How to join a thread which have fork in it? - c

Question

Following is the case.
{
...
pthread_create(thread_id, NULL, thread_fun, NULL);
pthread_join(thread_id, NULL);
...
}
void * thread_fun(void * arg)
{
if(fork())
{
printf("In Parent\n");
pthread_exit(NULL);
}
else
{
printf("In Child\n");
pthread_exit(NULL);
}
}
How to use pthread_join() in such a way that I can wait for both child and parent process in thread?

Answer 1

Put all threads into endless loops and then inspect the whole program state with ps, top, htop or similar tools. You will find that the child process only has a single thread, while the parent process has (at least) two.
Now, concerning your question how to use pthread_join() to wait for the child process, you simply can't, because you can only use it to wait for threads in the same process. You could wait for the child process to terminate (waitpid()).
If the above doesn't answer your question (it's a bit unclear because you want to "wait for both child and parent process", which is where I wonder from which process' context) take a step back instead and describe on a higher level what you're trying to achieve instead. In other words, this could be a so-called "XY Problem". Do a bit of research on that term, it's a good thing to learn and understand in any case.

Answer 2

There's basically two ways you can do it. You can either have the child thread wait for the child process before it exits, or you can pass the PID of the child process to the parent thread so it can wait for it. Here's the first approach:
#include <sys/types.h>
#include <sys/wait.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <pthread.h>
void *thread_fun(void *arg);
int main(void) {
pthread_t thread;
pthread_create(&thread, NULL, thread_fun, NULL);
pthread_join(thread, NULL);
return 0;
}
void *thread_fun(void *arg) {
pid_t pid = fork();
if(pid) {
printf("In Parent\n");
waitpid(pid, NULL, 0);
pthread_exit(NULL);
} else {
printf("In Child\n");
exit(0);
}
}
And here's the second:
#include <sys/types.h>
#include <sys/wait.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <pthread.h>
#include <semaphore.h>
struct locked_pid {
sem_t sem;
pid_t pid;
};
void *thread_fun(void *arg);
int main(void) {
pthread_t thread;
struct locked_pid s;
sem_init(&s.sem, 0, 0);
pthread_create(&thread, NULL, thread_fun, &s);
sem_wait(&s.sem);
waitpid(s.pid, NULL, 0);
pthread_join(thread, NULL);
sem_destroy(&s.sem);
return 0;
}
void *thread_fun(void *arg) {
pid_t pid = fork();
if(pid) {
struct locked_pid *s = arg;
s->pid = pid;
sem_post(&s->sem);
printf("In Parent\n");
pthread_exit(NULL);
} else {
printf("In Child\n");
exit(0);
}
}
Note that the first is much simpler than the second. This is because in the second, you need a semaphore so that the parent process knows when the child PID gets written.
By the way, there's two things you should know about this code before you use it in anything important:
It doesn't have any error-checking, even though most of the functions it uses can fail.
POSIX says "If a multi-threaded process calls fork() ... the child process may only execute async-signal-safe operations until such time as one of the exec functions is called." This means that you shouldn't be doing printf("In Child\n");.