Question

I am working through the excellent The C Programming Language at the moment, and have got stuck while trying to open and read a file. The program compiles, but seg faults on execution:

$ ./a.out 
Segmentation fault

Here is the code:

#include <stdio.h>

main()
{
  FILE *fp;
  fp=fopen("/home/c-sandbox/index.html", "r");
  fprintf(fp, "Testing...\n");
  fclose(fp);
}

Note that the path points to a real file containing the string “hello, world”.

Any ideas on where I am going wrong?

Answer 1

You opened the file for reading only, and are attempting to write to it.

Use "a" if you want to append to the end of the existing file.

Edit: As others have noted, you’re also not checking to see if the file was opened. fopen will return NULL if it fails and set the global variable errno to a value that indicates why it failed. You can get a human-readable explanation using strerror(errno)

if( fp == NULL ) {
    printf( "Error opening file: %s\n", strerror( errno ) );
}