Page History

...

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 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 failureTake care when reading errno.

Compliant Solution (POSIX)

...

#include <errno.h>
#include <stdio.h>

#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 (IEEE Std 1003.1, 2013 Edition) but not by the C Standard.

...

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.

Automated Detection

Related Vulnerabilities

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

Related Guidelines

Bibliography

...

Bibliography

...

Image Modified Image Modified Image Modified