C multithread shared variable - c

Question

I need help with multithreading in C.
Initially, I have a variable named client in the main function.
void *my_function (void *arg) {
int *client = (int*)arg;
*client = 5;
return 0;
}
void *my_function2 (void *arg) { ... }
void *my_function3 (void *arg) { ... }
int main()
{
int client = 0;
pthread_t connect_thread, func2_thread, func3_thread;
pthread_create(&connect_thread, NULL, my_function, (void *)&client);
pthread_create(&func2_thread, NULL, my_function2, (void *)&client);
pthread_create(&func3_thread, NULL, my_function3, (void *)&client);
// how can i make it such that after executing my_function, client's value = 5 in main()??
pthread_join(my_function, NULL);
// once the value is updated, i will pass this updated value into another thread
pthread_join(my_function2, NULL);
pthread_join(my_function3, NULL);
return 0;
}
How can I change the value of client in main from 0 to 5 after the execution of my_function?

Answer 1

The problem with your code is that when main() completes, it terminates the entire program, all other threads included. Your code will indeed modify the value of client, but it isn't doing it in a safe way (in general), as you should be using mutexes to protect data accessed in multiple threads. It's most likely your program is terminating before the thread is fully created.
You need to add mutexes to your code, and use pthread_join() to wait for the thread to complete before allowing main() to complete and return. Please refer to the corrected example below.
Code Listing
/*******************************************************************************
* Preprocessor directives
******************************************************************************/
#include <stdio.h>
#include <pthread.h>
/*******************************************************************************
* Globals
******************************************************************************/
pthread_mutex_t mDataMutex = PTHREAD_MUTEX_INITIALIZER;
/*******************************************************************************
* Function prototypes
******************************************************************************/
void *my_function(void *arg);
/*******************************************************************************
* Function definitions
******************************************************************************/
/*----------------------------------------------------------------------------*/
void *my_function(void *arg)
{
int *client = (int*)arg;
printf("Thread started.\n");
pthread_mutex_lock(&mDataMutex);
*client = 5;
pthread_mutex_unlock(&mDataMutex);
printf("Thread completed.\n");
return 0;
}
/*----------------------------------------------------------------------------*/
int main(void)
{
int client = 0;
int rc;
void *status;
pthread_t connect_thread;
pthread_attr_t attr;
// Print initial value of variable.
printf("Initial value of client:%d.\n", client);
/* Initialize and set thread detached attribute */
pthread_attr_init(&attr);
pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_JOINABLE);
// Create the thread and start it.
rc = pthread_create(&connect_thread, &attr, my_function, (void *)&client);
if (rc)
{
printf("ERROR; return code from pthread_create() is %d\n", rc);
return (-1);
}
// Clean up the attribute struct, don't need it anymore
pthread_attr_destroy(&attr);
// Wait for the thread to complete.
rc = pthread_join(connect_thread, &status);
// Print update value of client
pthread_mutex_lock(&mDataMutex);
printf("Updated value of client:%d.\n", client);
pthread_mutex_unlock(&mDataMutex);
return 0;
}
Sample Output
Initial value of client:0.
Thread started.
Thread completed.
Updated value of client:5.

Answer 2

Your code already changes the value. The error in your code is in the pthread_join call which takes as input a thread id of type pthread_t (connect_thread variable in your case) while you passed to it the function.
#include<pthread.h>
#include<stdio.h>
void *my_function (void *arg) {
int *client = (int*)arg;
*client = 5;
return 0;
}
int main()
{
int client = 0;
pthread_t connect_thread, func2_thread, func3_thread;
pthread_create(&connect_thread, NULL, my_function, (void *)&client);
// how can i make it such that after executing my_function, client's value = 5 in main()??
pthread_join(connect_thread, NULL);
// once the value is updated, i will pass this updated value into another thread
printf("%d\n", client);
return 0;
}
Another alternative is to use a global variable and mutex to change the value.