Get stdout of already running process in Linux in C

You can use posix_spawn for this, without having to use race-condition-prone, Linux-specific /proc/<pid>/fd/N. You can keep all the benefits of posix_spawn.

You were on the right track thinking about file_actions. Below is an example that prints out the child's stdout in Python-style triple quotes, as well as the child's exit code, from the parent process using posix_spawn and file_actions.

Here is an example of the example output.

child pid: 17468
child exit status: 0
child stdout:
"""Hello World!
"""

Here is the example.

#define _DEFAULT_SOURCE
#include <spawn.h>
#include <stdio.h>
#include <unistd.h>
#include <string.h>
#include <sys/wait.h>

extern char **environ;

static void dump_child_stdout(int filedes)
{
    ssize_t num_read;
    char buf[1];

    printf("child stdout:\n\"\"\"");
    for (;;)
    {
        num_read = read(filedes, buf, sizeof(buf));
        if (num_read > 0)
        {
            printf("%c", buf[0]);
        }
        else
        {
            break;
        }
    }
    printf("\"\"\"\n");
}

int main(int argc, char *argv[])
{
    int status;
    pid_t pid;
    int out[2];
    posix_spawn_file_actions_t action;
    char *args[] = {"/bin/echo", "Hello World!", NULL };

    posix_spawn_file_actions_init(&action);

    pipe(out);

    posix_spawn_file_actions_adddup2(&action, out[1], STDOUT_FILENO);
    posix_spawn_file_actions_addclose(&action, out[0]);

    status = posix_spawn(&pid, args[0], &action, NULL, args, environ);
    if (status == 0)
    {
        printf("child pid: %d\n", pid);
        if (waitpid(pid, &status, 0) < 0)
        {
            perror("waitpid");
        }
        else
        {
            if (WIFEXITED(status))
            {
                printf("child exit status: %d\n", WEXITSTATUS(status));
            }
            else
            {
                printf("child died an unnatural death.\n");
            }

            close(out[1]);
            dump_child_stdout(out[0]);
        }
    }
    else
    {
        fprintf(stderr, "posix_spawn: %s\n", strerror(status));
        close(out[1]);
    }

    posix_spawn_file_actions_destroy(&action);

    return 0;
}