Passing arguments to thread in C

[Business_Woman]

Hi,

Im trying to figure out how to successfully pass a struct as an argument to a thread.
But i can't seem to get it right. Ideas?

#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
#include <string.h>
    
/* Prototypes */
void *run(void *thread_arg);



struct thread_args {
       
      int arraylength;
      int *ptr;
      char *name;
};
        
int main() {
        
  pthread_t thread_id;
  struct thread_args  pthread_data;
  struct thread_args *thread_data = &pthread_data;
   
    
  thread_data->arraylength = 10;
 
  strcpy(thread_data->name,"David");
  
  pthread_create(&thread_id,NULL,run,(void *)&thread_data);
  pthread_join(thread_id,NULL);
  pthread_exit(NULL);
  return 0;
  }    
       

       
 void *run(void *thread_arg) {
        
        struct thread_args *my_data;
        my_data = (struct thread_args *)thread_arg;
        printf("%s",my_data->name);   //Supposed to print "David"
         
}

 

[fonz]

Hi,

Im trying to figure out how to successfully pass a struct as an argument to a thread.
But i can't seem to get it right.

Actually, you do - more or less. The error is somewhere else. Read on.

struct thread_args  pthread_data;
struct thread_args *thread_data = &pthread_data;

Although not incorrect, I find this ugly. Why declare a seperate pointer thread_data when you can simply use &pthread_data? See the next point...

pthread_create(&thread_id,NULL,run,(void *)[red]&[/red]thread_data);

Ditch the ampersand (or just write &pthread_data in the first place), thread_data is already a pointer and you're passing a pointer to that pointer but in run() you cast it back to the pointer itself.

strcpy(thread_data->name,"David");

But this is your actual mistake. Can you tell why? (hint: think memory allocation)

Hope this helps,

Fonz

 

[Business_Woman]

Actually, you do - more or less. The error is somewhere else. Read on.

strcpy(thread_data->name,"David");

But this is your actual mistake. Can you tell why? (hint: think memory allocation)

Hope this helps,

Fonz

Hm, you have to malloc some memory for the pointer to point to?

 

[fonz]

Hm, you have to malloc some memory for the pointer to point to?

Yep. Either explicitly allocate memory for pthread_data.name (the destination string) by using malloc(3) or calloc(3) manually, or do it implicitly by instead of strcpy(3) using another function that does the allocation for you, such as strdup(3) or asprintf(3).

Hope this helps,

Fonz

 

[expl]

Simply changing to:

thread_data->name = "David";

Will do the trick. (In current context)


Edit:

By the way, you should call pthread_exit() in your children threads and not in main().

 

[fonz]

Plus the change in the call to pthread_create(3), that is. Use either thread_data or &pthread_data, not &thread_data since that will still segfault as explained above.

Fonz

 

[Business_Woman]

Thank you Fonzie and expl!

It seems to be working now

 

[fonz]

Thank you Fonzie and expl!

Eeey, you're welcomundo

Fonz