Passing arguments to thread in C

  1. Hi,

    I am trying to figure out how to pass arguments to a thread via a struct.
    It's not really working out for me.

    What am i doing wrong here?
    Code (Text):
    #include <stdio.h>
    #include <stdlib.h>
    #include <pthread.h>
    #include <string.h>


    void *run(void *thread_arg);

    struct thread_args {
     
          int arraylength;
          int *ptr;
          char *name;
    };
           
    int main() {
           
      pthread_t thread_id;
      struct thread_args *thread_data;
     

      int array[10] = { 1,2,3,4,5,6,7,8,9,10 };
     
     
      thread_data->arraylength = 10;
      thread_data->ptr = &array[0];
     
      strcpy(thread_data->name,"David"); /* Is this correct ? */
      printf("%s",thread_data->name); //This prints nothing

     
      pthread_create(&thread_id,NULL,run,(void *)&thread_data);
      pthread_join(thread_id,NULL);
      return 0;
      }    
           

     void *run(void *thread_arg) {
            printf("hi");
            struct thread_args *my_data;
            my_data = (struct thread_args *)thread_arg;
            printf("%s",my_data->name);
             
             
      }
     
     
  2. jcsd
  3. rcgldr

    rcgldr 7,611
    Homework Helper

    You declared an instance of a pointer to struct thread_data, but you never declared or allocated an instance of struct thread_data. Looking at your code I think you just need to remove the pointer reference:

    Code (Text):

        struct thread_args thread_data;
        ...  
        thread_data.arraylength = 10;
        thread_data.ptr = &array[0];
     
    or if you want to use a pointer

    Code (Text):

        struct thread_args thread_data;
        struct thread_args *pthread_data = &thread_data;
       
        ...  
        pthread_data->arraylength = 10;
        pthread_data->ptr = &array[0];
        ...
        pthread_create(&thread_id,NULL,run,(void *)pthread_data);

     
     
    Last edited: Jul 28, 2011
  4. Oops, stupid mistake :redface:

    But why is there no output from the thread routine?

    It is supposed to print the name "David"
     
    Last edited: Jul 28, 2011
  5. I'm not familiar with the syntax of p_threads, but if you passed a bad pointer as a result of your (thread_data) variable, then the thread has most likely not been created at the very least due to a pointer problem and the thread won't have been allocated, let alone run.

    Assuming there are no problems in this regard (and the program didn't crash), then the only thing I could think of is that you didn't make the right call or pass the right argument for the thread to automatically start.

    To be honest though, I'm surprised you didn't get any crashes due to your bad code that doesn't even instantiate or allocate the data for the struct you are using.
     
  6. I like Serena

    I like Serena 6,194
    Homework Helper

    This looks like homework.
    Is it?
     
  7. rcgldr

    rcgldr 7,611
    Homework Helper

    Because you added yet another level of indirection in the function call. Thread_data pointer was allocated on the stack, but probably not initialized or initialized to zero, so you just stored the strings into some random location in memory or location zero. Then you called the function with &thread_data, which is the address within the stack for the thread_data pointer.
     
  8. No, it's not. It's home work, play for fun :)
     
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