Dynamically allocating struct inside a loop - c

Question

I have wrote a simple program to play with pthreads .
Each thread receives as thread data a pointer to a struct and prints it. Then I have wrote a loop to dynamically allocate structs and then use the point in the thread creation.
The problem is, Why does all threads receive the same stuct although I have allocated different loops?
#include <pthread.h>
#include <stdio.h>
#define NUM_THREADS 5
struct thread_data {
long thread_id;
int thread_sum;
};
void *PrintHello(void *threadData)
{
struct thread_data *data_input = (struct thread_data *)threadData;
printf("Hello World! It's me, thread #%ld! ,%d , pointer %d\n", data_input->thread_id, data_input->thread_sum, data_input);
pthread_exit(NULL);
}
int main(int argc, char *argv[])
{
pthread_t threads[NUM_THREADS];;
struct thread_data allData[NUM_THREADS];
int rc;
long t;
for (t = 0; t < NUM_THREADS; t++){
struct thread_data* data = malloc(sizeof(struct thread_data));
//allData[t].thread_id = t;
//allData[t].thread_sum = 100;
// struct thread_data data;
data->thread_id = t;
data->thread_sum = 0;
printf("In main: creating thread %ld , %d , %d\n", data->thread_id, data->thread_sum, &data);
rc = pthread_create(&threads[t], NULL, PrintHello, (void *)&data);
if (rc){
printf("ERROR; return code from pthread_create() is %d\n", rc);
exit(-1);
}
}
/* Last thing that main() should do */
pthread_exit(NULL);
return 0;
}

Answer 1

on your code &data is of type struct thread_data **
replace this
rc = pthread_create(&threads[t], NULL, PrintHello, (void *)&data);
by this
rc = pthread_create(&threads[t], NULL, PrintHello, (void *)data);
also, you have to call free for each allocated memory inside (and just before pthread_exit) the PrintHello function

Answer 2

Directly use data
#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
#define NUM_THREADS 5
struct thread_data {
long thread_id;
int thread_sum;
};
void *PrintHello(void *threadData)
{
struct thread_data *data_input = (struct thread_data *)threadData;
printf("Hello World! It's me, thread #%ld! ,%d , pointer %d\n", data_input->thread_id, data_input->thread_sum, data_input);
pthread_exit(NULL);
}
int main(int argc, char *argv[])
{
pthread_t threads[NUM_THREADS];;
struct thread_data allData[NUM_THREADS];
int rc;
long t;
for (t = 0; t < NUM_THREADS; t++){
struct thread_data* data = malloc(sizeof(struct thread_data));
//allData[t].thread_id = t;
//allData[t].thread_sum = 100;
// struct thread_data data;
data->thread_id = t;
data->thread_sum = 0;
printf("In main: creating thread %ld , %d , %d\n", data->thread_id, data->thread_sum, &data);
rc = pthread_create(&threads[t], NULL, PrintHello, (void *)data);
if (rc){
printf("ERROR; return code from pthread_create() is %d\n", rc);
exit(-1);
}
}
/* Last thing that main() should do */
pthread_exit(NULL);
return 0;
}
Will ouput:
In main: creating thread 0 , 0 , -273609272
In main: creating thread 1 , 0 , -273609272
Hello World! It's me, thread #0! ,0 , pointer 13357072
In main: creating thread 2 , 0 , -273609272
In main: creating thread 3 , 0 , -273609272
In main: creating thread 4 , 0 , -273609272
Hello World! It's me, thread #2! ,0 , pointer 13357712
Hello World! It's me, thread #1! ,0 , pointer 13357392
Hello World! It's me, thread #4! ,0 , pointer 13359632
Hello World! It's me, thread #3! ,0 , pointer 13358032