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CS 162Introduction to Computer Science

Chapter 18argc, argv, envp, system

Herbert G. Mayer, PSUStatus 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 lineint 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 entered4 command line args passed.arg 3 = "55"arg 2 = "r"arg 1 = "3"

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Sample argc, argva.out 12 '.' "345" E "+" 0Number of command line arguments passed = 7This includes program nameargv[0] = a.outargv[1] = 12argv[2] = . -- without single quote ‘argv[3] = 345 -- without double quote “argv[4] = Eargv[5] = + -- without double quote “argv[6] = 0Note 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 envpThe Unix shell command man setenv generates this output:

NAMEset, 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 whileprintf( "Number of environment vars = %d\n", index );exit( 0 );

} //end main

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Sample envp, Counting from 1environment 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 1stchar * 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/index.jsp?topic=%2Fcom.ibm.xlcpp8a.doc%2Flanguage%2Fref%2Fmainf.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()

#include <stdlib.h>

int main(){ // main system( "man system" );

. . .} //end main

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:

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Sample Uses of system()

#include <stdlib.h>int main( void ){ // main

// assume execution in directory arg_test system( "cd ..; ls -la; cd arg_test; ls -la" );} //end main

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

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Sample Uses of system()

#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

C program that prints the current working directory by using system(), specifying string literal “pwd”

Or by passing-in the command through argv[1]

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Tricky Use of system()

#include <stdlib.h>#include <time.h>// file name: sys7.cint 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

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:

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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. ps command2. setenv command3. signal.h lib4. stdlib.h lib5. sys/types.h lib6. time.h lib -- use for HW37. time command -- 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

USER=herbLOGNAME=herbHOME=/u/herbPATH=.:/usr/lang:/bin ... long list of pathsMAIL=/var/mail//herbSHELL=/bin/cshTZ=US/PacificLC_CTYPE=en_US.ISO8859-1LC_COLLATE=en_US.ISO8859-1 ... etc.

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:

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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 64-

bit target systems Key Functions:

clock_t

time_t

pid_t

size_t

_CHAR_IS_SIGNED (or UNSIGNED)

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C Library time

time_t cur_time = time( '\0' ); printf( "%s", asctime( localtime( & cur_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*

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C Library time

clock_t start, end, total; // to measure start time etc.. . .start = clock(); // take first measurement. . . // do compute-intensive workend = clock(); // take final measurementtotal = end - start; // track time for workprintf( "ticks = %10u, seconds = %f\n", total,

(double)( (double)total / (double)CLOCKS_PER_SEC ) );

clock_t is type to track system ticks

treat as if it were a numeric type

clock() function, returning ticks clock_t

CLOCKS_PER_SEC clock_t constant used to convert

clock ticks into seconds

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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 1m41.911suser 1m39.816ssys 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 101.76user 99.67sys 1.94

Real time: Elapsed wall clock time 101.76sec.

User time: User CPU time 99.67sec

System time: System overhead CPU time 1.94sec.

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Unix C Shell Command time> time a.out

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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 shows architecture detail, 32/64 bit –v 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