I'm trying to pass pointers to struct lower_hyper_id from a thread to the main thread, by the means of pthread_exit() function, that would compare and output the value in the struct. However, i receive an error (Segmentation fault) when i am trying to use the returned value and cast it to the struct.
thread that creates and returns the struct:
void *compute(void *arg){
lower_hyper_id *data = (lower_hyper_id *)malloc(sizeof(lower_hyper_id));
//some code
//i debug the program, and at this point, the struct i want
//to return has the values i want.
pthread_exit((void *)data);
}
in the main:
lower_hyper_id l_hyper_id;
int main(){
void *ap_state;
lower_hyper_id values;
void *ret;
//some code
for (int i = 0; i < NUMBER_OF_FILTERING_THREADS; i++)
{
s = pthread_join(filtering_threads[i], (void *)&ret);
//some error checking
values = *((lower_hyper_id *)ret); //this is where i receive the error
if (values.lowest_cost <= l_hyper_id.lowest_cost)
{
l_hyper_id.hyper_id = values.hyper_id;
l_hyper_id.lowest_cost = values.lowest_cost;
}
free(ret);
}
I have already looked at answers in the stackoverflow such as this question , but it hasn't helped me resolving this. I actually changed the code to be exactly equal to the code in this answer , but still it gives me an error.
You're not testing if malloc returned NULL. That could be an issue if you're allocing a large chunk and the allocation can fail. Other than that, I don't think the problem is in the return value passing.
pthread_exit()
ing with a malloc
d pointer should work just fine.
A minimial working example:
#include <errno.h>
#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
void *compute (void *arg)
{
printf("thread=%llx\n", (unsigned long long)pthread_self());
size_t sz = strlen("hello world")+1;
char *ret = malloc(sz+1);
if(ret) memcpy(ret, "hello world", sz+1);
return ret;
}
int main()
{
printf("thread=%llx\n", (unsigned long long)pthread_self());
pthread_t ptid;
int er;
if((er=pthread_create(&ptid,0,compute,0))) return errno=er,perror(0),1;
void *retval;
if((er=pthread_join(ptid,&retval))) return errno=er,perror(0),1;
printf("thread returned: %s\n", (char*)retval);
free(retval);
}
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