CERT C: Rec. DCL09-C

Declare functions that return errno with a return type of errno_t

Since R2026a

Description

Declare functions that return errno with a return type of errno_t¹

Polyspace Implementation

The rule checker checks for the issue errno returning function not declared with errno_t return type.

Examples

errno returning function not declared with errno_t return type

Issue

This issue occurs when a function returns errno in at least one execution path but the function is not declared with the return type errno_t.

Risk

Consider a function that returns errno in at least one execution path. If the return type of such a function is not errno_t, then it is unclear from the function prototype if the function returns a error status in any of its execution paths. The meaning and nature of the returned value is not clear.

Fix

If a function checks for errors and returns errno, declare the function with the return type errno_t. The type errno_t is introduced in C11 Annex K. For older version of the C language, use a typedef to create this type:

#ifndef __STDC_LIB_EXT1__
  typedef int errno_t;
#endif

Example

In this example, the function func() checks for an error and returns errno. Because the return type of this function is int, Polyspace^® reports a violation.

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

int func() {    //Noncompliant
	FILE *file;
	const char *filename = "filename.txt";

	// Attempt to open a file
	file = fopen(filename, "r");

	// Check if the file pointer is NULL
	if(file == NULL) {
		fprintf(stderr, "Error opening file '%s': %s\n", filename, strerror(errno));
		// Return errno to indicate the specific error
		return errno;
	}

	// If the file was opened successfully, close it
	fclose(file);

	return 0;
}

Correction

To fix this violation, use errno_t as the return type of func(). If necessary, use a typedef to define the type errno_t.

#include <stdio.h>
#include <errno.h>
#include <string.h>
typedef int errno_t;  //Define errno_t

errno_t func() {    //Compliant
	FILE *file;
	const char *filename = "filename.txt";

	// Attempt to open a file
	file = fopen(filename, "r");

	// Check if the file pointer is NULL
	if(file == NULL) {
		fprintf(stderr, "Error opening file '%s': %s\n", filename, strerror(errno));
		// Return errno to indicate the specific error
		return errno;
	}

	// If the file was opened successfully, close it
	fclose(file);

	return 0;
}

Check Information

Group: Rec. 02. Declarations and Initialization (DCL)

PQL Name: std.cert.DCL09_C

Version History

Introduced in R2026a