c - 如何获取子进程的退出状态？

Question

两个示例输出(由我的教授提供)是(这些在 Linux 终端中输入):

 ibrahim@ibrahim-latech:~$ ./prog2 .
 Current working directory: /home/ibrahim
 Executing ls . --all -l --human-readable
 total 24M
 drwxr-xr-x 74 ibrahim ibrahim 20K Oct 26 16:08 .
 drwxr-xr-x 6 root root 4.0K Apr 10 2014 ..
 -rw-r--r-- 1 ibrahim ibrahim 4.1K Sep 13 12:09 .bashrc
 drwxr-xr-x 7 ibrahim ibrahim 4.0K Oct 11 14:51 Desktop/
 ...snip...
 Exit status: 0

因此，如果代码有效，退出状态为 0，但此示例:

 ibrahim@ibrahim-latech:~$ ./prog2 /root
 Current working directory: /home/ibrahim
 Executing ls /root --all -l --human-readable
 Can't chdir to /root
 Exit status: 1 

退出状态为 1，因为它是 false。

这是我的代码:

#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>

int main(int argc, char *argv[]){
    pid_t pid;
    pid = fork();
    char cwd[255];
    if(pid == 0){
        printf("Current working directory: %s\n",getcwd(cwd, sizeof(cwd)));
        printf("Executing ls %s --all -l --human-readable\n",argv[1]);
        if(chdir(argv[1])!=0){
            printf("this is not a valid directory");
        }
        else{
            execl("/bin/ls","ls","--all","-l","--human-readable", NULL);
        }
    }
    else{
        wait(NULL):
        printf("Exit status:");// not sure how to put the 1 or 0 in this
    }
}

退出状态必须由父进程输出。我不知道如何让退出状态发挥作用。

Answer 1

^{将评论中的关键信息转化为答案。}

wait()函数告诉您 PID 和退出状态 - 您只需捕获它们，而不是忽略它们。您需要使用 WIFEXITED 和 WEXITSTATUS (如果 WIFEXITED 报告 false，还有其他宏可用于信号和核心转储等)。

修改您的代码:

#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/wait.h>
#include <unistd.h>

#ifndef LS_PATH
#define LS_PATH "/bin/ls"
#endif

int main(int argc, char **argv)
{
    if (argc != 2)
    {
        fprintf(stderr, "Usage: %s directory\n", argv[0]);
        return 1;
    }
    pid_t pid = fork();
    if (pid == 0)
    {
        char cwd[255];
        printf("Current working directory: %s\n", getcwd(cwd, sizeof(cwd)));
        printf("Executing 'ls -a -l' in %s\n", argv[1]);
        if (chdir(argv[1]) != 0)
        {
            fprintf(stderr, "%s: %s is not a valid directory\n", argv[0], argv[1]);
            return 1;
        }
        //execl(LS_PATH, "ls", "--all", "-l", "--human-readable", (char*) NULL);
        execl(LS_PATH, "ls", "-a", "-l", (char*) NULL);
        fprintf(stderr, "Failed to execute %s\n", LS_PATH);
        return 1;
    }
    else
    {
        int status;
        int corpse = wait(&status);
        if (corpse < 0)
            printf("Failed to wait for process %d (errno = %d)\n", (int)pid, errno);
        else if (corpse != pid)
            printf("Got corpse of process %d (status 0x%.4X) when expecting PID %d\n",
                   corpse, status, (int)pid);
        else if (WIFEXITED(status))
            printf("Process %d exited with normal status 0x%.4X (status %d = 0x%.2X)\n",
                   corpse, status, WEXITSTATUS(status), WEXITSTATUS(status));
        else if (WIFSIGNALED(status))
            printf("Process %d exited because of a signal 0x%.4X (signal %d = 0x%.2X)\n",
                   corpse, status, WTERMSIG(status), WTERMSIG(status));
        else
            printf("Process %d exited with status 0x%.4X which is %s\n",
                   corpse, status, "neither a normal exit nor the result of a signal");
    }
    return 0;
}

我在不使用 GNU ls 的 Mac 上工作，因此我已将代码编译时配置为 ls 的位置，并且它仅使用可移植的选项(-a 和 -l 而不是双破折号选项)。我更改了消息传递并检测到更多错误。与您的代码一样，这假设 fork() 不会失败 - 它也应该检查那里是否有错误。

示例输出(缩写) - 来自从 es61.c 编译的程序 es61:

$ es61 /orthography
Current working directory: /Users/jonathanleffler/soq
Executing 'ls -a -l' in /orthography
es61: /orthography is not a valid directory
Process 64463 exited with normal status 0x0100 (status 1 = 0x01)
$ es61 $PWD
Current working directory: /Users/jonathanleffler/soq
Executing 'ls -a -l' in /Users/jonathanleffler/soq
total 1016
drwxr-xr-x  100 jonathanleffler  staff   3200 Oct 31 15:53 .
drwxr-xr-x+  69 jonathanleffler  staff   2208 Oct 31 15:53 ..
…
-rwxr-xr-x    1 jonathanleffler  staff   9028 Oct 31 15:53 es61
-rw-r--r--    1 jonathanleffler  staff   1806 Oct 31 15:53 es61.c
drwxr-xr-x    3 jonathanleffler  staff     96 Oct 31 15:43 es61.dSYM
drwxr-xr-x    9 jonathanleffler  staff    288 Nov  5  2018 etc
…
-rw-r--r--    1 jonathanleffler  staff    390 Nov 16  2017 makefile
…
Process 64557 exited with normal status 0x0000 (status 0 = 0x00)
$