Linux - Syscall to Iterate over children threads - linux

Question

I am creating a syscall to return some information about a process and its children threads. I am testing this in Ubuntu 14.04. Here is the code that I have written:
#include <linux/list.h>
#include <linux/linkage.h>
#include <linux/sched.h>
#include <linux/uaccess.h>
#define NUM_THREADS 5
struct thread_info_prj {
int pid;
int nthreads;
int tid[NUM_THREADS];
};
asmlinkage void sys_threadinfo_prj(void *ptr)
{
struct task_struct *task;
struct thread_info_prj t_info;
struct list_head *list;
int num_threads = 0;
t_info.pid = current->pid;
list_for_each(list, &current->children) {
//task = list_entry(&p_task->children, struct task_struct, sibling);
task = list_entry(list, struct task_struct, sibling);
t_info.tid[num_threads] = task->pid;
num_threads++;
}
t_info.nthreads = num_threads;
copy_to_user(ptr, &t_info, sizeof(struct thread_info_prj));
}
This code is built into the kernel as a syscall , and tested with the following program.
#include <pthread.h>
#include <stdio.h>
#include <sys/syscall.h>
#include <errno.h>
#define NUM_THREADS 5
struct thread_info_prj {
int pid;
int nthreads;
int tid[NUM_THREADS];
};
void * thread_fn(void *ptr) {
printf("Im a thread!\n");
sleep(4);
pthread_exit(NULL);
}
int main() {
pthread_t threads[NUM_THREADS];
pthread_attr_t attr;
int i;
struct thread_info_prj t_info;
void *status;
pthread_attr_init(&attr);
//pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_JOINABLE);
for (i = 0; i < NUM_THREADS; i++)
pthread_create(&(threads[i]), &attr, thread_fn, NULL);
sleep(1);
syscall(352, &t_info);
//for (i = 0; i < NUM_THREADS; i++)
// pthread_join(threads[i], &status);
printf("PID: %d\n", t_info.pid);
printf("Num Threads: %d\n", t_info.nthreads);
for (i = 0; i < t_info.nthreads; i++) {
printf("Thread ID: %d\n", t_info.tid[i]);
}
printf("%s\n",strerror(errno));
return 0;
}
Unfortunately, the only output is the proper parent pid. Otherwise, it returns that there are no threads and does not print any additional PIDs. By adding a printk inside the for loop, I discovered that it never even enters the for loop. Any suggestions?
Thanks so much.