CS 162
Introduction to Computer Science
Chapter 18
argc, argv, envp, system
Herbert G. Mayer, PSU
Status 11/17/2014
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argc argv envp
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Syllabus
 Definitions
 Samples of argc argv envp
 Possible argv[] Implementation?
 Sample envp
 Concatenate
 References
 system()
 Sample Uses of system()
 Unix Commands & C Libraries
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Definitions
 In C, the main() function returns an int result; the
formal parameter list may be empty, or else contain
the specification of command line parameters
 In C++, the main() function also returns an int
type result; yet the formal parameter list must be
specified as void in C++, if main() does not
specify command line parameters
 These are traditionally named argc, argv, and
envp; on Apple platforms a fourth parameter can be
specified, not surprisingly named “apple”
 Environment variable names may be chosen freely,
but “argc” and “argv” are tradition 
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Definitions
int main(
int argc,
// specifies # in argv[]
char * argv[], // list of parameters
char * envp[] ) // all environment vars
{ // main
. . .
} //end main
• Why specify argc?
• To allow empty-string command line parameters
• Cannot have a null environment variable
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Definitions
 argc



Specified as int argc, is main()’s first formal parameter
argc counts the number of command line arguments that
argv[] holds for this current program execution
Includes the object program itself, hence must be >= 1
 argv



Specified as char ** argv, equivalently char * argv[]
is 2nd formal parameter of main(); optional; if specified, the
first parameter argc must already be specified
argv is a pointer to the list of actual const string arguments
 envp



Specified as char ** envp, equivalently char * envp[]
is 3rd formal and optional parameter of main()
Holds environment variables as string constants, plus the
header: PATH=
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C Quirks
 Note that for argv[] an ancillary data structure is
provided: argc
 Thus it is possible to pass an empty string as
one/some of the various command line arguments

And if your program scans for the “end of the list” by
checking for the null string, your loop terminates checking
the argv[] values too early; hence argc
 This is not possible with envp[], as no empty (null
string) environment variable can be specified
 Delicate point, but that is what system programming
is all about!
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Reverse-Print Command Line Args
// output all command line arguments of a.out:
// but in reverse order of command line
int main( int argc, char * argv[] ) // ignore envp
{ // main
printf( "%d command line args passed.\n", argc );
while( --argc > 0 ) { // pre-decrement skips a.out
printf( "arg %d = \"%s\"\n", argc, argv[ argc ] );
} //end while
} //end main
[args] a.out 3 r 55  the command line entered
4 command line args passed.
arg 3 = "55"
arg 2 = "r"
arg 1 = "3"
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Sample argc, argv
a.out 12 '.' "345" E "+" 0
Number of command line arguments passed = 7
This includes program name
argv[0] = a.out
argv[1] = 12
argv[2] = .
-- without single quote ‘
argv[3] = 345
-- without double quote “
argv[4] = E
argv[5] = +
-- without double quote “
argv[6] = 0
Note that all matching “ and ‘ pairs on command line are
stripped away; all command args concatenated are
"12.345E+0”; looks like float number 12.345
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Possible argv[] Implementation?
How are such actual parameters passed to main() ?
Which tool passes them from the command line to the
run-time-system?
Students think, design, discuss:
1.) Will on-the-fly creation of assembler source program
help? Assumes an assembly step to create another *.o
2.) Will manipulation of stack, heap, code help, before or
while loading provide the actual parameters?
3.) Other possibilities? Discuss or email good ideas, and
get extra credit!
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Possible argv[] Implementation?
Your Unix shell executes the a.out command
Let’s say: a.out 100000 “9/12/2012”
The shell sees the command line, verifies that a.out
exists and is executable, loads your program, does ??
magic, executes your program, continues after exit
At the point of ?? some magic happens
All work to provide command line parameters is
accomplished between shell command and execution
Clearly a.out may not be modified from run to run
So, sometimes argc, argv, envp are accessed
during execution, other times not
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Sample envp
The Unix shell command man setenv generates this output:
NAME
set, unset, setenv, unsetenv, export -shell built-in functions to
determine the characteristics for environmental variables of the
current shell and its descendents . . Much more
Current environment variables used in your Unix environment are
tracked and can be output for any of your processes
Similar to char * argv[], envp is also a pointer to an array of 0 or
more strings, followed by a null string
Each such string contains one of your process’ environment variables
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Sample envp Version in C
int main( int argc, char ** argv, char * envp[] )
{ // main
int index = 0;
while( envp[ index ] ) { // OK to assume null-terminated strings
// print one environment variable at a time
printf( "envp[%d] = \"%s\"\n", index, envp[ index++ ] );
} //end while
printf( "Number of environment vars = %d\n", index );
exit( 0 );
} //end main
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Sample envp, Counting from 1
environment variable 1 = "USER=herb"
environment variable 2 = "LOGNAME=herb"
environment variable 3 = "HOME=/u/herb"
environment variable 4 =
"PATH=.:/usr/lang:/bin/sun4:/usr/etc:/vol/local/rhost/sun4/bin:/usr/ucb:/bin:/vol/
local:/vol/local/bin:/vol/local/sun4/bin:/usr/ccs/bin:/usr/ccs:/vol/userlocal/bin:
/usr/local/bin:/home/vads/VADS_location/bin"
environment variable 5 = "MAIL=/var/mail//herb"
environment variable 6 = "SHELL=/bin/csh"
environment variable 7 = "TZ=US/Pacific"
. . . And many more
environment variable 12 = "LC_MONETARY=en_US.ISO8859-1"
environment variable 13 = "LC_MESSAGES=C"
environment variable 14 = "LANG=en_US.UTF-8"
environment variable 15 = "SSH_CLIENT=50.53.47.189 50625 22"
environment variable 16 = "SSH_CONNECTION=50.53.47.189 50625 131.252.208.127 22"
environment variable 17 = "SSH_TTY=/dev/pts/33"
environment variable 18 = "TERM=xterm-256color"
environment variable 19 = "PWD=/u/herb/progs/args"
environment variable 20 = "term=vt100=”
Number of environment vars = 20
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Concatenate
char g_string[ 100000 ];
// large enough to hold command line arguments
// return string pointing to concatenation of command line args, not 1st
char * cat_not_first( int argc, char * argv[] )
{ // cat_not_first
g_string[ 0 ] = (char)(0);
// place null into first character
for ( int i = 1; i < argc; i++ ) { // don’t start at 0, for “a.out”
strcat( g_string, argv[ i ] );
} //end for
return g_string;
} //end cat_not_first
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References
1. http://crasseux.com/books/ctutorial/argc-and-argv.html
2. http://en.wikipedia.org/wiki/Main_function
3. http://publib.boulder.ibm.com/infocenter/comphelp/v8v101/inde
x.jsp?topic=%2Fcom.ibm.xlcpp8a.doc%2Flanguage%2Fref%2F
mainf.htm
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system()
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system( “string” )
 The system() function is a C function, NOT a shell
command; causes error as a shell command
 Available in library #include <stdlib.h>
 Passes its sole string parameter to the shell, as if
that same string had been typed via a shell
command
 Exit status of last completed command is passed
back to the invoking C program
 If a null string is given, system( “” ) checks if
the shell exists and then terminates
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system( “string” )
 Completed (last) command returns with exit status
in waitpid(3) format, which is passed to the
invoking C program
 waitpid() reqires: #include <sys/types.h>
and #include <sys/wait.h>
 waitpid suspends execution of calling program
until status of terminated child process is available -any of its child processes
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Sample Uses of system()
 Write a C program, using system() that prints the
Unix man page for system()
 Just as if command “man system” had been typed
as a Unix shell command
 Sample shown below:
#include <stdlib.h>
int main()
{ // main
system( "man system" );
. . .
} //end main
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Sample Uses of system()
Write a C program using system() that:

