How to cast an integer to void pointer?

Question

While working with Threads in C, I'm facing the warning

"warning: cast to pointer from integer of different size"

The code is as follows

#include<stdio.h> #include<sys/types.h> #include<stdlib.h> #include<pthread.h> void *print(void *id) {  int a=10;  printf("My thread id is %ld\n",pthread_self());  printf("Thread %d is executing\n",id);  return (void *) 42; }  int main() {  pthread_t th[5];  int t;  int i;  int status;  void *ret;  for(i=0;i<5;i++)  {    status=pthread_create(&th[i],NULL,print,(void *)i); //Getting warning at this line    if(status)    {     printf("Error creating threads\n");     exit(0);    }    pthread_join(th[i],&ret);    printf("--->%d\n",(int *)ret);  }  pthread_exit(NULL); } 

Can anybody explain how to pass an integer to a function which receives (void * ) as a parameter?

Answer 1

This is a fine way to pass integers to new pthreads, if that is what you need. You just need to suppress the warning, and this will do it:

#include <stdint.h>  void *threadfunc(void *param) {     int id = (intptr_t) param;     ... }  int i, r; r = pthread_create(&thread, NULL, threadfunc, (void *) (intptr_t) i); 

Discussion

This may offend your sensibilities, but it's very short and has no race conditions (as you'd have if you used &i). No sense in writing a few dozen lines of extra code just to get a bunch of numbered threads.

Data races

Here is a bad version with a data race:

#include <pthread.h> #include <stdio.h>  #define N 10  void *thread_func(void *arg) {     int *ptr = arg;     // Has *ptr changed by the time we get here?  Maybe!     printf("Arg = %d\n", *ptr);     return NULL; }  int main() {     int i;     pthread_t threads[N];     for (i = 0; i < N; i++) {         // NO NO NO NO this is bad!         pthread_create(&threads[i], NULL, thread_func, &i);     }     for (i = 0; i < N; i++) {         pthread_join(threads[i], NULL);     }     return 0; } 

Now, what happens when I run it with the thread sanitizer?

(Also, check out how it prints "5" twice...)

 ================== WARNING: ThreadSanitizer: data race (pid=20494)   Read of size 4 at 0x7ffc95a834ec by thread T1:     #0 thread_func /home/depp/test.c:9 (a.out+0x000000000a8c)     #1 <null> <null> (libtsan.so.0+0x000000023519)    Previous write of size 4 at 0x7ffc95a834ec by main thread:     #0 main /home/depp/test.c:17 (a.out+0x000000000b3a)    Location is stack of main thread.    Thread T1 (tid=20496, running) created by main thread at:     #0 pthread_create <null> (libtsan.so.0+0x0000000273d4)     #1 main /home/depp/test.c:18 (a.out+0x000000000b1c)  SUMMARY: ThreadSanitizer: data race /home/depp/test.c:9 thread_func ================== Arg = 1 Arg = 2 Arg = 3 Arg = 4 Arg = 5 Arg = 6 Arg = 7 Arg = 8 Arg = 9 Arg = 5 ThreadSanitizer: reported 1 warnings