C- Unique Thread Identifier

[undone wren]

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I am aiming for each thread to have a unique identifier which I require for a bigger piece of code, but when ran it only pr```cpp
ints out the same identifier for each thread. What's the solution for this?

Code:

#include <pthread.h>
#include <stdio.h>

#define NTHREADS 10

void *thread_function(void *);

pthread_mutex_t mutex1 = PTHREAD_MUTEX_INITIALIZER;

int counter = 0;

int main() {
pthread_t thread_id[NTHREADS];
int i, j;

for (i = 0; i < NTHREADS; i++) {
pthread_create(&thread_id[i], NULL, thread_function, (void*)&i);
}

for (j = 0; j < NTHREADS; j++) {
pthread_join(thread_id[j], NULL);
}
}

void* thread_function(void* identifier) {
int threadIdentifier = ((int)identifier);

printf("Thread ID %d\n", threadIdentifier);
}

orange juice

[pale crypt]

(void*)&i you're passing every thread a pointer to the same variable i on the stack

You'll need NTHREADS different ints in memory, either an array or allocated with malloc or whatever.
Or you could do something kind of hacky: (void*)i and then (int)identifier in the thread

The "(void*)i and then (int)identifier in the thread" thing works

;compile c -lpthread

#include <pthread.h>
#include <stdio.h>

#define NTHREADS 10

void *thread_function(void *);

pthread_mutex_t mutex1 = PTHREAD_MUTEX_INITIALIZER;

int counter = 0;

int main() {
  pthread_t thread_id[NTHREADS];
  int i, j;

  for (i = 0; i < NTHREADS; i++) {
    pthread_create(&thread_id[i], NULL, thread_function, (void*)i);
  }

  for (j = 0; j < NTHREADS; j++) {
    pthread_join(thread_id[j], NULL);
  }
}

void* thread_function(void* identifier) {
  int threadIdentifier = (int*)identifier;

  printf("Thread ID %d\n", threadIdentifier);
}

[rocky slate]

Program Output

Thread ID 0
Thread ID 1
Thread ID 2
Thread ID 3
Thread ID 4
Thread ID 5
Thread ID 6
Thread ID 7
Thread ID 8
Thread ID 9

Compiler Output

<source>: In function 'main':
<source>:17:58: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
   17 |     pthread_create(&thread_id[i], NULL, thread_function, (void*)i);
      |                                                          ^
<source>: In function 'thread_function':
<source>:26:26: warning: initialization of 'int' from 'int *' makes integer from pointer without a cast [-Wint-conversion]
   26 |   int threadIdentifier = (int*)identifier;
      |                          ^

jr-#6677 | 41ms | c | x86-64 gcc 12.2 | godbolt.org

[pale crypt]

You could also take a look at

!man pthread_self

[undone wren]

man7

pthread_self(3)

pthread_self - obtain ID of the calling thread

Synopsis

#include <pthread.h>

pthread_t pthread_self(void);

Compile and link with -pthread.

[pale crypt]

Np

[undone wren]

@bright zodiac Has your question been resolved? If so, run !solved :)