How to make a memcpy inside of an thread of pthread? - c++

Question

I'm trying to do a sum of 2 matrices using pthreads in c++. I'm stuck at trying to pass the result of the sum calculated inside a thread to my main function.
The 2 values to be added are inside a struct:
struct sum{
int value1;
int value2;
int result;
}typedef struct_sum;
And the struct containing the values is passed as an argument to pthread_create() so that the operation is excuted inside a thread.
Here's my routine:
void * routine(void * sum) {
std::cout<<((struct_sum *)sum)->value1 + ((struct_sum *)sum)->value2<<std::endl;
std::cout<<((struct_sum *)sum)->value1<<std::endl;
std::cout<<((struct_sum *)sum)->value2<<std::endl;
int i = (((struct_sum *) sum)->value1 + ((struct_sum *) sum)->value2);
// memcpy(&(((struct_sum *)sum)->result), reinterpret_cast<const void *>(i), sizeof(i));
((struct_sum *)sum)->result = i;
std::cout<<&(((struct_sum *)sum)->result)<<std::endl;
pthread_exit(nullptr);
}
In the first 3 cout I check if my values are coming correctly to the thread.
In the last cout (before exiting the thread) I check the memory address of the result element of the struct (so I can see that it has the same address inside the main function).
Here's the main function:
int main(int argc, char * argv[]) {
int mat_1[ROW_SIZE][COLUMN_SIZE] = {{1, 2},
{6, 7}};
int mat_2[ROW_SIZE][COLUMN_SIZE] = {{3, 15},
{9, 14}};
int mat_result[ROW_SIZE][COLUMN_SIZE];
int mat_size = sizeof(mat_1) / sizeof(int);
int row_size = sizeof(mat_1) / sizeof(mat_1[0]);
int column_size = sizeof(mat_1[0]) / sizeof(int);
pthread_t threads[mat_size];
int thread_number = 0;
int thread_handler;
for (int row = 0; row < row_size; row++) {
for (int column = 0; column < column_size; column++) {
struct_sum *result;
result = static_cast<struct_sum *>(malloc(sizeof(struct_sum)));
result->value1 = mat_1[row][column];
result->value2 = mat_2[row][column];
result->result = 0;
thread_handler = pthread_create(&threads[thread_number], nullptr, routine, result);
if(thread_handler) return(-1);
std::cout << &(result->result)<<std::endl;
thread_number++;
mat_result[row][column] = result->result;
// free(result);
}
}
pthread_exit(nullptr);
}
I'm having two problems:
Even though the result has the same address in the main and in the thread, when I copy the value of i to ((struct_sum *)sum)->result, in the main function, result->result is still 0.
When I uncomment the memcpy() line the thread simply don't run, so I don't know how I'm doing it wrong.
I was expecting that in my main function the statement std::cout << (result->result) <<std::endl would return me the result of the operation, but the current value is 0.
So, how do I perform the memcpy() correctly in the thread?

Answer 1

You have to JOIN your threads. It means, wait them to finish.
The way you were doing is basically starting the thread and not giving it a guaranteed time do to anything. Besides that, some important change to the API, check the comments below:
void * routine(void * sum) {
int i = (((struct_sum *) sum)->value1 + ((struct_sum *) sum)->value2);
((struct_sum *)sum)->result = i;
// notice you don't need memcpy(), in fact...
// but you could use it here if you want... it won't fail.
// you have to use this function so it return your result to the main thread.
pthread_exit(sum);
}
int main(int argc, char * argv[]) {
int mat_1[ROW_SIZE][COLUMN_SIZE] = {{1, 2},
{6, 7}};
int mat_2[ROW_SIZE][COLUMN_SIZE] = {{3, 15},
{9, 14}};
int mat_result[ROW_SIZE][COLUMN_SIZE];
int mat_size = sizeof(mat_1) / sizeof(int);
int row_size = sizeof(mat_1) / sizeof(mat_1[0]);
int column_size = sizeof(mat_1[0]) / sizeof(int);
pthread_t threads[mat_size];
int thread_number = 0;
int thread_handler;
for (int row = 0; row < row_size; row++) {
for (int column = 0; column < column_size; column++) {
struct_sum *result;
result = static_cast<struct_sum *>(malloc(sizeof(struct_sum)));
result->value1 = mat_1[row][column];
result->value2 = mat_2[row][column];
result->result = 0;
thread_handler = pthread_create(&threads[thread_number], nullptr, routine, result);
if(thread_handler) return(-1);
// std::cout << &(result->result)<<std::endl;
thread_number++;
// forget this line below
// mat_result[row][column] = result->result;
}
}
// here you wait for the threads to JOIN
// here it means they actually "finished" their job
for (int i = 0; i < mat_size; i++)
{
struct_sum *result;
// here you wait for the threads to finish their job
// add something to your struct to "identify" the thread, so
// you can figure out where in the final matrix you put the result
pthread_join(threads[i], (void**)&result);
std::cout << result->result << "\n";
// this will print the correct sums: 4, 17, 15 and 21
// notice: it will be printed in ANY order, once you
// don't know which thread will finish first
// but result->result has the... result you need!
// you have to figure out how to fit this result in your matrix.
// but this is out of scope of the question
// and you can do yourself. have fun! :-)
// here you can free the result, you already got the value!
free(result);
}
// you don't need this line below... this goes to routine()
// pthread_exit(nullptr);
return 0;
}