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Many existing functions that return an errno error code are declared as returning a value of type int. It is semantically unclear by looking at the function declaration or prototype if these functions return an error status or a value (or worse, some combination of the two).

TR 24731-1 defines a introduces the new type of errno_t that is defined to be type int in <errno.h> and elsewhere. Many of the functions defined in TR 24731-1 return values of this type. As a matter of programming style, errno_t should be used as the type of something that deals only with the values that might be found in errno. For example, a function that returns the value of errno should be declared as having the return type errno_t.

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This non-compliant code example illustrates a function called opener() that returns errno error codes.  However, the function is declared as returning a value of type int. The function, however, uses this return value to indicate the return status of the function by returning values of errno. an int.  Consequently, the meaning of the return value is not as clear as it could be.

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In this compliant solution, the opener() function returns a value of type errno_t, providing a clear indication that this function returns an errnoerror code.

#include <errno.h>

enum { NO_FILE_POS_VALUES = 3 };

errno_t opener(FILE* file, int *width, int *height, int *data_offset) {
  int file_w;
  int file_h;
  int file_o;
  int rc;
  fpos_t offset;

  if (file == NULL) { return EINVAL; }
  if (fgetpos(file, &offset) != 0 ) { return errno; }
  if (fscanf(file, "%i %i %i", &file_w, &file_h, &file_o)  != NO_FILE_POS_VALUES) { return EIO; }
  if (fsetpos(file, &offset) != 0 ) { return errno; }

  *width = file_w;
  *height = file_h;
  *data_offset = file_o;

  return 0;
}

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