.
eg: if(a != 10)
printf("a is not equal to 10");
Intro to DDS MICRO-C Page: 27
4.2.15 Greater Than: >
The GREATER THAN operator performs a test of the two values,
and returns a one (1) is the left value is higher than the
right value. If the left value is equal to or less than the
right value, zero (0) is returned.
eg: if(a > 10)
printf("a is bigger than 10");
4.2.16 Less Than: <
The LESS THAN operator performs a test of the two values,
and returns a one (1) is the left value is lower than the right
value. If the left value is equal to or greater than the right
value, zero (0) is returned.
eg: if(a < 10)
printf("a is smaller than 10");
4.2.17 Greater Than or Equals: >=
The GREATER THAN OR EQUALS operator performs a test of the
two values, and returns a one (1) is the left value is higher
than OR equal to the right value. If the left value is less
than the right value, zero (0) is returned.
eg: if(a >= 10)
printf("a is bigger than or equal to 10");
4.2.18 Less Than or Equals: <=
The LESS THAN OR EQUALS operator performs a test of the two
values, and returns a one (1) is the left value is lower than
OR equal to the right value. If the left value is greater than
the right value, zero (0) is returned.
eg: if(a <= 10)
printf("a is smaller than or equal to 10");
Intro to DDS MICRO-C Page: 28
4.2.19 Assignment: =
The ASSIGNMENT operator takes the value to the right, and
STORES it in the left value. The left value must be ASSIGNABLE.
eg: a = 10; /* Store 10 in variable a */
4.2.20 Self Assignment Operators
'C' provides special operators which implement a shorthand
method of performing an operation on two values when the result
is stored back into the left value. These operators are called
SELF ASSIGNMENT operators.
Shorthand: Equivalent to:
--------------------------------------
a += 1; a = a + 1;
b -= 2; b = b - 2;
c *= 3; c = c * 3;
d /= 4; d = d / 4;
e %= 5; e = e % 5;
f &= 6; f = f & 6;
g |= 7; g = g | 7;
h ^= 8; h = h ^ 8;
i <<= 9; i = i << 9;
j >>= 10; j = j >> 10;
With many compilers, the self assignment operators do not
produce any better code when using simple variables. They
simply allow you to state what you want done in a more clear
and concise manner.
With MICRO-C, and most non-optimizing compilers, there is an
advantage to using the self assignment operators when
referencing a CALCULATED ADDRESS (Such as when accessing an
indexed array).
array[a] += b;
Is often more efficient than:
array[a] = array[a] + b;
Intro to DDS MICRO-C Page: 29
4.3 Other Operators
There are a few 'C' operators which do not fall into one of the
above clearly defined classes. Those operators are presented here:
4.3.1 Statement terminator: ;
We have already seen the ';' STATEMENT TERMINATOR, this is
just a reminder of how important it is. Leaving out the ';' at
the end of a statement is a good way to convince the compiler
to produce lots of error messages.
4.3.2 Comma Operator: ,
The function performed by the COMMA operator ',' depends on
where it occurs in the 'C' source file.
When used within the arguments to a function call, its
function is to separate each argument that is passed:
eg: function(a, b, c);
When used in a DECLARATION statement, its function is to
separate each variable name to be defined:
eg: int a, b, c;
When used in a global variable initialization, its function
is to separate the initial elements:
eg: int array[] = { 1, 2, 3 };
When used in any expression other than those mentioned
above, a comma allows several expressions to be used in a place
where only one expression is expected. The value returned in
that of the RIGHTMOST expression:
eg: return a=4, 10; /* Set a = 4, and return 10 */
Intro to DDS MICRO-C Page: 30
4.3.3 Conditional Operator: ?
The CONDITIONAL operator allows you to create a single
expression which evaluates one of two SUB-EXPRESSIONS depending
on a logical condition. It is coded in the 'C' source program
in the form:
<condition> ? <TRUE expression> : <FALSE expression>
Consider, the standard library function "toupper", which
converts any lower case character to upper case. If the
original character was lower case, no conversion is made, and
the character is returned unchanged.
