1 / 39

CS 201 Computer Systems Programming Chapter 2 argc, argv, envp, system(), libs

Herbert G. Mayer, PSU CS Status 6/13/2014. CS 201 Computer Systems Programming Chapter 2 argc, argv, envp, system(), libs. argc argv envp. Syllabus. Definitions Samples of argc argv envp Possible argv[] Implementation? Sample envp Concatenate References system()

annora
Télécharger la présentation

CS 201 Computer Systems Programming Chapter 2 argc, argv, envp, system(), libs

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Herbert G. Mayer, PSU CS Status 6/13/2014 CS 201Computer Systems ProgrammingChapter 2argc, argv, envp, system(),libs

  2. argc argv envp

  3. Syllabus • Definitions • Samples of argc argv envp • Possible argv[]Implementation? • Sample envp • Concatenate • References • system() • Sample Uses of system() • Unix Commands & C Libraries

  4. 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 inttype 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 

  5. 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

  6. 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=

  7. 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!

  8. 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. arg3 = "55" arg2 = "r" arg1 = "3"

  9. 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

  10. 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!

  11. 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.outmay not be modified from run to run • So, sometimes argc, argv, envp are accessed during execution, other times not

  12. 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

  13. Sample envpVersion 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

  14. 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

  15. 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

  16. References • http://crasseux.com/books/ctutorial/argc-and-argv.html • http://en.wikipedia.org/wiki/Main_function • http://publib.boulder.ibm.com/infocenter/comphelp/v8v101/index.jsp?topic=%2Fcom.ibm.xlcpp8a.doc%2Flanguage%2Fref%2Fmainf.htm

  17. system()

  18. 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

  19. 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> • waitpidsuspends execution of calling program until status of terminated child process is available --any of its child processes

  20. 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

  21. 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

  22. 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

  23. 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

  24. Unix Commands & C Libraries

  25. 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.: • ps command • setenv command • signal.h lib • stdlib.h lib • sys/types.h lib • time.h lib -- use for HW3 • time command -- use for HW3

  26. 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]

  27. 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.

  28. $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 setenvwithout arguments will provide information that your program outputs, when scanning through envp[] • What if you wish to add directory /u/herb/progsto your search path $PATH? • Issue command setenv PATH $PATH\:/u/herb/progs, will add /u/herb/progsat the end of existing path

  29. 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

  30. C Library stdlib • Name: #include <stdlib.h> • Purpose: defines key functions and macros • Key Functions: exit() malloc() printf() ...

  31. 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)

  32. 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 ) ) );

  33. C Library time 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 • 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 ) );

  34. 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-clocktime • user CPU time, in float format • system CPU time, in float format • and more . . .

  35. 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.

  36. Traditional Unix Command time > time -p ./a.out...real 101.76user 99.67sys 1.94 Real time: Elapsed wall clock time 101.76 sec. User time: User CPU time 99.67 sec System time: System overhead CPU time 1.94 sec.

  37. 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 ...

  38. 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

  39. References The C Programming Language, Second Edition: Brian W Kernighan and Dennis M. Richie, Prentice Hall, Englewood Cliffs, New Jersey 07632 time(1) man page: http://linux.die.net/man/1/time csh(1) man page: http://linux.die.net/man/1/csh

More Related