Thread: Pthreads help

  1. #1
    Registered User
    Join Date
    Jan 2011

    Pthreads help

    Hi I have built a simple web server in C, and I now have to make it multi threaded. I also have to produce a multi threaded command line client to test the multi-threaded capability of the server. I have finished the client and the server however I get read errors when making more than 1 request to the server. I believe it is because I need to look some resources with mutexes but I'm not quite sure how and where to do it.

    while (1) {
        //Accept new connection and fork
        remote_len = sizeof(remote_addr);
        sd = accept(lsd, (struct sockaddr *) &remote_addr, &remote_len);
        printf("Connection Received!\n");
        //Store remote IP in "inetadddr" for error logging
        inet_ntop(AF_INET, &(remote_addr.sin_addr), inetaddr, INET_ADDRSTRLEN);
        if (sd < 0) {
          logerrors("Socket Error: Cannot accept connection from",inetaddr);
        params->sd = sd;
        params->inetaddr = inetaddr;
        threadr = pthread_create(&thread[i++], NULL, web, (void*) params);
    That is the accept code in main and the new thread starts with web function:

    void *web (void *arg) 
      int n, sd;
      char buffer[BUFFERSIZE];
      char *inetaddr;
      struct funcparams *params = (void *) arg;
      sd = params->sd;
      inetaddr = params->inetaddr;
      bzero(buffer, BUFFERSIZE);
      //Read browser request
      n = read(sd,buffer,BUFFERSIZE);
    So this read request fails, well it works for the first connection then fails on the rest. Like I said i'm sure its because I need to lock the resources sd and inetaddr but i'm not sure where to lock and unlock. Any help would be greatly appreciated

  2. #2
    and the hat of int overfl Salem's Avatar
    Join Date
    Aug 2001
    The edge of the known universe
    You really need

    params = malloc( sizeof *params );

    If your params is anything else (pointer to a local, an array of some sort), then MULTIPLE threads are going to end up referencing the same (or possibly non-existent) block of memory.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.

  3. #3
    Registered User
    Join Date
    Jan 2011
    The malloc is before the accept loop, it is there:

    params = (struct funcparams *) malloc(sizeof(struct funcparams));
    The struct is:

    struct funcparams {
    	int sd;
    	char *inetaddr;

  4. #4
    Registered User Codeplug's Avatar
    Join Date
    Mar 2003
    >> The malloc is before the accept loop
    It needs to be in the loop - each thread gets its own parameters. You're passing a shallow copy of 'inetaddr' too. That needs to be a unique pointer per-thread as well.


  5. #5
    Registered User
    Join Date
    Jan 2011
    I don't understand what you mean, can you elaborate please?

  6. #6
    Registered User
    Join Date
    Jan 2011
    OK I got it working now, thanks

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