FIO44-C. Only use values for fsetpos() that are returned from fgetpos()

Calling fsetpos() sets the file position indicator of a file stream. By C99 definition, the only values that are correct for the position parameter for fsetpos() are values returned from the fgetpos(). Using any other values will result in undefined behavior and should be avoided.

Non-Compliant Code Example

The following non-compliant code attempts to read two parameters from a file, then set the cursor position back to the beginning of the file and return to the caller.

int opener(FILE* file, int *width, int *height, int *data_offset) {
  int w;
  int h;
  int o;
  int offset = 0;

  if(file == NULL) { return -1; }

  if (fscanf(file, "%i %i %i", &w, &h, &o)  != 3) { return -1; }

  if (fsetpos(file, &offset) != 0) { return -1; }

  *width = w;
  *height = h;
  *data_offset = o;

  return 0; 
}

int main(int argc, char *argv[]) {
  int width;
  int height;
  int data_offset;
  FILE *file;
  ...

  file = fopen("myfile", "rb");
  if(opener(file, &width, &height, &data_offset) != 0 ) { return 0; }

  ...
}

However, since only the return value of a getpos() call is valid to be used with setpos(), passing a specified int in instead may not work. It is possible that the position will be set to an arbitrary location in the file.

Compliant Solution

In this compliant solution, the initial file position indicator is stored by first calling fgetpos(), which is used to restore the state back to the beginning of the file in the later call to fsetpos().

int opener(FILE* file, int *width, int *height, int *data_offset) {
  int w;
  int h;
  int o;
  fpos_t offset;

  if(file == NULL) { return -1; }

  if(fgetpos(file, &offset) != 0) { return -1; }

  if (fscanf(file, "%i %i %i", &w, &h, &o)  != 3) { return -1; }

  if (fsetpos(file, &offset) != 0) { return -1; }

  *width = w;
  *height = h;
  *data_offset = o;

  return 0; 
}

int main(int argc, char *argv[]) {
  int width;
  int height;
  int data_offset;
  FILE *file;
  ...

  file = fopen("myfile", "rb");
  if(opener(file, &width, &height, &data_offset) != 0 ) { return 0; }

  ...
}

Risk Assessment

The misuse of fsetpos() could move a file stream read to a undesired location in the file. If this location held input from user the user would then gain control of the variables being read from the file.

References

[[ISO/IEC 9899-1999:TC2]] Section 7.19.9.3, "The fsetpos function"