Assumes the current working directory is arg_test

Changes directory from . named arg_test, to .. one level up

Then lists all files in that .. directory via: ls –la

Changes back to directory arg_test

And finally shows all files listed in arg_test
#include <stdlib.h>
int main( void )
{ // main
// assume execution in directory arg_test
system( "cd ..; ls -la; cd arg_test; ls -la" );
} //end main
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Sample Uses of system()
 C program that prints the current working directory
by using system(), specifying string literal “pwd”
 Or by passing-in the command through argv[1]
#include <stdlib.h>
#define command "pwd"
// invoke via: a.out "pwd”, or via: a.out pwd
int main( int argc, char * argv[] )
{ // main
printf( "using string const \"pwd\"\n" );
system( command );
printf( "using argv[1] = %s\n”, argv[1] );
system( argv[1] );
system( "argv[1]" );
// this would be an error!
// the string "argv[1]" is NOT a valid shell command
} //end main
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Tricky Use of system()
 Write a C program using system() that prints the
current date/time
 Then compiles and runs its own source sys7.c
 Watch out what happens with C file sys7.c:
#include <stdlib.h>
#include <time.h>
// file name: sys7.c
int main( /* void */ )
{ // main
time_t cur_time = time( '\0' );
printf( "%s", asctime( localtime( & cur_time ) ) );
printf( ”Compile and execute sys7.c\n" );
system( "gcc sys7.c; a.out" );
printf( "U r rich or old when u get here \n" );
} //end main
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Unix Commands & C Libraries
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Unix Commands, C Libraries
 C and C++ are mature languages, widely used, yet
offering limited high-level language facilities



