如何处理execvp（...）错误后fork（）？ [英] How to handle execvp(...) errors after fork()?

问题描述

我执行常规操作：

• fork（）


• execvp

如果execvp失败，因为找不到cmd，我如何在父进程中注意到此错误？

If execvp fails because no cmd is found, how can I notice this error in parent process?

推荐答案

知名的自我管理技巧可以适应此用途。

#include <errno.h>
#include <fcntl.h>
#include <stdio.h>
#include <string.h>
#include <sys/wait.h>
#include <sysexits.h>
#include <unistd.h>

int main(int argc, char **argv) {
    int pipefds[2];
    int count, err;
    pid_t child;

    if (pipe(pipefds)) {
        perror("pipe");
        return EX_OSERR;
    }
    if (fcntl(pipefds[1], F_SETFD, fcntl(pipefds[1], F_GETFD) | FD_CLOEXEC)) {
        perror("fcntl");
        return EX_OSERR;
    }

    switch (child = fork()) {
    case -1:
        perror("fork");
        return EX_OSERR;
    case 0:
        close(pipefds[0]);
        execvp(argv[1], argv + 1);
        write(pipefds[1], &errno, sizeof(int));
        _exit(0);
    default:
        close(pipefds[1]);
        while ((count = read(pipefds[0], &err, sizeof(errno))) == -1)
            if (errno != EAGAIN && errno != EINTR) break;
        if (count) {
            fprintf(stderr, "child's execvp: %s\n", strerror(err));
            return EX_UNAVAILABLE;
        }
        close(pipefds[0]);
        puts("waiting for child...");
        while (waitpid(child, &err, 0) == -1)
            if (errno != EINTR) {
                perror("waitpid");
                return EX_SOFTWARE;
            }
        if (WIFEXITED(err))
            printf("child exited with %d\n", WEXITSTATUS(err));
        else if (WIFSIGNALED(err))
            printf("child killed by %d\n", WTERMSIG(err));
    }
    return err;
}

这是一个完整的程序。

$ ./a.out foo
child's execvp: No such file or directory
$ (sleep 1 && killall -QUIT sleep &); ./a.out sleep 60
waiting for child...
child killed by 3
$ ./a.out true
waiting for child...
child exited with 0

如何工作：

创建管道，使得写端点 CLOEXEC ：当成功执行 exec 时，它自动关闭。

Create a pipe, and make the write endpoint CLOEXEC: it auto-closes when an exec is successfully performed.

在孩子中，尝试 exec 。如果它成功，我们不再有控制，但管道是关闭的。如果失败，请将失败代码写入管道并退出。

In the child, try to exec. If it succeeds, we no longer have control, but the pipe is closed. If it fails, write the failure code to the pipe and exit.

在父级中，尝试从其他管道端点读取。如果读取返回零，则管道关闭，并且子成功必须具有 exec 。如果读取返回数据，则是我们的孩子写的失败代码。

In the parent, try to read from the other pipe endpoint. If read returns zero, then the pipe was closed and the child must have exec successfully. If read returns data, it's the failure code that our child wrote.

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