Knowing that in ASCII, lower case characters have a value
equal to the equivalent upper case character + 64, we could
code the following function:
/* Function to convert lower case to upper case */
char toupper(chr)
char chr;
{
if((chr >= 'a') && (chr <= 'z')) /* lower case ? */
return chr - 64; /* Convert */
else
return chr; /* Leave alone */
}
The same function coded using a conditional expression is:
/* Function to convert lower case to upper case */
char toupper(chr)
char chr;
{
return (chr >= 'a') && (chr <= 'z') ? chr - 64 : chr;
}
Although the difference between the above two functions may
seem trivial, remember that you can use ANY expression
(including a CONDITIONAL expression) anywhere that you could
place a simple value. Once you get the hang of that, you will
find that conditional expressions are a very powerful feature
of the 'C' language.
Intro to DDS MICRO-C Page: 31
4.3.4 Round Brackets: ( )
The ROUND BRACKETS are used by 'C' to perform two functions.
First, as already mentioned, they identify function calls, and
hold any arguments which are being passed:
eg: function1();
function2(a);
function3(a, b);
The round brackets are also used to set up a SUB EXPRESSION,
which can be used within another expression as a simple value:
For example:
b = a + 5;
c = b / 2;
Can be replaced by:
c = (a + 5) / 2;
Or, if you want 'b' assigned:
c = (b = a + 5) / 2;
4.3.5 Square Brackets: [ ]
The SQUARE BRACKETS are used to perform indexing. Variables
which have been declared as ARRAYS may be indexed.
MULTIDIMENSIONAL arrays may be indexed by using a pair of
square brackets for each dimension:
eg: int array1[4], array2[2][2];
array1[0] = 10;
a = array1[1];
b = array2[1][0];
If a multidimensional array is indexed with a number of
square bracket pairs which is less that the number of
dimensions of the array, the value returned is a POINTER VALUE
to the beginning address of the next dimension. A pointer may
be indexed as it they were single dimension array, the memory
offsets added to the pointer will be calculated based on the
size of the data type to which it has been declared to point:
eg: int array[3][3], *ptr;
array[2][1] = 10;
ptr = array[2];
a = ptr[1]; /* a = 10 */
Intro to DDS MICRO-C Page: 32
5. CONTROL STATEMENTS
There are a number of statements in 'C', which serve to control
the flow of program execution.
5.1 The IF statement
The IF statement controls the execution of one other statement,
based of the logical value (TRUE or FALSE) of a CONDITION
EXPRESSION:
if(condition)
statement;
The "statement" in the above example would only be executed if
"condition" evaluated to a logically TRUE (non-zero) value.
An optional "else" clause may be added to the IF statement. In
this example, "statement-1" is executed if "condition" evaluates
to TRUE, and "statement-2" is executed if "condition" evaluates to
FALSE.
if(condition)
statement-1;
else
statement-2;
5.2 The WHILE Loop
The WHILE statement is similar to the first form of IF
statement (without an else), except that the "statement" is
repeated until the condition becomes FALSE. If the "statement"
does not effect the condition, the statement will be executed over
and over forever (ie: infinite loop).
while(condition)
statement;
Try entering, compiling and executing the following program:
#include stdio.h
/* Our old familiar "count to 10" program, with a few
improvements to let us see the numbers */
main()
{
int a;
a = 0;
while(a < 10)
printf("A=%d\n", a++);
}
Intro to DDS MICRO-C Page: 33
Note the '\n' at the end of the prompt in the call to "printf".
'C' operates on a concept called STREAM I/O, which means that all
input and output is considered to occur in a never ending
"stream", like a stream of water. There are no "lines" in the
stream unless we cause them. The '\n' sequence at the end of the
prompt is a NEWLINE character, which tells the compiler to insert
the necessary code into the string to cause the output to go to a
NEW LINE on a terminal or printer when the string is displayed.
Try removing the '\n' from the prompt and see what happens.
5.3 The DO/WHILE Loop
The DO/WHILE statement is similar to the WHILE statement,
except that the condition is tested AFTER the statement is
executed, not BEFORE. Note that when using DO/WHILE, you are
guaranteed that the statement will always be executed at least
ONCE.
do
statement;
while(condition);
Here is another example of our "count" program, which uses our
own function to output the number in decimal:
#include stdio.h
/* Main "count" program */
main()
{
int a;
a = 0;
do
outnum(a);
while(++a < 10);
}
/* Function to output a number in decimal */
outnum(value)
unsigned value;
{
char stack[6]; /* Small stack to hold digits */
unsigned sp; /* Our own stack pointer */
/* calculate each digit of the number (in reverse order) */
sp = 0;
do
stack[sp++] = (value % 10) + '0';
while(value /= 10);
/* Display the stack of calculated digits */
while(sp)
putc(stack[--sp], stdout);
putc('\n', stdout); /* move to new line */
}
Intro to DDS MICRO-C Page: 34
The "putc" function is another function from the STANDARD
LIBRARY. The name "stdout" is defined in the "stdio.h" file, and
tells "putc" that the character should be written to "standard
output" (The PC console).
