Pass variable into pthread_create function

Question

I'm building a multi-threaded network chat program, but I can't figure out how to pass variables into pthread_create.

I have the following line of code that creates a thread:

pthread_create(&thread, NULL, receive, (void *) socket_fd);

My receive function looks like this:

void * receive(void * socket) {
    int socket_fd, response;
    char message[MESSAGE_BUFFER];
    socket_fd = (int) socket;

    while(true) {
        response = recvfrom(socket_fd, message, MESSAGE_BUFFER, 0, NULL, NULL);
        if (response) {
            printf("\nServer> %s", message);
            printf("%s", prompt);
        }
    }
}

How can I pass a prompt variable into this receive function, when calling receive in pthread_create?

Do you just want to pass a pointer to a struct? Just see the example here. — chrisaycock
– chrisaycock, Commented Feb 8, 2017 at 18:20

Daniel · Accepted Answer · 2017-02-08 18:38:55Z

1

You can pack all of the data you want to pass to your thread on creation in a single struct and pass its pointer through the last parameter of pthread_create. in short:

define a struct:

typedef struct{
     char* prompt;
     int socket;
} thread_data;

and then call pthread_create:

thread_data data;
// place code here to fill in the struct members...
pthread_create(&thread, NULL, receive, (void *) &data);

in your receive function:

void * receive(void * threadData) {
    int socket_fd, response;
    char message[MESSAGE_BUFFER];

    thread_data* pData = (thread_data*)threadData;
    socket_fd = pData->socket;
    char* prompt = pData->prompt;

    while(true) {
        response = recvfrom(socket_fd, message, MESSAGE_BUFFER, 0, NULL, NULL);
        if (response) {
            printf("\nServer> %s", message);
            printf("%s", prompt);
        }
    }
}

Hope this helps.

edited Feb 8, 2017 at 18:38

answered Feb 8, 2017 at 18:25

Daniel

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2 Comments

John Bollinger Over a year ago

But one must be careful with this if one passes (a pointer to) the same struct in more than one call to pthread_create(), and any of those threads or the parent thread also modifies the struct. Proper synchronization is required in such a case.

Daniel Over a year ago

You are absolutely correct. This is simply the most basic solution and it does not show any synchronization and locking mechanism that's required in the case of a shared resource.

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