C捕获分叉儿童cout

2023年6月10日 184次阅读

我有以下代码片段：

stringstream userOut;
streambuf* old = cout.rdbuf(userOut.rdbuf());

pid_t pid;

if (!(pid = fork())) {

    cout << "hello from child!" << endl;
    exit(0);


} else {
    int status;

    // wait for child to finish
    wait(&status);

    // give cout old buffer back
    cout.rdbuf(old);

    // prints nothing!
    cout << "child content: " << userOut.str() << endl;
}

我希望能够从子进程中捕获和重定向cout以在父进程中使用,但到目前为止,重定向的输出始终为空.可能导致这种情况的原因是什么？

最佳答案 如评论中所述,分叉进程获取流的副本.所以它写入输出流的副本.

通常,您希望将管道用于此类问题.
如果你咨询man 2 pipe,你会在最后找到这个例子：

The following program creates a pipe, and then fork(2)s to create a
child process; the child inherits a duplicate set of file descriptors
that refer to the same pipe. After the fork(2), each process closes
the file descriptors that it doesn’t need for the pipe (see pipe(7)).
The parent then writes the string contained in the program’s command-
line argument to the pipe, and the child reads this string a byte at
a time from the pipe and echoes it on standard output.

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

   int
   main(int argc, char *argv[])
   {
       int pipefd[2];
       pid_t cpid;
       char buf;

       if (argc != 2) {
           fprintf(stderr, "Usage: %s <string>\n", argv[0]);
           exit(EXIT_FAILURE);
       }

       if (pipe(pipefd) == -1) {
           perror("pipe");
           exit(EXIT_FAILURE);
       }

       cpid = fork();
       if (cpid == -1) {
           perror("fork");
           exit(EXIT_FAILURE);
       }

       if (cpid == 0) {    /* Child reads from pipe */
           close(pipefd[1]);          /* Close unused write end */

           while (read(pipefd[0], &buf, 1) > 0)
               write(STDOUT_FILENO, &buf, 1);

           write(STDOUT_FILENO, "\n", 1);
           close(pipefd[0]);
           _exit(EXIT_SUCCESS);

       } else {            /* Parent writes argv[1] to pipe */
           close(pipefd[0]);          /* Close unused read end */
           write(pipefd[1], argv[1], strlen(argv[1]));
           close(pipefd[1]);          /* Reader will see EOF */
           wait(NULL);                /* Wait for child */
           exit(EXIT_SUCCESS);
       }
   }
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