fwrite write an integer

Question

I'm trying to write a word to a file using this function:

extern void write_int(FILE * out, int num) {
   fwrite(&num,sizeof(int),1, out);
   if(ferror(out)){
      perror(__func__);
      exit(EXIT_FAILURE);
   }
}

But I get a segmentation fault whenever it tries to run the fwrite. I looked at the man page for fwrite(3) and I feel like I used it correctly, is there something I'm missing?

Answer 1

What platform/compiler? Issues of alignment might matter.

Can you write anything else to the file successfully? I suspect your out-file may not be opened properly.

Answer 2

Try this instead:

void write_int(FILE * out, int num) {
   if (NULL==out) {
       fprintf(stderr, "I bet you saw THAT coming.\n");
       exit(EXIT_FAILURE);
   }
   fwrite(&num,sizeof(int),1, out);
   if(ferror(out)){
      perror(__func__);
      exit(EXIT_FAILURE);
   }
}

And why was your original function extern?

Answer 3

Is the file handle valid? Did you fopen() with "w"? fwrite() will segfault if it's not.

The function itself really does nothing, so it's obviously the fwrite call that's the problem. Examine the arguments.