Homework
• HW6 On line – due next class
• Starting K&R Chapter 7 and Appendix B
• Also, UNIX – Various chapters in Glass
1
Standard Library Functions
• Learn as much as possible about all standard
library functions  K&R Appendix B
• On line, you can get documentation about
standard library functions with man command
% man printf
• For more information, use grep on standard
library function names in standard .h files
% grep strcmp /usr/include/*
2
UNIX grep, Glass Pg 219+
• UNIX command “grep” finds matches for a
specified string in identified filenames
• If no filenames present, grep searches stdin
• Some options in grep:
-i
-n
-v
-w
ignore case (“TEXT” same as “text”)
adds line numbers to display
gives only lines that don’t match
only matches complete words
3
UNIX “Pipes”, Glass Pg 84+
• Remember redirection for stdin and stdout?
% command <filename1 >filename2
• In general, you can invoke a program and
pass stdout from that program as stdin to
another program via UNIX pipe – symbol |
% command1 arguments | command2 arguments
% grep pattern filename(s) | more
4
Standard Input /Output, 7.1
• C code for reading stdin
char getchar (void) gets a character from stdin
• C code for writing to stdout
void putchar(char) puts a character to stdout
• stdin and stdout can be redirected or piped
% ./tail <tail.in | more
% cat tail.in | ./tail >filename2
5
Formatted Output, printf, K&R 7.2
• Formats and prints out internal values.
int printf(char *format, arg1, arg2, . . .);
• printf has a variable length argument list (as many
arguments after the first one as % conversions in
the format string)
• We will learn how to do this shortly
• Return from printf is number of characters printed
• Haven't used this up till now, but it may be useful
if there is some error or limit truncation
6
Formatted Output, printf
• Between the % and the conversion character, there
are a number of other characters which may exist. In
the order they must be placed, they are:
- (minus sign)
+ (plus sign)
0 (zero)
m (number m)
. (dot)
p (integer p)
h or l (letters)
left adjust printing of argument
print with a leading sign character
pad with leading zeros instead of spaces
minimum field width
separates min field width & precision
precision:
max chars for string
min digits for int
h for short int, l for long int
• ORDER for %d is: %[-][+][0][m][.][p][h|l]d,
Note: No embedded spaces allowed!
7
Formatted Output, printf
• Figure out what these would do:
%10d, %-10d, %hd, %e, %10ld, %10.p
• Experiment for 10 minutes with a program, using
different formats
• Learn string precision given on pg. 154
• Also, to print at most max characters from string s
(max is int type var or const), use * after % and
include the int max as an argument before s:
printf("%.*s", max, s);
8
Formatted Output, printf
• Can print string literal as format string with no “%s”
printf("hello, world!\n");
• Could also print a string variable as format string:
char s[ ] = "hello, world";
printf(s);
• If string s has a % character in it, this is unsafe!
• printf will look for another argument after format
string s. Better to write out a variable string s as:
printf("%s", s);
9
Formatted Output, sprintf
• See sprintf in K&R Appendix B, pg 245
• Function sprintf works same as printf, but it writes to
a specified string, e.g. char array[ ], with trailing ‘\0’
int sprintf(char *string, char *format, arg1, arg2, …);
• Note: int return value does not include trailing ‘\0’
• Recall how we wrote itoa() and itox() functions
• No functions like this in C library!
• Use sprintf() to print int into a string using %d or %x
10
Formatted Input, scanf
• This is the opposite of printf. Reads in variables
from stdin using conversion format string. See pg.
246 (and prior pg 245 which explains everything).
int scanf(char *format, …);
• Return value from scanf( ) is number of successfully
scanned tokens
• Not successful if scanf can't parse any value brought
in from stdin according to the specified format
11
Formatted Input, scanf
• Must call scanf with a POINTER to each variable so
that values can be set by scanf which is a function!
int age, weight;
char lname[100];
while(some condition) {
printf("Input your last name, age, and weight”);
cnt = scanf("%s %d %d", lname, &age, &weight);
}
• Note: lname is an array and is already a pointer 12
Formatted Input, scanf
• Scanf is useful to allow you to read in int or
double value AS A NUMBER (instead of a
character string leaving you to do your own
conversion in your code)
• scanf() always see a character sequence in stdin: it
just does its own conversion to int or double
13
Formatted Input, scanf
• However, scanf is FLAWED, because it ignores
'\n' characters.
• Can get very confusing if the user enters too few
arguments on an input line being parsed by scanf
• (Prompt) Input your last name, age, and weight:
• (User input) Clinton 52
• User gets no response after carriage return.
• User retries, remembers to enter weight this time
• (User input) Clinton 52 200
14
Formatted Input, scanf
• scanf sees Clinton 52 Clinton since the user entered
carriage return is seen as white space
• scanf thinks weight has bad value and returns 2 as
number of successfully scanned tokens
• scanf does not “consume” the token Clinton and the
value of age will be uninitialized
• The re-entry of Clinton 52 200 will be processed and
returned after the incomplete Clinton 52
• Always check the return value from scanf!
15
Formatted Input, scanf
• Use scanf only for programs needing only ONE
input item, usually "quick and dirty" programs
with no input checking.
• Can't code defensively with scanf( ): can't count
number of tokens parsed ON A LINE - scanf
doesn't care about input lines
• Best approach is to read a line into an array s[ ]
and use "sscanf( )" to parse the arguments in line
• This also allows you to try to interpret things in
more than one way
16
Formatted Input, sscanf
• See sscanf in K&R Appendix B, pg 246
• Function sscanf works same as scanf, but it reads from
a specified string, e.g. char array[ ], with a trailing ‘\0’
int sscanf(char *string, char *format, &arg1, &arg2, …);
• Recall how we wrote function atoi, axtoi to convert a
decimal or hex character string s to an integer i?
– Use sscanf(s, "%d", &i) for atoi
– Use sscanf(s, "%x", &i) for axtoi
17
Formatted Input, scanf/sscanf
• Note: with both scanf and sscanf, if you put specific
characters in the format string, the functions must see
exactly those specific characters in the user input
cnt = sscanf(s, "%d/%d/%d", &month, &day, &year);
• Expects input to look exactly like this: 07/23/96
• If not, cnt value returned by sscanf is less than 3
18
Variable Length Argument Lists
• Both printf and scanf have an argument (the
format string) that defines the number and
type of the remaining arguments in the list
• Concept similar to “Overloading” in Java
• C does not support multiple declarations of
the same function each with different lists
• How is it supported in C?
• Look at stack frame after a function call!
19
Typical Stack frame
Stack Pointer
After Call
Decreasing Addresses
Function’s
Automatic
Variables
Return Data A
e.g.PC, other r
Registers, etc g
1
Code provides the location
of the last fixed argument in
call sequence to va_start
Stack Pointer
Before Call
A
r
g
2
A
r
g
3
Address
0xffffffff
From fixed arguments, the code must determine the number of
additional arguments to access via offsets from stack pointer and
va_arg can work its way back up the stack to get each argument
20
Variable Length Argument Lists
• Use va_ macro package inside function with
a variable length argument list to get args
void foo (int n, …) /* note ellipsis …
*/
{
va_list ap;
/* variable name ap */
va_start(ap, n); /* n is last named arg */
• Now ap points just before first unnamed arg
21
Variable Length Argument Lists
• Each call to va_arg( ) advances pointer ap
by one argument and returns value by type:
ival = va_arg(ap, int);
fval = va_arg(ap, float);
sval = va_arg(ap, char *);
• Function must clean up before returning:
va_end(ap);
22