Thread: pthreads join problem

  1. #1
    Registered User
    Join Date
    Jun 2013
    Jodhpur, India

    Question pthreads join problem

    I am trying to emulate callback mechanism in C. The code I have is follows:

    #include <stdio.h>
    #include <pthread.h>
    struct fopen_struct {
        char *filename;
        char *mode;
        void *(*callback) (FILE *);
    void *fopen_callback(FILE *);
    pthread_t fopen_t(void *(*callback)(FILE *), const char *, const char *);
    void *__fopen_t__(void *);
    pthread_t fopen_t(void *(*callback)(FILE *), const char *filename, const char *mode) {
        struct fopen_struct args;
        args.filename = filename;
        args.mode = mode;
        args.callback = callback;
        pthread_t thread;
        pthread_create(&thread, NULL, &__fopen_t__, &args);
        //pthread_join ??
        return thread;
    void *__fopen_t__(void *ptr) {
        printf("I am done!\n");
        struct fopen_struct *args = (struct fopen_struct *)ptr;
        FILE *result = fopen(args -> filename, args -> mode);
        args -> callback(result);
    int main() {
        pthread_t thread = fopen_t(&fopen_callback, "test.txt", "r");
        //pthread_join ??
    void *fopen_callback(FILE *stream) {
        if (stream != NULL)
            printf("Opened file successfully\n");
    If I do a pthread_join in fopen_t() it works perfectly fine. But if I do that in main() I get a segmentation fault error. I want to call it in main() because I want it to be non-blocking, is there any solution?

  2. #2
    Hurry Slowly vart's Avatar
    Join Date
    Oct 2006
    Rishon LeZion, Israel
    in line 20 you are passing pointer to local var args to pthread_create

    you fopen_t could exit before __fopen_t__ will read this memory
    as a result __fopen_t__ will access uninitialized memory.

    you need to use malloc to allocate args in fopen_t and free it in the __fopen_t__ after the data was read from the struct
    All problems in computer science can be solved by another level of indirection,
    except for the problem of too many layers of indirection.
    David J. Wheeler

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