The DO/WHILE loop in the "outnum" function demonstrates a case
where a simple WHILE would not suffice. Try to recode the function
using only WHILE loops. You might try:
sp = 0;
while(value)
stack[sp++] = (value % 10) + '0', value /= 10;
This appears to work, however, try executing "outnum(0)". The loop
is never executed, "sp" is never incremented from zero, and
NOTHING is output. Not even a single '0'.
5.4 The FOR Loop
Notice that in the example above, we have to initialize "sp",
test "value", and perform an operation on value at the end of the
loop. 'C' provides a special loop construct, just for doing such
operations. It is called a FOR loop:
for(initialization; condition; operation)
statement;
Using the FOR loop, we can rewrite the above example as:
for(sp = 0; value; value /= 10)
stack[sp++] = (value % 10) + '0';
Actually, the FOR loop is more often used to implement simple
loops which execute a predetermined number of times:
/* A count loop, which executes 10 times */
for(a=0; a < 10; ++a)
printf("A=%d\n", a);
Sometimes, it is possible to do all the computations that are
required in the "initialization", "condition" and "operation"
expressions to the FOR loop. In this case, you could use the
SEMICOLON ';' as the entire "statement" to tell FOR that and you
don't want it to perform any other operations:
for(a=0; a < 10; printf("A=%d\n", a++))
; /* This is a NULL statement */
Intro to DDS MICRO-C Page: 35
This semicolon by itself is called a NULL statement, and can be
used anywhere that any other statement would be used.
a = 0;
while(a++ < 100)
;
if(a == b)
;
else
printf("a is not equal to b");
Note that the expressions in a FOR statement end with ';'. This
means that they are also optional, and could be replaced by NULL
statements.
for(; a < 10; ++a) ; /* No initialization */
for(a = 0;; ++a) ; /* No condition (infinite loop) */
for(a = 0; ++a < 10;) ; /* No operation at end of loop */
for(;;) ; /* Standard way to do infinite loop */
5.5 Compound Statements
This is all well and good you say, but IF, WHILE, DO/WHILE and
FOR seem quite limited in that the "condition" or "loop" applies
only to a single statement. What if I want to perform more complex
calculations in my loop?
Another very powerful feature of the 'C' language, is that any
number of ordinary statements may be placed together in a group
and treated as a single large statement. All you have to do is to
enclose them in CURLY BRACKETS '{ }'. For example:
if(a == 10) { /* '{' begins compound statement */
printf("A was equal to 10... ");
a = 99;
printf("But its not anymore"); } /* '}' ends */
else
printf("A was never equal to 10");
You have already seen an example of compound statements, in the
curly brackets which surround the body of functions. Technically,
each function consists of only one statement, but thanks to 'C's
compounding capability, you can actually use as many statements as
you wish.
Intro to DDS MICRO-C Page: 36
5.6 BREAK and CONTINUE
Two more 'C' statements are available which are designed
specially for extending the capabilities of loops.
The BREAK statement causes the program to skip any remaining
statements in the loop, and to break out of the loop. Execution
will proceed with the first statement which follows the loop, just
as if the loop had terminated in its normal fashion:
for(a=0; a < 10; ++a) {
if(a == 5)
break;
printf("A=%d\n", a); }
The above loop will never reach its terminal count of 10,
because the "break" statement will be executed when 'a' reaches
five. Thus, the BREAK statement allows you to sprinkle additional
exit conditions throughout the body of the loop.
The CONTINUE statement causes the loop to skip any remaining
statements in the loop, but to continue looping. Execution will
proceed as if all the statements in the loop had executed
normally:
for(a=0; a < 10; ++a) {
if(a != 5)
continue;
printf("A=%d\n", a); }
The above example will execute all 10 iterations of the loop,
but the "printf" statement will only be executed for the loop in
which 'a' is equal to 5.
