调用 fork() 后函数的 return 值的可见性

Question

当使用fork()创建一个新的子进程时，我们得到一个pid_t返回表明我们是在子进程（pid == 0）还是父进程（pid > 0) 在调用 fork() 完成后。总的来说，我在解决这个问题时遇到了一些麻烦，但特别难以找到以下问题的答案：

在函数中包装对 fork() 的调用时，该函数的调用代码（永远）是否会看到 child 中的 return 值fork()?

为了说明，请参阅我编写的以下函数。我不确定调用 process/code 是否会收到 -1 return 值之一，这些值可以从代码。

/*
 * Opens the process `cmd` similar to popen() but does not invoke a shell.
 * Instead, wordexp() is used to expand the given command, if necessary.
 * If successful, the process id of the new process is being returned and the 
 * given FILE pointers are set to streams that correspond to pipes for reading 
 * and writing to the child process, accordingly. Hand in NULL for pipes that
 * should not be used. On error, -1 is returned.
 */
pid_t popen_noshell(const char *cmd, FILE **out, FILE **err, FILE **in)
{
    if (!cmd || !strlen(cmd)) return -1;

    // 0 = read end of pipes, 1 = write end of pipes
    int pipe_stdout[2];
    int pipe_stderr[2];
    int pipe_stdin[2];

    if (out && (pipe(pipe_stdout) < 0)) return -1;
    if (err && (pipe(pipe_stderr) < 0)) return -1;
    if (in  && (pipe(pipe_stdin)  < 0)) return -1;

    pid_t pid = fork();
    if (pid == -1)
    {
        return -1;
    }
    else if (pid == 0) // child
    {    
        // redirect stdout to the write end of this pipe
        if (out)
        {
            if (dup2(pipe_stdout[1], STDOUT_FILENO) == -1) return -1;
            close(pipe_stdout[0]); // child doesn't need read end
        }
        // redirect stderr to the write end of this pipe
        if (err)
        {
            if (dup2(pipe_stderr[1], STDERR_FILENO) == -1) return -1;
            close(pipe_stderr[0]); // child doesn't need read end
        }
        // redirect stdin to the read end of this pipe
        if (in)
        {
            if (dup2(pipe_stdin[0], STDIN_FILENO) == -1) return -1;
            close(pipe_stdin[1]); // child doesn't need write end
        }

        wordexp_t p;
        if (wordexp(cmd, &p, 0) != 0) return -1;

        execvp(p.we_wordv[0], p.we_wordv);
        _exit(1);
    }
    else // parent
    {
        if (out)
        {
            close(pipe_stdout[1]); // parent doesn't need write end
            *out = fdopen(pipe_stdout[0], "r");
        }
        if (err)
        {
            close(pipe_stderr[1]); // parent doesn't need write end
            *err = fdopen(pipe_stderr[0], "r");
        }
        if (in)
        {
            close(pipe_stdin[0]); // parent doesn't need read end
            *in = fdopen(pipe_stdin[1], "w");
        }
        return pid;
    }
}

Answer 1

fork 复制整个调用堆栈，因此如果其中一个 return -1 语句在 child 中执行，它将返回给 popen_noshell 的调用者在 child 中（以及在 parent 中）。这可能不是您想要的，它们可能应该替换为 _exit(-1)

调用 fork() 后函数的 return 值的可见性

Visibility of a function's return value after a call to fork()

c

fork

child-process