Question

Can anyone tell me what’s wrong with this code? I’m getting a seg fault. I’m trying to read just the first line of a file into a newly-created file.

char *buffer;

int main(int argc, char *argv[])
{
  FILE *source = fopen(argv[0], "r");
  FILE *destination = fopen("destination", "w");

  fgets(buffer, 500, source);
  fwrite(buffer, 1, sizeof(buffer), destination);

}

Answer 1

You haven’t allocated anything for buffer.

Change:

char *buffer;

to

char buffer[500];

As your code is right now, buffer is just an uninitialized pointer. Attempting to dereference it will cause undefined behavior. (and seg-fault in your case)

---

Alternatively, you can dynamically allocate memory for buffer:

buffer = (char*)malloc(500 * sizeof(char));

but you should remember to free the memory later on:

free(buffer);

If you go with this latter method, the code will look like this:

char *buffer;

int main(int argc, char *argv[])
{
  FILE *source = fopen(argv[0], "r");
  FILE *destination = fopen("destination", "w");

  //  Allocate
  buffer = (char*)malloc(500 * sizeof(char));

  fgets(buffer, 500, source);
  fwrite(buffer, 1, 500 * sizeof(char), destination); //  Fixed here

  //  Free memory
  free(buffer);

  //  Don't forget return value
  return 0;
}