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According to the C Standard, 7.1.3 [ISO/IEC 9899:2011],

All identifiers that begin with an underscore and either an uppercase letter or another underscore are always reserved for any use.

All identifiers that begin with an underscore are always reserved for use as identifiers with file scope in both the ordinary and tag name spaces.

Each macro name in any of the following subclauses (including the future library directions) is reserved for use as specified if any of its associated headers is included, unless explicitly stated otherwise.

All identifiers with external linkage (including future library directions) and errno are always reserved for use as identifiers with external linkage.

Each identifier with file scope listed in any of the following subclauses (including the future library directions) is reserved for use as a macro name and as an identifier with file scope in the same name space if any of its associated headers is included.

Additionally, subclause 7.31 defines many other reserved identifiers for future library directions.

Noncompliant Code Example (

...

Include Guard)

A common, but noncompliant, practice is to choose a reserved name for a macro used in a preprocessor conditional guarding against multiple inclusions of a header file. (see See also PRE06-C. Enclose header files in an inclusion include guard.) . The name may clash with reserved names defined by the implementation of the C standard library in its headers or with reserved names implicitly predefined by the compiler even when no C standard library header is included.

Code Block
bgColor#FFCCCC
langc
#ifndef _MY_HEADER_H_
#define _MY_HEADER_H_

/* Contents of <my_header.h> */

#endif /* _MY_HEADER_H_ */

Compliant Solution (

...

Include Guard)

This compliant solution avoids using leading underscores in the macro name of the header include guard:

Code Block
bgColor#ccccff
langc
#ifndef MY_HEADER_H
#define MY_HEADER_H

/* Contents of <my_header.h> */

#endif /* MY_HEADER_H */

...

In this noncompliant code example, the names of the file scope objects _max_limit and _limit both begin with an underscore. Because _max_limit is static, this declaration might seem to be impervious to clashes with names defined by the implementation. However, because the header <stddef.h> is included to define size_t, a potential for a name clash exists. (Note, however, that a conforming compiler may implicitly declare reserved names regardless of whether any C standard library header is explicitly included.)

In addition, because _limit has external linkage, it may clash with a symbol with of the same name defined in the language runtime library even if such a symbol is not declared in any header. Consequently, it is not safe to start the name of any file scope identifier with an underscore even if its linkage limits its visibility to a single translation unit.

...

In this noncompliant code example, because the C standard library header <inttypes.h> is specified to include <stdint.h>, the name SIZE_MAX conflicts with a standard macro of the same name, used which is used to denote the upper limit of size_t. In addition, although the name INTFAST16_LIMIT_MAX is not defined by the C standard library, it is a reserved identifier because it begins with the INT prefix and ends with the _MAX suffix. (see See the C Standard, 7.31.10.).

Code Block
bgColor#ffcccc
langc
#include <inttypes.h>
#include <stdio.h>

static const int_fast16_t INTFAST16_LIMIT_MAX = 12000;

void print_fast16(int_fast16_t val) {
  enum { SIZE_MAX = 80 };
  char buf[SIZE_MAX];
  if (INTFAST16_LIMIT_MAX < val) {
    sprintf(buf, "The value is too large");
  } else {
    snprintf(buf, SIZE_MAX, "The value is %" PRIdFAST16, val);
  }
}

...

Noncompliant Code Example (Identifiers with External Linkage)

In addition to symbols defined as functions in each C standard library header, identifiers with external linkage include errno and math_errhandling, among others, regardless of whether any of them are masked by a macro of the same name.

This noncompliant example provides definitions for the C standard library functions malloc() and free(). Although this practice is permitted by many traditional implementations of UNIX (for example, the Dmalloc library), it is undefined behavior according to the C Standard. Even on systems that allow replacing malloc(), doing so without also replacing aligned_alloc(), calloc(), and realloc() is likely to cause problems.

...

Noncompliant Code Example (errno)

In addition to symbols defined as functions in each C standard library header, identifiers with external linkage include errno and math_errhandlingAccording to the C Standard, 7.5, paragraph 2 [ISO/IEC 9899:2011], the behavior of a program is undefined when

a macro A macro definition of errno is suppressed in order to access an actual object, or the program defines an identifier with the name errno

...

DCL37-C-EX2: For compatibility with other compiler vendors or language standard modes, it is acceptable to create a macro identifier the that is the same as a reserved identifier so long as the behavior is idempotent, as in this example:

...