5.7 The SWITCH Statement
Sometimes, you have a large number of conditions, which are to
be executed depending on the value of a certain variable. One way
that you could do this is with a series of "IF" statements, using
a popular "else if" construct:
if(a == 1)
statement-1; /* Executed if a == 1 */
else if(a == 2)
statement-2; /* Executed if a == 2 */
else if(a == 3)
statement-3; /* Executed if a == 3 */
else if(a == 4) { /* Note compound statement */
statement-4; /* Executed if a == 4 */
statement-5; } /* Executed if a == 4 */
else
statement-6; /* Executed if a == anything else */
Intro to DDS MICRO-C Page: 37
'C' has a built in statement to implement this kind of
structure. It is more readable, and usually generates more
efficient code than such a series of "IF" statements. It is called
a SWITCH statement:
switch(a) {
case 1 :
statement-1; /* Executed if a == 1 */
break;
case 2 ;
statement-2; /* Executed if a == 2 */
break;
case 3 :
statement-3; /* Executed if a == 3 */
break;
case 4 :
statement-4; /* Executed if a == 4 */
statement-5; /* Executed if a == 4 */
break;
default:
statement-6; } /* Executed of a == anything else */
The "BREAK" statement at the end of every CASE causes the
program to proceed to the statement immediately following the end
of the entire switch construct. If it were not present, the
statements in a case would "fall through", and execute the
statements in the following case as well. Since the case values do
not have to be presented in any particular order, this behavior
can often be used to your advantage by careful placing of the case
statements relative to each other. Note also that the code in the
last case ("Default") does not need a "break" since it is already
at the end of the switch construct.
5.8 Labels and GOTO
If none of the above constructs does exactly what you need to
structure a particular program. 'C' also has available the old
familiar GOTO command. Although the use of "GOTO" is often frowned
upon, it can save you much programming effort in some cases. In
order to use goto, you must have a LABEL. Any statement may be
labeled by preceding it with a name, followed immediately by ':'.
For example:
#include stdio.h
/* Our "count to 10" program using goto looping */
main()
{
int a;
a = 0;
count: printf("A=%d\n", a); /* Labeled "count" */
if(++a < 10)
goto count;
}
Intro to DDS MICRO-C Page: 38
6. RECURSION
RECURSION is the ability of a function to call itself, and is a
powerful capability of the 'C' language.
In a previous section, I explained how memory is allocated on the
stack to a function when it begins execution. This allows each
function to have its own local variables. It also means that if a
function calls itself (by referencing its own name in an expression
contained within it), the function will begin executing with its own
local memory, which is distinct from the local memory of the calling
function (which is itself!). This means that when the "lower"
instance of the function terminates, it will not have affected the
memory or execution state of the "higher" version of the function.
The classic example of a recursive algorithm is the FACTORIAL,
which may be defined as: factorial(n) is equal the product of all
numbers from one to n.
Thus:
factorial(1) == 1 /* 1 */
factorial(2) == 2 /* 1*2 */
factorial(3) == 6 /* 1*2*3 */
factorial(4) == 24 /* 1*2*3*4 */
factorial(5) == 120 /* 1*2*3*4*5 */
Using this algorithm, we can define an ITERATIVE (non-recursive)
function to calculate the factorial of a passed value:
/*
* ITERATIVE factorial function
*/
unsigned factorial(value)
unsigned value;
{
unsigned result;
result = 1; /* Begin with one */
while(value > 1) /* For each value */
result *= value--; /* Include in product & reduce */
return result; /* Send back result */
}
You can see that in this above example, the function simply loops
the required number of times to perform the appropriate number of
multiply operations to calculate the factorial. (The factorial of 0
is defined as 1, NOT 0, so the above function will return the correct
result in this case).
Intro to DDS MICRO-C Page: 39
Another way to define factorial is: factorial(n) = the product of
n and factorial(n-1). (Remember that the factorial of zero is defined
as 1).
