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TR 24731-1 defines a new type of errno_t which that is 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 which 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 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. Consequently, the meaning of the return value is not as clear as it could be.

enum { NO_FILE_POS_VALUES = 3 };

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

  if (file == NULL) { return -1; }
  if (fgetpos(file, &offset) != 0) { return -1; }
  if (fscanf(file, "%i %i %i", &file_w, &file_h, &file_o)  != NO_FILE_POS_VALUES) { return -1; }
  if (fsetpos(file, &offset) != 0) { return -1; }

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

  return 0; 
}

Compliant Solution

In this compliant solution, the opener() function returns a value of type errno_t, providing a clear indication that this returns a values value that might be found in errno.

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 ((rc = fgetpos(file, &offset)) != 0 ) { return rc; }
  if (fscanf(file, "%i %i %i", &file_w, &file_h, &file_o)  != NO_FILE_POS_VALUES) { return EIO; }
  if ((rc = fsetpos(file, &offset)) != 0 ) { return rc; }
 
  *width = file_w;
  *height = file_h;
  *data_offset = file_o;

  return 0; 
}

NOTE: Neither EINVAL nor and EIO are not defined in C99, but they are defined in most implementations.

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Failing to test for error conditions can lead to vulnerabilities of varying severity. Declaring functions that return an errno with a return type of errno_t will not eliminate this problem , but will help mitigate in its mitigationit.

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