Introduction to fork in multithreading under Linux

Question:

Recall that when a program has only one main thread and fork is called, the child process created by fork will also have only one thread;

What if we put fork into a multi-threaded program?

Let's try it out:

Case (1) fork before creating a child thread

Code:

#include <stdio.h>
#include <pthread.h>
#include <unistd.h>
void* pthread_fun(void* arg)
{
	printf("fun = %d\n", getpid());
	pthread_exit(NULL);
}
int main()
{
	fork();

	pthread_t id;
	pthread_create(&id, NULL, pthread_fun, NULL);
	
	printf("main_pid = %d\n", getpid());
	pthread_join(id, NULL);

	return 0;
}

Result: The forked child process will also create its own child thread (兩個進程：四個線程)

Case (2) fork after creating a child thread

Code:

#include <stdio.h>
#include <pthread.h>
#include <unistd.h>

void* pthread_fun(void* arg)
{
	printf("fun = %d\n", getpid());
	pthread_exit(NULL);
}
int main()
{

	pthread_t id;
	pthread_create(&id, NULL, pthread_fun, NULL);
	fork();
	
	printf("main_pid = %d\n", getpid());
	pthread_join(id, NULL);
	return 0;
}

Result: After creating a child thread, a child process is created. At this time, the fork child process will only execute the code after fork (兩個進程：三個線程)

Case (3) fork in child thread

Code:

#include <stdio.h>
#include <pthread.h>
#include <unistd.h>

void* pthread_fun(void* arg)
{
	fork();
	printf("fun = %d\n", getpid());
	pthread_exit(NULL);
}
int main()
{

	pthread_t id;
	pthread_create(&id, NULL, pthread_fun, NULL);
	
	printf("main_pid = %d\n", getpid());
	pthread_join(id, NULL);

	return 0;
}

result:

in conclusion:

In which thread is fork, the child process created after fork will use this thread as its main thread and execute the code after this thread

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