trusty (3) system.3.gz

NAME

       system - execute a shell command

SYNOPSIS

       #include <stdlib.h>

       int system (const char * string);

DESCRIPTION

       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

       The  value  returned is 127 if the execve() call for /bin/sh fails, -1 if there was another error and the
       return code of the command otherwise.

       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

       ANSI C, POSIX.2, BSD 4.3

BUGS

       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);
               exit(127);
           }
           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 sh.)

       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 implemented.

       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 sure.

SEE ALSO

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