linux下system函数的简单分析

int
__libc_system (const char *line)
{
 if (line == NULL)
  /* Check that we have a command processor available. It might
    not be available after a chroot(), for example. */
  return do_system ("exit 0") == 0;

 return do_system (line);
}
weak_alias (__libc_system, system)

static int
do_system (const char *line)
{
 int status, save;
 pid_t pid;
 struct sigaction sa;
#ifndef _LIBC_REENTRANT
 struct sigaction intr, quit;
#endif
 sigset_t omask;

 sa.sa_handler = SIG_IGN;
 sa.sa_flags = 0;
 __sigemptyset (&sa.sa_mask);

 DO_LOCK ();
 if (ADD_REF () == 0)
  {
   if (__sigaction (SIGINT, &sa, &intr) < 0)
  {
   (void) SUB_REF ();
   goto out;
  }
   if (__sigaction (SIGQUIT, &sa, &quit) < 0)
  {
   save = errno;
   (void) SUB_REF ();
   goto out_restore_sigint;
  }
  }
 DO_UNLOCK ();

 /* We reuse the bitmap in the 'sa' structure. */
 __sigaddset (&sa.sa_mask, SIGCHLD);
 save = errno;
 if (__sigprocmask (SIG_BLOCK, &sa.sa_mask, &omask) < 0)
  {
#ifndef _LIBC
   if (errno == ENOSYS)
  __set_errno (save);
   else
#endif
  {
   DO_LOCK ();
   if (SUB_REF () == 0)
    {
     save = errno;
     (void) __sigaction (SIGQUIT, &quit, (struct sigaction *) NULL);
    out_restore_sigint:
     (void) __sigaction (SIGINT, &intr, (struct sigaction *) NULL);
     __set_errno (save);
    }
  out:
   DO_UNLOCK ();
   return -1;
  }
  }

#ifdef CLEANUP_HANDLER
 CLEANUP_HANDLER;
#endif

#ifdef FORK
 pid = FORK ();
#else
 pid = __fork ();
#endif
 if (pid == (pid_t) 0)
  {
   /* Child side. */
   const char *new_argv[4];
   new_argv[0] = SHELL_NAME;
   new_argv[1] = "-c";
   new_argv[2] = line;
   new_argv[3] = NULL;

   /* Restore the signals. */
   (void) __sigaction (SIGINT, &intr, (struct sigaction *) NULL);
   (void) __sigaction (SIGQUIT, &quit, (struct sigaction *) NULL);
   (void) __sigprocmask (SIG_SETMASK, &omask, (sigset_t *) NULL);
   INIT_LOCK ();

   /* Exec the shell. */
   (void) __execve (SHELL_PATH, (char *const *) new_argv, __environ);
   _exit (127);
  }
 else if (pid < (pid_t) 0)
  /* The fork failed. */
  status = -1;
 else
  /* Parent side. */
  {
   /* Note the system() is a cancellation point. But since we call
   waitpid() which itself is a cancellation point we do not
   have to do anything here. */
   if (TEMP_FAILURE_RETRY (__waitpid (pid, &status, 0)) != pid)
  status = -1;
  }

#ifdef CLEANUP_HANDLER
 CLEANUP_RESET;
#endif

 save = errno;
 DO_LOCK ();
 if ((SUB_REF () == 0
    && (__sigaction (SIGINT, &intr, (struct sigaction *) NULL)
    | __sigaction (SIGQUIT, &quit, (struct sigaction *) NULL)) != 0)
   || __sigprocmask (SIG_SETMASK, &omask, (sigset_t *) NULL) != 0)
  {
#ifndef _LIBC
   /* glibc cannot be used on systems without waitpid. */
   if (errno == ENOSYS)
  __set_errno (save);
   else
#endif
  status = -1;
  }
 DO_UNLOCK ();

 return status;
}

do_system

#ifdef FORK
 pid = FORK ();
#else
 pid = __fork ();
#endif
 if (pid == (pid_t) 0)
  {
   /* Child side. */
   const char *new_argv[4];
   new_argv[0] = SHELL_NAME;
   new_argv[1] = "-c";
   new_argv[2] = line;
   new_argv[3] = NULL;

   /* Restore the signals. */
   (void) __sigaction (SIGINT, &intr, (struct sigaction *) NULL);
   (void) __sigaction (SIGQUIT, &quit, (struct sigaction *) NULL);
   (void) __sigprocmask (SIG_SETMASK, &omask, (sigset_t *) NULL);
   INIT_LOCK ();

   /* Exec the shell. */
   (void) __execve (SHELL_PATH, (char *const *) new_argv, __environ);
   _exit (127);
  }
 else if (pid < (pid_t) 0)
  /* The fork failed. */
  status = -1;
 else
  /* Parent side. */
  {
   /* Note the system() is a cancellation point. But since we call
   waitpid() which itself is a cancellation point we do not
   have to do anything here. */
   if (TEMP_FAILURE_RETRY (__waitpid (pid, &status, 0)) != pid)
  status = -1;
  }

#define  SHELL_PATH  "/bin/sh"  /* Path of the shell. */
#define  SHELL_NAME  "sh"    /* Name to give it. */ 

# define LIBC_START_MAIN __libc_start_main

STATIC int
LIBC_START_MAIN (int (*main) (int, char **, char ** MAIN_AUXVEC_DECL),
     int argc, char **argv,
#ifdef LIBC_START_MAIN_AUXVEC_ARG
     ElfW(auxv_t) *auxvec,
#endif
     __typeof (main) init,
     void (*fini) (void),
     void (*rtld_fini) (void), void *stack_end)
{
 /* Result of the 'main' function. */
 int result;

 __libc_multiple_libcs = &_dl_starting_up && !_dl_starting_up;

#ifndef SHARED
 char **ev = &argv[argc + 1];

 __environ = ev;

 /* Store the lowest stack address. This is done in ld.so if this is
   the code for the DSO. */
 __libc_stack_end = stack_end;

　　　　......
 /* Nothing fancy, just call the function. */
 result = main (argc, argv, __environ MAIN_AUXVEC_PARAM);
#endif

 exit (result);
}