Thus:
factorial(1) == 1 /* 1*1 */
factorial(2) == 2 /* 2*1 */
factorial(3) == 6 /* 3*2 */
factorial(4) == 24 /* 4*6 */
factorial(5) == 120 /* 5*24 */
Using this algorithm, we can define a RECURSIVE function to
calculate the factorial of a passed value:
/*
* RECURSIVE factorial function
*/
unsigned factorial(value)
unsigned value;
{
if(value == 0) /* Factorial 0 is 1 */
return 1;
return value * factorial(value - 1);
}
In this example, you can see that the function will call itself,
each time passing a reduced value until a value of zero is
encountered. When the zero value is passed, "factorial" returns the
predefined result of one, and all other called versions of the
function will perform a single multiplication, and pass the new
result on. The value returned by the "highest" version of "factorial"
will be the factorial of the originally passed value.
This is the "classic" example of recursion, and it is a poor one.
Although it demonstrates the concept, it does not show a useful
application. In fact, the recursive factorial function will execute
slower and require much more memory than the iterative function.
(Remember each "version" of the function gets its own local memory).
The MICRO-C compiler itself relies heavily on recursion, and
provides much better examples of this programming technique since it
is used to accomplish feats which are not easily possible using a
non-recursive algorithm.
One such use of recursion in the compiler is to accomplish 'C's
COMPOUND STATEMENT capability. Recall that multiple statements in 'C'
may be grouped together and treated as a single statement by
enclosing them in CURLY BRACKETS '{}'. Any statement within this
"group" may also be a compound statement, and this "nesting" of
compound statement blocks may continue to ANY LEVEL.
Intro to DDS MICRO-C Page: 40
The compiler contains a function called "statement", which is
passed the first token from a 'C' statement, and processes that
statement. This function contains a "switch" statement which
processes the token, and decides the action to be performed for that
statement. Note: "tokens" are numeric representations of the
individual entities which may occur in the source file (such as
keywords, symbol names, operators etc.).
The "statement" function contains a fragment of code which is
similar to this:
/*
* Evaluate a language statement
*/
statement(token)
unsigned token;
{
/* ... Not shown ... */
switch(token) { /* act upon the token */
case OCB: /* '{' - begin a block */
while((token = get_token()) != CCB)
statement(token);
break;
case WHILE:
/* ... Not shown ... */
eval(CRB);
cond_jump(FALSE, b, -1);
statement(get_token());
test_jump(a);
/* ... Not shown ... */
break;
/* Other cases ... Not shown ... */ }
}
In this function, a "while" keyword causes the following
expression to be evaluated, followed by a conditional jump, after
which a single statement is compiled (by recursive call to
"statement"), followed by a jump back to the beginning of the
expression evaluation.
If "statement" finds an Opening Curly Bracket (OCB), it will
accept and compile more statements (recursively) until a Closing
Curly Bracket (CCB) is found. Therefore, if the statement compiled in
the "while" loop begins with OCB, that version of the "statement"
function will compile all subsequent statements up to CCB. When that
version of "statement" terminates, the original version of the
function (handling the "while") will then compile the closing jump.
This has the effect that all statements between OCB and CCB ('{' and
'}') will be included in the body of the "while" loop.
Another case where recursion is used within the MICRO-C compiler,
is to evaluate sub-expressions (contained in round brackets '()')
which are inside another expression. In this case, the expression
evaluation function calls itself (recursively) when an Opening Round
Bracket (ORB) is encountered.
Intro to DDS MICRO-C Page: 41
7. COMMAND LINE ARGUMENTS
When a 'C' program is invoked by typing its name at the DOS
command prompt, the remainder of the command line is broken down into
distinct "words" (based on separating spaces or tabs), and passed as
standard 'C' arguments to the main function.
Two values are passed to "main", the first is an integer count of
the number of command line arguments which were found, and the second
is an array of pointers to character strings which contain each of
the arguments. In order that the program may identify itself, the
first (zero'th) argument is always the name of the file containing
the programs executable image.
For example, consider this program, compiled and saved in a file
called "TEST.COM":
#include stdio.h /* Standard I/O definitions */
/*
* Command line arguments - DEMO program
*/
main(argc, argv)
int argc; /* Count of arguments */
char *argv[] /* Array of pointers to char strings */
{
int i; /* Temporary counter variable */
for(i=0; i < argc; ++i)
printf("argv[%d] = '%s'\n", i, argv[i]);
}
When executed with the following command line:
TEST the quick brown fox
The program will display the following output:
argv[0] = 'TEST.COM'
argv[1] = 'the'
argv[2] = 'quick'
argv[3] = 'brown'
argv[4] = 'fox'
NOTE1: The 'argc' value includes argv[0] (program name) in its
argument count.
