MISRA C:2012 Rule 22.8

The value of errno shall be set to zero prior to a call to an errno-setting-function

Description

Rule Definition

The value of errno shall be set to zero prior to a call to an errno-setting-function.¹

This rule comes from MISRA C™: 2012 Amendment 1.

Rationale

An errno-setting function sets errno to nonzero values to indicate error conditions.

If you do not set errno to zero before calling an errno-setting function,a nonzero value of errno might be left over from a previous call to an errno-setting function. Using errno to check errors can then lead you to falsely conclude that an error occurred from the most recent call.

errno is set to 0 at program startup but is not automatically reset after an error occurs. You must explicitly set errno to 0 when required.

Errno-setting functions include:

• ftell, fgetpos, fgetwc and related functions.

• strtoimax, strtol and related functions.

  The wide-character equivalents such as wcstoimax and wcstol are also covered.

Polyspace Implementation

Polyspace^® reports a violation of this rule if your code calls an errno-setting function without taking either of these steps first:

• Set errno to zero.

• Compare errno to zero in a conditional statement and call the function in the branch where errno is zero.

Polyspace reports violations of this rule only for the functions that the C standard specifies as errno-setting function.

Troubleshooting

If you expect a rule violation but do not see it, refer to Diagnose Why Coding Standard Violations Do Not Appear as Expected.

Examples

errno Not Reset Before Use

#include <stdlib.h>
#include <errno.h>

double val = 0.0;

void f ( void )
{
    val = strtod("1.0",NULL); /* Non-compliant*/ 
    if ( 0 == errno ) 
    {
        val = strtod("1.0",NULL); /* Compliant - case 1*/
        if ( 0 == errno ) /* Check errno for nonzero values */
        {
        }
    }
    else
    {
        errno = 0;
        val = strtod("1.0",NULL); /* Compliant - case 2*/
        if ( 0 == errno ) /* Check errno for nonzero values */
        {
        }
    }
}

In this example, the rule is violated when strtod is called but errno is not reset prior to the call.

The rule is not violated in the following cases:

• Case 1: errno is compared against zero and then strtod is called in the if( 0 == errno ) branch.

• Case 2: errno is explicitly set to zero and then strtod is called.

Check Information

Group: Resources

Category: Required

AGC Category: Required

Version History

Introduced in R2017a

R2024a: Rule violation displayed differently and more functions covered

Starting R2024a, the rule violation appears on the call to an errno-setting function if you do not set errno to zero before the call but test errno for error conditions after the call. Previously, instead of the function call, the rule violation appeared on the subsequent test of errno.

The checker also covers a wider range of errno-setting functions.