Return vector from pthread function? - c

Question

having some trouble finding out the syntax around returning a vector from a pthread function. Here's currently what I have:
typedef struct vectorData {
vector v1;
vector v2;
} vectorData;
void *vectorAddThread(void *arg) {
vectorData *data = (vectorData *)arg;
vector v1 = data->v1;
vector v2 = data->v2;
vector result = {v1->x + v2->x, v1->y + v2->y, v1->z + v2->z};
return (void*) result;
}
I'm getting errors on the last two lines about the return result, and the the adding part. Thanks

Answer 1

You can't convert a struct to a void * pointer. You need dynamically allocate a vector in your case with malloc, and return the pointer.
vector *result = malloc(sizeof(vector));
result->x = ?;
result->y = ?;
result->z = ?;
return (void *)result;
Now that would solve the issue of returning a struct from a function that returns void *. But if you're using pthreads you shouldn't be returning objects from them, you need to pass the vector to it as user data in arg.

Answer 2

This is probably easiest:
typedef struct vectorData {
vector v1;
vector v2;
vector result;
} vectorData;
Saves messing about with dubious thread-stack vars, mallocs etc. and ties the result output directly to the inputs.

Answer 3

The thread start function in pthread library returns a pointer to void. Can return any pointer type by type-casting it to (void *). The return value can be accessed by pthread_join(3).
However, a vector needs the type of element information also. Not sure if type-casting it to (void *) and accessing it again will work fine. Using it as an element of structure seems better.
In the following example, I am returning a pointer to structure from the thread start function. The structure contains a vector and two character arrays. You can change it according to your requirement. I am using two pointers to show that data is copied on the variable used for pthread_join(3).
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <pthread.h>
#include <errno.h>
#include <vector>
void *functionPthread(void *);
struct retStruct
{
std::vector<int> vecNumber;
char name[20];
char city[20];
};
struct retStruct *r2;
struct retStruct *r1;
int main()
{
int rc, i;
pthread_t th;
r2 = (retStruct*) malloc(sizeof(retStruct));
memset(r2, '\0', sizeof(r2));
if(rc = pthread_create(&th, NULL, &functionPthread, NULL))
{
printf("Thread creation failed, return code %d, errno %d", rc, errno);
}
pthread_join(th, (void**)&r2);
for(i = 0; i < r2->vecNumber.size(); i++)
printf("Vectore element at index %d = %d\n", i, r2->vecNumber.at(i));
printf("Name: %s, City: %s, Vector Number: %d\n", r2->name, r2->city, r2->vecNumber.front());
free(r1);
free(r2);
return 0;
}
void *functionPthread(void *)
{
r1 = (retStruct*) malloc(sizeof(retStruct));
memset(r1, '\0', sizeof(r1));
strcpy(r1->name, "xxxx");
strcpy(r1->city, "yyyy");
r1->vecNumber.push_back(11);
r1->vecNumber.push_back(12);
return r1;
}