Print threads of a running process. Response to http://stackoverflow.com/questions/21007804/how-do-i-find-threads-that-belong-to-the-same-process-in-linux.

threadsofpid.c

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <dirent.h>

int main(int argc, char** argv)
{
    if(argc != 2)
    {
        printf("usage: %s <pid>", argv[0]);
        return -1;
    }

    const int MAX_PID_DIGITS = 30; // Way more than enough.
    if(strlen(argv[1]) > MAX_PID_DIGITS)
    {
        perror("Are you sure that's a pid...?");
        return -1;
    }

    char pid[MAX_PID_DIGITS];
    strcpy(pid, argv[1]);
    char filename[MAX_PID_DIGITS + 13]; // As long as it can possibly be with "/proc/<pid>/task\0"
    sprintf(filename, "/proc/%s/task", pid);

    struct dirent* entry;
    DIR* dir = opendir(filename);

    if(dir == NULL)
    {
        perror("opendir failed");
        return -1;
    }

    // Iterate over the subdirectories.
    while((entry = readdir(dir)) != NULL)
    {
        // Ignore '.', '..' directories.
        // /proc/[pid]/task directory also has a subdirectory
        // whose name is the pid of /proc/[pid]/task, ignore
        // that as well.
        if(strcmp(entry->d_name, ".") == 0
           || strcmp(entry->d_name, "..") == 0
           || strcmp(entry->d_name, pid) == 0)
        {
            continue;
        }

        printf("%s\n", entry->d_name);
    }

    closedir(dir);
}