Using fwrite() deletes data

Question

I wrote a very simple file corruptor for fun, but to my surprise, the "corrupted" file ends up being smaller in size than the original.

Here is the corruption function that is supposed to replace bytes but not to delete them:

void
corruptor(char *inputname, int percent)
{
  FILE *input;
  FILE *output;
  int filesize;

  char *outputname = append_name(inputname);

  // Duplicate file
  cp(outputname, inputname);

  // Open input and output
  input = fopen(inputname, "r");
  if (input == NULL)
    {
      printf("Can't open input, errno = %d\n", errno);
      exit(0);
    }
  output = fopen(outputname, "w+");
  if (output == NULL)
    {
      printf("Can't open output, errno = %d\n", errno);
      exit(0);
    }

  // Get the input file size
  fseek(input, 0, SEEK_END);
  filesize = ftell(input);

  // Percentage
  int percentage = (filesize * percent) / 100;

  srand(time(NULL));

  for (int i = 0; i < percentage; ++i)
    {
      unsigned int r = rand() % filesize;

      fseek(output, r, SEEK_SET);
      unsigned char corrbyte = rand() % 255;
      fwrite(&corrbyte, 1, sizeof(char), output);

      printf("Corrupted byte %d\n", r);
    }

  fclose(input);
  fclose(output);
}

Answer 1

output = fopen(outputname, "w+");

This deletes the contents of the file, To open the file for reading and writing without deleting the contents, use mode "r+".