Linux system() 函数源代码分析

inux 下system函数原型：

int system(const char *command);
system() executes a command specified in command by calling /bin/sh -c command, and returns after the command has been completed. During execution of the command, SIGCHLD will be blocked, and SIGINT and SIGQUIT will be ignored.
system函数的返回值比较多，且存在相同的值却代表着不同的意思，针对以上问题对源码进行分析。
Libc-2.9 下sysdeps\posi\system.c源码：

    int system (const char *line)

    {

        return __libc_system (line);

    }

    int __libc_system (const char *line) /*system函数实际上调用的是do_system函数*/

    {

        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; /* 当line为NULL时，返回值为0，及执行bash –c exit 0*/

        if (SINGLE_THREAD_P)

            return do_system (line);
        /*GCC cleanup exception range can cover the
          LIBC_CANCEL_ASYNC() and LIBC_CANCEL_RESET():http://sourceware.org/ml/libc-alpha/2011-08/msg00063.html */
        int oldtype = LIBC_CANCEL_ASYNC ();

        int result = do_system (line);

        LIBC_CANCEL_RESET (oldtype);

        return result;

    }

    weak_alias (__libc_system, system) /* weak_alias 别名 */

        /*通过以上分析system函数实际上调用的是do_system函数， 以下对do_system函数进行分析，以下列出主要函数：*/

        /* Execute LINE as a shell command, returning its status. */

    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 (); /* mutex lock*/

        if (ADD_REF () == 0)

        {

            if (__sigaction (SIGINT, &sa, &intr) < 0) /*执行时 SIGINT被忽略*/
            {
                SUB_REF ();
                goto out;
            }
            if (__sigaction (SIGQUIT, &sa, &quit) < 0) /*执行时 SIGQUIT被忽略*/
            {
                save = errno;
                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) /*执行时设置SIG_BLOCK标志位，SIGCHLD被阻塞，执行失败，则恢复之前的信号的bitmap*/
        {
    #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
        /*执行成功，调用fork，生成子进程执行command命令*/
    #ifdef FORK
        pid = FORK (); /*调用SYS_CALL生成子进程*/
    #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); /*exec执行失败则返回127*/
        }
        else if (pid < (pid_t) 0)
            /* The fork failed. */
            status = -1;
        else /*父进程，waitpid*/
            /* 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; /*waitpid 失败返回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;
    }

    总结：system函数的返回值：分2大类：
    （1） 当参数为空时，调用do_system ("exit 0"),返回值为NULL /*网上都说是返回非0值，但我测的是NULL ！！！！*/
    （2） 调用result = do_system (line)；
    当执行忽略SIGINT和SIGQUIT信号时，错误返回-1
    fork子进程时出现错误 返回值为-1
    当exec执行错误或命令无效时返回值为127
    当waitpid错误时 返回值为-1
    if(NULL == cmdstring) //如果cmdstring为空趁早闪退吧，尽管system()函数也能处理空指针
        以下可以得到返回值错在什么位置，相关知识详见APUE（摘录的具体位置忘了，作者看到后请留言）
    if(NULL == cmdstring) //如果cmdstring为空趁早闪退吧，尽管system()函数也能处理空指针
    {
        return XXX;
    }
    status = system(cmdstring);
    if(status < 0)
    {
        printf("cmd: %s\t error: %s", cmdstring, strerror(errno)); // 这里务必要把errno信息输出或记入Log
        return XXX;
    }

    if(WIFEXITED(status))
    {
        printf("normal termination, exit status = %d\n", WEXITSTATUS(status)); //取得cmdstring执行结果
    }
    else if(WIFSIGNALED(status))
    {
        printf("abnormal termination,signal number =%d\n", WTERMSIG(status)); //如果cmdstring被信号中断，取得信号值
    }
    else if(WIFSTOPPED(status))
    {
        printf("process stopped, signal number =%d\n", WSTOPSIG(status)); //如果cmdstring被信号暂停执行，取得信号值
    }