NOTE2: Like any other 'C' function, "main" does not have to
declare its arguments unless it is going to use them.
Intro to DDS MICRO-C Page: 42
8. FILE ACCESS
As mentioned before, all I/O in 'C' is performed by STANDARD
LIBRARY FUNCTIONS. Before any file access can be performed, the
header file "stdio.h" must be included in the source (Via #include
pre-processor directive), in order to properly define the various
functions and types which are used on your particular system.
For detailed information on the standard library functions
mentioned below, see the MICRO-C technical manual. NOTE that cross
compilers for embedded systems (or any other environment which does
not have an general "operating system") do not usually provide file
I/O functions. Refer to the library documentation for your compiler.
8.1 File Pointers
All files in 'C' are identified by a FILE POINTER, which is a
value returned by the operating system when the file is OPENED.
Since the type of value used to identify files may vary from one
operating system to another, the "stdio.h" header file defines a
data type called "FILE" which contains the correct definition for
your particular operating system. This allows the declaration of
the file pointers to be portable from one system to another
without changing the program source code.
8.2 File I/O Functions
The standard library function "fopen" is used to open a file by
name, and obtain the file pointer value.
The functions "getc", "fget", "fgets", "fread" and "fscanf" may
be used to read information from an open file.
The functions "putc", "fput", "fputs", "fwrite" and "fprintf"
may be used to write information to an open file.
The function "fclose" is used to close the file, which informs
the operating system that you are finished with it.
Intro to DDS MICRO-C Page: 43
/*
* Sample program to copy a file called "input"
* To a file called "output". Note: To keep this
* example simple, no error checking is performed.
*/
main()
{
char buffer[1000]; /* Declare a copy buffer */
int nbytes; /* Records # bytes read */
FILE *ifp, *ofp /* Declare file pointers */
/* Open the files */
ifp = fopen("input","r"); /* Open for Read access */
ofp = fopen("output", "w"); /* Open for Write access */
/* Copy data in 1000 byte blocks */
do {
nbytes = fget(buffer, 1000, fp); /* Read data */
fput(buffer, nbytes, fp); } /* Write data */
while(nbytes == 1000);
fclose(ifp); /* Close the input file */
fclose(ofp); /* Close the output file */
}
8.3 Standard I/O
Whenever a 'C' program is executed, three file pointers are
automatically established which allow access to the console
keyboard and display:
stdin - Reads input from the console keyboard. Note that stdin
may be REDIRECTED (using '<filename'). For example:
program <input.dat
executes "program", and causes it to read its standard
input from "input.dat" instead of the keyboard.
stdout - Writes to the console display. Note that stdout may be
REDIRECTED (using '>filename). For example:
program >output.dat
executes "program", and causes it to write standard
output to "output.dat" instead of the display.
stderr - Also writes to the console display, but CANNOT BE
REDIRECTED, usually used to insure that error messages
will be displayed on the console even if the output
has been redirected.
The stdin, stdout and stderr file pointers are defined in the
"stdio.h" header file, and are always available. They do not have
to be opened or closed.
Since "stdin" and "stdout" are used very often within 'C'
programs, the functions "scanf" and "printf" are available. These
behave exactly the same as the general "fscanf" and "fprintf"
functions, except that they do not accept a file pointer argument,
and always access stdin and stdout.
Intro to DDS MICRO-C Page: 44
9. SAMPLE FUNCTIONS
Here are some sample 'C' functions, demonstrating features of 'C'
which we have discussed:
Any function used in an example but not defined therein is a
Library Function. Refer to the MICRO-C Technical Manual for a
description.
9.1 Prime Number Generator
#include stdio.h /* Standard I/O definitions */
/*
* This program tests a range of numbers, and prints out any
* values which it finds to be prime. Each value is divided
* by increasing values from 2 to (num/2). The "modulus" ('%')
* operator is used, resulting in a zero result (no remainder)
* if a factor is found. The main loop is incremented by two,
* to skip even numbers which are never prime (except for 2
* which is not shown by this program).
