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NAME    [Toc]    [Back]

       system - execute a shell command

SYNOPSIS    [Toc]    [Back]

       #include <stdlib.h>

       int system (const char * string);

DESCRIPTION    [Toc]    [Back]

       system()  executes  a command specified in string by calling /bin/sh -c
       string, and returns after the command has been completed.  During  execution
  of the command, SIGCHLD will be blocked, and SIGINT and SIGQUIT
       will be ignored.

RETURN VALUE    [Toc]    [Back]

       The value returned is -1 on error (e.g. fork failed),  and  the	return
       status  of  the command otherwise.  This latter return status is in the
       format specified in wait(2).  Thus, the exit code of the  command  will
       be  WEXITSTATUS(status).   In  case  /bin/sh could not be executed, the
       exit status will be that of a command that does exit(127).

       If the value of string is NULL, system() returns nonzero if  the  shell
       is available, and zero if not.

       system() does not affect the wait status of any other children.

CONFORMING TO    [Toc]    [Back]

       ANSI C, POSIX.2, BSD 4.3

BUGS    [Toc]    [Back]

       It  is  extremely unfortunate that the libc version of system() ignores
       interrupts.  This makes programs that call it from a loop  uninterruptable.
   This  means  that for such purposes one should not use system()
       but a private version like (warning: untested code!)

       int my_system (const char *command) {
	   int pid, status;

	   if (command == 0)
	       return 1;
	   pid = fork();
	   if (pid == -1)
	       return -1;
	   if (pid == 0) {
	       char *argv[4];
	       argv[0] = "sh";
	       argv[1] = "-c";
	       argv[2] = command;
	       argv[3] = 0;
	       execve("/bin/sh", argv, environ);
	   do {
	       if (waitpid(pid, &status, 0) == -1) {
		   if (errno != EINTR)
		       return -1;
	       } else
		   return status;
	   } while(1);

       Do not use system() from  a  program  with  suid  or  sgid  privileges,
       because	strange values for some environment variables might be used to
       subvert system integrity.  Use the exec(3) family of functions instead,
       but not execlp(3) or execvp(3).	system() will not, in fact, work properly
 from programs with suid or sgid privileges	on  systems  on  which
       /bin/sh	is  bash  version 2, since bash 2 drops privileges on startup.
       (Debian uses a modified bash which does not do  this  when  invoked  as

       The check for the availability of /bin/sh is not actually performed; it
       is always assumed to be available.  ISO	C  specifies  the  check,  but
       POSIX.2	specifies  that  the  return shall always be non-zero, since a
       system without the shell is not conforming, and	it  is	this  that  is

       It is possible for the shell command to return 127, so that code is not
       a sure indication that the execve() call failed; check  errno  to  make

SEE ALSO    [Toc]    [Back]

       sh(1), signal(2), exec(3)

GNU				  1998-05-11			     SYSTEM(3)
[ Back ]
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