No array assignments
No lexically nested functions or procedures
Limited built-in functions, hence the numerous libraries
 Libraries still render the C language rich and
versatile in use; some Unix libs, commands are
ubiquitous, e.g.:
1.
2.
3.
4.
5.
6.
7.
ps
setenv
signal.h
stdlib.h
sys/types.h
time.h
time
command
command
lib
lib
lib
lib
command
-- use for HW3
-- use for HW3
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Unix Command ps
 Name:
ps
 Purpose:
print status of current processes
 Functions: prints information about active
processes. Without options, lists processes with
same effective user ID and controlling terminal as
invoker
 Output contains process ID, terminal ID, cumulative
execution time, and command name
 Options are numerous: ps [-aAcdefjlLPyZ] [-g
grplist] [-n namelist] [-o format] [-p
proclist] [-s sidlist] [-t term] [-u
uidlist] [-U uidlist] [-G gidlist] [-z
zonelist]
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Unix Command setenv
 Name:
setenv
 Purpose:
reports of environment variables for
the current process and all its descendents
 Function: various commands are set unset
unsetenv export
 Sample use without arguments:
USER=herb
LOGNAME=herb
HOME=/u/herb
PATH=.:/usr/lang:/bin ... long list of paths
MAIL=/var/mail//herb
SHELL=/bin/csh
TZ=US/Pacific
LC_CTYPE=en_US.ISO8859-1
LC_COLLATE=en_US.ISO8859-1 ... etc.
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$PATH, etc. on PSU’s Odin, Sirius

On Unix shell, issue the command man shell_builtins and
learn about several interesting commands, e.g.:










alias
echo $PATH
setenv
Command echo $PATH shows the one PATH= line that your
program did output –plus many more– scanning through envp[]
That was the line starting with PATH= . /user/ . . .
Or just issue command $PATH and observe the error at the end
Again you will see a path for all directories searched, in order, to
resolve a command --or finding a file-- you specified
Command setenv without arguments will provide information
that your program outputs, when scanning through envp[]
What if you wish to add directory /u/herb/progs to your search
path $PATH?
Issue command setenv PATH $PATH\:/u/herb/progs, will
add /u/herb/progs at the end of existing path
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C Library signal
 Name:
#include <signal.h>
 Purpose:
provide signal management for
application processes
 Key Functions with 1st argument sig: signal()
sigset() sighold() sigrelse() sigignore()
sigpause()
 Signals are tools for asynchronous interprocess
communication in Unix. Signal is sent to process or
thread of the same process to notify that a specific
event (the signal) has occurred. Some of the
functions set, others remove etc. the event
recording. If process has registered a signal
handler, that handler is executed upon signal set
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C Library stdlib
 Name:
#include <stdlib.h>
 Purpose:
defines key functions and macros
 Key Functions: exit() malloc() printf() ...
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C Library types
 Name:
#include <sys/types.h>
 Purpose:
defines 100s of types for 32-bit and 64bit target systems
 Key Functions:
clock_t
time_t
pid_t
size_t
_CHAR_IS_SIGNED (or UNSIGNED)
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C Library time
 Name:
#include <time.h>
 Purpose:
tracking run-time cycles
 Key Functions:
localtime() returns: struct tm*
single argument: const * clock
asctime()
returns: char *
single argument: const struct tm*
time_t cur_time = time( '\0' );
printf( "%s", asctime( localtime( & cur_time ) ) );
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C Library time
clock_t
clock()
CLOCKS_PER_SEC
is type to track system ticks
treat as if it were a numeric type
function, returning ticks clock_t
clock_t constant used to convert
clock ticks into seconds
clock_t start, end, total;
// to measure start time etc.
. . .
start = clock();
// take first measurement
. . .
// do compute-intensive work
end
= clock();
// take final measurement
total = end - start;
// track time for work
printf( "ticks = %10u, seconds = %f\n", total,
(double)( (double)total / (double)CLOCKS_PER_SEC ) );
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Unix Command time arg
 Command name: time arg
 Purpose:
measures the time of any optional Unix
command arg
 Function: outputs the timing statistics to
standard error, as
elapsed time, ie. wall-clock time
user CPU time, in float format
system CPU time, in float format
and more . . .
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Unix Command time a.out
> time ./a.out
...
real
user
sys
1m41.911s
1m39.816s
0m1.957s
Real time: Elapsed wall clock time
1 min. 41 sec.
User time: User CPU time
1 min. 39 sec
System time: System overhead CPU time 0 min. 1.9 sec.
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Traditional Unix Command time
> time -p ./a.out
...
real 101.76
user 99.67
sys 1.94
Real time: Elapsed wall clock time
User time: User CPU time
System time: System overhead CPU time
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101.76
sec.
99.67
sec
1.94
sec.
Unix C Shell Command time
> time a.out
...
99.0u 1.0s 1:41 98% 0+0k 0+0io 0pf+0w
Default format: '%Uu %Ss %E %P %I+%Oio %Fpf+%Ww' where:

%U: User CPU time in seconds

%S: System CPU time in seconds

%E: Elapsed wall-clock time in minutes and seconds

and more ...
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Other Unix Commands


isainfo –v
–v
shows architecture detail, 32/64 bit
verbose version gives max isainfo

uname –p
prints product name, e.g.: sparc

arch
manufacturer marketing name, e.g.: sun4
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References
1. The C Programming Language, Second Edition: Brian
W Kernighan and Dennis M. Richie, Prentice Hall,
Englewood Cliffs, New Jersey 07632
2. time(1) man page: http://linux.die.net/man/1/time
3. csh(1) man page: http://linux.die.net/man/1/csh
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