*/
main()
{
int num, test, limit;
char flag;
for(num=1; num < 1000; num += 2) { /* Test range */
limit = num/2; /* Only test to 1/2 */
flag = 1; /* Assume prime */
for(test = 2; test < limit; ++test) { /* Test factors */
if(!(num%test)) { /* No remain: factor */
flag = 0; /* Not prime */
break; } } /* Waste no more time */
if(flag) /* Prime, display */
printf("%d\n", num); }
}
Intro to DDS MICRO-C Page: 45
9.2 A Simple Sort
#include stdio.h /* Standard I/O definitions */
/*
* This is an array of unsorted numbers
*/
int numbers[10] = { 13, 25, 22, 7, 16, 91, 11, 41, 18, 0 };
/*
* This main program calls a function "sort" which rearranges
* the elements of an integer array to place them in ascending
* order. It then prints out the resultant array using a simple
* loop.
*/
main()
{
int i;
sort(numbers, 10); /* Perform the sort */
for(i=0; i < 10; ++i) /* Display contents of array */
printf("[%d]=%d\n", i, numbers[i]);
}
/*
* Function to sort an array of numbers. It is passed the
* address of an integer array, and the size (in elements).
*
* Note: The declaration 'int array[]' identifies "array" as a
* single dimension integer array of unspecified size. Since
* arrays in 'C' are passed as a pointer value to the array
* address, accesses to "array" will access the actual contents
* of the passed array variable, allowing that variable to be
* modified directly by this function.
*/
sort(array, size)
int array[], size;
{
int i, j, lowest;
for(i=0; i < size; ++i) { /* For each element */
lowest = i;
for(j=i+1; j < size; ++j) /* Search higher elems */
if(array[j] < array[lowest])
lowest = j; /* And remember lowest */
j = array[lowest]; /* Swap with original */
array[lowest] = array[i];
array[i] = j; }
}
Intro to DDS MICRO-C Page: 46
9.3 Text Display of Value
This program accepts any number of command line arguments,
which it evaluates as unsigned numbers, and displays the result
for each argument using english text.
For example, if this program were saved in a file called
"TEXTNUM.COM", and the following command is executed:
TEXTNUM 1000 1100 1111 31415 9265 358
The program will display the following output:
One Thousand
One Thousand, One Hundred
One Thousand, One Hundred and Eleven
Thirty One Thousand, Four Hundred and Fifteen
Nine Thousand, Two Hundred and Sixty Five
Three Hundred and Fifty Eight
Here is a listing of the program:
#include stdio.h
/*
* Main program which processes all of its arguments,
* interpreting each one as a numeric value, and
* displaying that value as english text.
*/
main(argc, argv)
int argc;
char *argv[];
{
int i;
if(argc < 2) /* No arguments given */
abort("\nUse: textnum <value> ...\n");
for(i=1; i < argc; ++i) { /* Display all arguments */
textnum(atoi(argv[i]));
putc('\n', stdout); }
}
/* >>> Continued on Next Page >>> */
Intro to DDS MICRO-C Page: 47
/*
* Text tables and associated function to display an
* unsigned integer value as a string of words.
* Note the use of RECURSION to display the number
* of thousands and hundreds.
*/
/* Table of single digits and teens */
char *digits[] = {
"Zero", "One", "Two", "Three", "Four", "Five", "Six",
"Seven", "Eight", "Nine", "Ten", "Eleven", "Twelve",
"Thirteen", "Fourteen", "Fifteen", "Sixteen",
"Seventeen", "Eighteen", "Nineteen" };
/* Table of tens prefixes */
char *tens[] = {
"Ten", "Twenty", "Thirty", "Forty", "Fifty",
"Sixty", "Seventy", "Eighty", "Ninety" };
/* Function to display number as string */
textnum(value)
unsigned value;
{
char join_flag;
join_flag = 0;
if(value >= 1000) { /* Display thousands */
textnum(value/1000);
fputs(" Thousand", stdout);
if(!(value %= 1000))
return;
join_flag = 1; }
if(value >= 100) { /* Display hundreds */
if(join_flag)
fputs(", ", stdout);
textnum(value/100);
fputs(" Hundred", stdout);
if(!(value %= 100))
return;
join_flag = 1; }
if(join_flag) /* Separator if required */
fputs(" and ", stdout);
if(value > 19) { /* Display tens */
fputs(tens[(value/10)-1], stdout);
if(!(value %= 10))
return;
putc(' ', stdout); }
fputs(digits[value], stdout); /* Display digits */
}