Question

I am trying to spawn several threads and for each thread I write to a different file (thread 1 writes to file 1, etc…). However, after the threads execute ferror() is set, preventing me from doing further file operations in the main process. I tried clearing the error, but it did not solve. This is the code I currently have:

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

void * bla (void *arg) {
    fprintf((FILE *) arg, "Hey, printing to file");
}

int main() {
    FILE *f1 = fopen("out0", "rw");
    FILE *f2 = fopen("out1", "rw");

    pthread_t t[2];
    pthread_create(&t[0], NULL, bla, f1);
    pthread_create(&t[1], NULL, bla, f2);

    pthread_join(t[0], NULL);
    pthread_join(t[1], NULL);
    printf("%d\n", ferror(f2));   // ERROR: ferror() is set to 1 here!

    //fseek(f1, 0, SEEK_END);
    fseek(f2, 0, SEEK_END);
    long pos = ftell(f2);         // This still works
    printf("%ld\n", pos);
    clearerr(f2);                 // Trying to clear the error, flag clears, but further operations fail
    char *bytes = malloc(pos);
    int err = fread(bytes, 1, 4, f2);  // fread returns 0
    printf("%d\n", ferror(f2));
    printf("%d\n", err);
    bytes[pos-1] = '\0';
    printf("%s", bytes);
    free (bytes);

    fclose(f1);
    fclose(f2);

    return 0;

Note that the file opened by the threads should not exist, and if it exists it should be cleared.
Any help would be greatly appreciated. Thanks!

Answer 1

mode argument for fopen should be "r+" (if the files should exist) or "w+" (or maybe even "a+") instead of "rw". The string "rw", which is not a valid mode, is probably interpreted as "r" mode, and you can’t fprintf to such a FILE*.