When developing new code, declare functions that return errno with a return type of errno_t.  Many existing functions that return errno are declared as returning a value of type int. It is semantically unclear by inspecting the function declaration or prototype if these functions return an error status or a value or, worse, some combination of the two. (See ERR02-C. Avoid in-band error indicators.)

C11 Annex K  introduced the new type errno_t that is defined to be type int in errno.h and elsewhere. Many of the functions defined in C11 Annex K return values of this type [ISO/IEC 9899:2011]. The errno_t type should be used as the type of an object that may contain only 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.

This recommendation depends on C11 Annex K being implemented. The following code can be added to remove this dependency:

#ifndef __STDC_LIB_EXT1__
  typedef int errno_t;
#endif

This noncompliant code example shows a function called opener() that returns errno error codes. However, the function is declared as returning an int. Consequently, the meaning of the return value is not readily apparent.

#include <errno.h>
#include <stdio.h>
 
enum { NO_FILE_POS_VALUES = 3 };

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

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

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

  if (width != NULL) { *width = file_w; }
  if (height != NULL) { *height = file_h; }
  if (data_offset != NULL) { *data_offset = file_o; }

  return 0;
}

This noncompliant code example nevertheless complies with ERR30-C. Set errno to zero before calling a library function known to set errno, and check errno only after the function returns a value indicating failure.

Compliant Solution (POSIX)

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

#define __STDC_WANT_LIB_EXT1__ 1
 
#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
    
enum { NO_FILE_POS_VALUES = 3 };

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

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

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

  if (width != NULL) { *width = file_w; }
  if (height != NULL) { *height = file_h; }
  if (data_offset != NULL) { *data_offset = file_o; }

  return 0;
}

This compliant solution is categorized as a POSIX solution because it returns EINVAL and EIO , which are defined by POSIX but not by the C Standard.

Risk Assessment

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 may reduce errors caused by programmers' misunderstanding the purpose of a return value.

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

Related Guidelines

Bibliography