how to create a thread calling an external function

[jfvg71]

Hello,

I have a following problem: i want to create 2 threads using pthreads but the function i must to call in pthread_create() accept two parameters and is implemented in a shared library.
The question is how i can call this function in the moment of thread creation.

Example:

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

extern double foo(int, char**);

int main(int argc, char** argv){

 pthread_t thread1, thread2;
 int  iret1, iret2;

 iret1 = pthread_create( &thread1, NULL, ?, ?); <- How to complete the arguments list for foo function
 iret2 = pthread_create( &thread2, NULL,?, ?); <- How to complete the arguments list  foo function

     pthread_join( thread1, NULL);
     pthread_join( thread2, NULL); 

     printf("Thread 1 returns: %d\n",iret1);
     printf("Thread 2 returns: %d\n",iret2);
     exit(0);

}

Thanks in advance,
Jose

[gerard4143]

About passing multi parameters, pthread_create takes

int pthread_create(pthread_t *thread, const pthread_attr_t *attr,void *(*start_routine) (void *), void *arg);

where

void *arg

allows you can pass a pointer to anything.

About the shared library...that shouldn't be a problem, just make sure to load the shared library before you call the functions.

[Cabhan]

The short answer is that you can't do it directly. pthread_create requires a pointer to a function that accepts only one argument.

Therefore, you have two choices:
- If you have written the function foo(), and it is only used for this purpose, you can rewrite it to accept only one argument. Because this argument is a void *, you can have the argument be a pointer to a struct that contains as much data as you want.

- Otherwise, you can have pthread_create begin by calling a function of your own that calls the foo function. The parameters can be hardcoded into this function, or may be passed in as described above.

[Rubberman]

As was pointed out, the start function can only take one argument, the one that is passed to pthread_create(). However, what is usually done is that arg is a pointer to a structure that has the actual arguments you will use, and the start function is simple a pointer to a wrapper function that takes the components contained in the structure arg points to and then calls your real start function with the arguments in place. This way, you can pass any number of arguments you want and pthread_create() is happily oblivious to that.

[Rubberman]

Here is an example based on your code:

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

extern double foo(int, char**);

struct foo_args {
    int arg1;
    char** arg2;
};

int start_foo( void* arg )
{
    struct foo_args* fargs = (struct foo_args*)arg;
    double foo_ret_val = foo( fargs->arg1, fargs->arg2);

    /* Do something with foo_ret_val here */
    return (int)foo_ret_val;
}

int main(int argc, char** argv)
{
    pthread_t thread1, thread2;
    int  iret1, iret2;
    struct foo_args fargs1 = {1, argv};
    struct foo_args fargs2 = {2, argv};
    iret1 = pthread_create( &thread1, NULL, start_foo, (void*)&fargs1);
    iret2 = pthread_create( &thread2, NULL, start_foo, (void*)&fargs2);

    pthread_join( thread1, NULL);
    pthread_join( thread2, NULL); 

    printf("Thread 1 returns: %d\n",iret1);
    printf("Thread 2 returns: %d\n",iret2);
    exit(0);
}