How to return a value from pthread threads in C?
Solution 1:
You are returning the address of a local variable, which no longer exists when the thread function exits. In any case, why call pthread_exit
? why not simply return a value from the thread function?
void *myThread()
{
return (void *) 42;
}
and then in main:
printf("%d\n", (int)status);
If you need to return a complicated value such a structure, it's probably easiest to allocate it dynamically via malloc()
and return a pointer. Of course, the code that initiated the thread will then be responsible for freeing the memory.
Solution 2:
Here is a correct solution. In this case tdata is allocated in the main thread, and there is a space for the thread to place its result.
#include <pthread.h>
#include <stdio.h>
typedef struct thread_data {
int a;
int b;
int result;
} thread_data;
void *myThread(void *arg)
{
thread_data *tdata=(thread_data *)arg;
int a=tdata->a;
int b=tdata->b;
int result=a+b;
tdata->result=result;
pthread_exit(NULL);
}
int main()
{
pthread_t tid;
thread_data tdata;
tdata.a=10;
tdata.b=32;
pthread_create(&tid, NULL, myThread, (void *)&tdata);
pthread_join(tid, NULL);
printf("%d + %d = %d\n", tdata.a, tdata.b, tdata.result);
return 0;
}
Solution 3:
You've returned a pointer to a local variable. That's bad even if threads aren't involved.
The usual way to do this, when the thread that starts is the same thread that joins, would be to pass a pointer to an int, in a location managed by the caller, as the 4th parameter of pthread_create. This then becomes the (only) parameter to the thread's entry-point. You can (if you like) use the thread exit value to indicate success:
#include <pthread.h>
#include <stdio.h>
int something_worked(void) {
/* thread operation might fail, so here's a silly example */
void *p = malloc(10);
free(p);
return p ? 1 : 0;
}
void *myThread(void *result)
{
if (something_worked()) {
*((int*)result) = 42;
pthread_exit(result);
} else {
pthread_exit(0);
}
}
int main()
{
pthread_t tid;
void *status = 0;
int result;
pthread_create(&tid, NULL, myThread, &result);
pthread_join(tid, &status);
if (status != 0) {
printf("%d\n",result);
} else {
printf("thread failed\n");
}
return 0;
}
If you absolutely have to use the thread exit value for a structure, then you'll have to dynamically allocate it (and make sure that whoever joins the thread frees it). That's not ideal, though.
Solution 4:
I think you have to store the number on heap. The int ret
variable was on stack and was destructed at the end of execution of function myThread
.
void *myThread()
{
int *ret = malloc(sizeof(int));
if (ret == NULL) {
// ...
}
*ret = 42;
pthread_exit(ret);
}
Don't forget to free
it when you don't need it
Another solution is to return the number as value of the pointer, like Neil Butterworth suggests.