Program is not generating random number and I am not sure why - c

Question

#include<pthread.h>
#include<stdio.h>
#include<stdint.h>
#include<stdlib.h>
#include<time.h>
#include<unistd.h>
#define ERROR_CREATE 1
#define ERROR_JOIN 2
// create the function to be executed as a thread
void *thread(void *ptr)
{
uintptr_t type = (uintptr_t) ptr; // thread number
srand(time(NULL) + getpid());
int wait = rand() % 10; // randomizes numbers from 0 to 10
sleep(wait); // waits in time intervals of seconds
printf("Thread - %ld waiting for %d seconds\n",type, wait);
return ptr; // returns the thread number
}
int main(int argc, char **argv) {
if (argc != 2) {
fprintf(stderr, "Error with command line arguments");
}
int num_threads = atoi(argv[1]);
pthread_t threads[num_threads]; // array of thread types
for (long i = 1; i <= num_threads; i++) {
if (pthread_create(&threads[i], NULL, thread, (void *)i) != 0)
// if there's an error creating thread
{
fprintf(stderr,"Error: could not create thread");
return ERROR_CREATE;
}
}
// terminate each thread assigned
for (int i = 1; i <= num_threads; i++) {
if (pthread_join(threads[i], NULL) != 0)
// if there's an error ending each thread
{
fprintf(stderr, "Error: could not terminate thread");
return ERROR_JOIN;
}
}
return 0;
}
Seeding the rand function, I am still getting the same number outputted. I understand the hardware is fast and therefore is getting the same answer as the clock speed is faster than seeding the rand function. Does anyone know another way of getting more variety from the rand function?

Answer 1

Return if argv[1] is not populated otherwise it segfaults.
srand() resets the sequence. As you call it multiple times with the same value this is not what you want. Moved it main().
The array threads is accessed out of bounds in the two loops.
#include <pthread.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <unistd.h>
#define ERROR_CREATE 1
#define ERROR_JOIN 2
// create the function to be executed as a thread
void *thread(void *ptr) {
uintptr_t type = (uintptr_t) ptr; // thread number
int wait = rand() % 10; // randomizes numbers from 0 to 10
sleep(wait); // waits in time intervals of seconds
printf("Thread - %ld waiting for %d seconds\n",type, wait);
return ptr; // returns the thread number
}
int main(int argc, char **argv) {
if (argc != 2) {
fprintf(stderr, "Error with command line arguments\n");
return 1;
}
srand(time(NULL));
int num_threads = atoi(argv[1]);
pthread_t threads[num_threads]; // array of thread types
for (long i = 0; i < num_threads; i++) {
if (pthread_create(&threads[i], NULL, thread, (void *)i) != 0)
// if there's an error creating thread
{
fprintf(stderr,"Error: could not create thread");
return ERROR_CREATE;
}
}
// terminate each thread assigned
for (int i = 0; i < num_threads; i++) {
if (pthread_join(threads[i], NULL) != 0)
// if there's an error ending each thread
{
fprintf(stderr, "Error: could not terminate thread");
return ERROR_JOIN;
}
}
}
and here is a couple of sample runs:
$ ./a.out 2
Thread - 1 waiting for 3 seconds
Thread - 2 waiting for 7 seconds
$ ./a.out 2
Thread - 1 waiting for 3 seconds
Thread - 2 waiting for 6 seconds