linux system 函数源代码分析

inux 下system函数原型：
#include
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
}

有不对之处，还请大家指正。