DCL37-C-EX3: As a compiler vendor or standard library developer, it is acceptable to use identifiers reserved for your implementation. Reserved identifiers may be defined by the compiler, in standard library headers , or headers included by a standard library header, as in this example declaration from the glibc standard C library implementation:

...

Using reserved identifiers can lead to incorrect program operation.

Rule

Severity

Likelihood

Remediation Cost

Priority

Level

DCL37-C

Low

Unlikely

Low

P3

L3

Automated Detection

Tool

Version

Checker

Description

Astrée
Include Page
Astrée_V
Astrée_V

future-library-use

language-override

language-override-c99

reserved-declaration

reserved-declaration-c99

reserved-identifier

Partially checked
Axivion Bauhaus Suite

Include Page
Axivion Bauhaus Suite_V
Axivion Bauhaus Suite_V

CertC-DCL37Fully implemented. Reserved identifiers, as in DCL37-C-EX3, are configurable.
CodeSonar
Include Page
CodeSonar_V
CodeSonar_V

LANG.STRUCT.DECL.RESERVED

Declaration of reserved name
Compass/ROSE
 




Coverity
 
Include Page

 

Fully implemented
Coverity_V
Coverity_V

MISRA C 2004 Rule 20.1

MISRA C 2004 Rule 20.2

MISRA C 2012 Rule 21.1

MISRA C 2012 Rule 21.2

Implemented
ECLAIR
Include Page
ECLAIR_V
ECLAIR_V
CC2.DCL37Fully implemented
Klocwork
Include Page
Klocwork_V
Klocwork_V
MISRA.DEFINE.WRONGNAME.UNDERSCORE
MISRA.STDLIB.WRONGNAME.UNDERSCORE
MISRA.STDLIB.WRONGNAME

LDRA tool suite
Include Page
LDRA_V
LDRA_V

86 S, 218 S, 219 S, 580 S, 626 S

Fully Implemented

Parasoft C/C++test
9.5MISRA2004-20_1_a
Include Page
Parasoft_V
Parasoft_V

CERT_C-DCL37-a

Do not #define or #undef identifiers with names which start with underscore
Polyspace Bug Finder

Include Page
Polyspace Bug Finder_V
Polyspace Bug Finder_V

CERT C: Rule DCL37-C


Checks for:

  • Defining and undefining reserved identifiers or macros
  • Declaring a reserved identifier or macro name

Rule partially covered

PRQA QA-C
Include Page
PRQA QA-C_v
PRQA QA-C_v

0602, 0603, 4600, 4601, 4602,

4603, 4604, 4605, 4606,


4607,

4608

 

Related Guidelines

, 4620, 4621, 4622, 4623,

4624, 4640, 4641, 4642, 4643,

4644, 4645


SonarQube C/C++ Plugin
Include Page
SonarQube C/C++ Plugin_V
SonarQube C/C++ Plugin_V
S978
RuleChecker
Include Page
RuleChecker_V
RuleChecker_V

future-library-use

language-override

language-override-c99

reserved-declaration

reserved-declaration-c99

reserved-identifier

Partially checked

Related Guidelines

Key here (explains table format and definitions)

Taxonomy

Taxonomy item

Relationship

CERT C Secure Coding StandardPRE00-C. Prefer inline or static functions to function-like macrosPrior to 2018-01-12: CERT: Unspecified Relationship
CERT C Secure Coding StandardPRE06-C. Enclose header files in an
inclusion
include guardPrior to 2018-01-12: CERT: Unspecified Relationship
CERT C Secure Coding StandardPRE31-C. Avoid side effects in arguments to unsafe macros
SEI CERT C++ Coding Standard
Prior to 2018-01-12: CERT: Unspecified Relationship
CERT CDCL51-CPP. Do not declare or define a reserved identifierPrior to 2018-01-12: CERT: Unspecified Relationship
ISO/IEC TS 17961Using identifiers that are reserved for the implementation [resident]Prior to 2018-01-12: CERT: Unspecified Relationship
MISRA C:2012Rule 21.1 (required)
,
Prior to 2018-01-12: CERT: Unspecified Relationship
MISRA C:2012Rule 21.2 (required)Prior to 2018-01-12: CERT: Unspecified Relationship

Bibliography

[IEEE Std 1003.1-2013]Section 2.2, "The Compilation Environment"
[ISO/IEC 9899:2011]7.1.3, "Reserved Identifiers"
7.31.10, "Integer Types <stdint.h>"

 


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