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According to ISO/IEC 9899:TC3, Section 7.1.3 on reserved identifiers,

• 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 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) are always reserved for use as identifiers with external linkage
• Each identifier with file scope listed in any of the above 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

No other identifiers are reserved ^#1. The behavior of a program that declares or defines an identifier in a context in which it is reserved, or defines a reserved identifier as a macro name, is undefined . See also undefined behavior 100 of Annex J of C99. Trying to define a reserved identifier can result in its name conflicting with that used in implementation, which may or may not be detected at compile time.

<ac:structured-macro ac:name="anchor" ac:schema-version="1" ac:macro-id="c722f986448c910e-6b5ea2ec-456d4d22-9015a1a1-1d55eb9134c0806e624d6389"><ac:parameter ac:name="">1</ac:parameter></ac:structured-macro> \[1\] Note that the POSIX ^&#xAE;^ standard extends the set of identifiers reserved by C99 to include an open-ended set of its own. See section [2.2 Compilation Environment|http://www.opengroup.org/onlinepubs/9699919799/functions/V2_chap02.html#tag_15_02] in [\[IEEE Std 1003.1-2008\]|AA. References#IEEE Std 1003.1-2008].

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A common but noncompliant practice is to choose a reserved name for a macro used in a preprocessor conditional guarding against multiple inclusion of a header file. See also guideline PRE06-C. Enclose header files in an inclusion 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. A typical manifestation of such a clash is a compilation error.

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In this noncompliant code example, the names of the file scope objects _max_limit and _limit both begin with an underscore. Since it is static, the declaration of _max_limit 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. (noteNote, however, that a conforming compiler may implicitly declare reserved names regardless of whether or not any C standard library header has been explicitly included.) . In addition, because _limit has external linkage, it may clash with a symbol with the same name defined in the language runtime library, even if such a symbol is not declared in any header. Consequently, it is unsafe to start the name of any file scope identifier with an underscore, even if its linkage limits its visibility to a single translation unit. Common effects of such clashes range from compiler errors, to linker errors, to abnormal program behavior at runtime.

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In the noncompliant code example below, because the C standard library header <inttypes.h> is specified to include <stdint.h>, the name MAX_SIZE conflicts with the name of the <stdint.h> header macro used to denote the upper limit of size_t. In addition, while the name INTFAST16_LIMIT_MAX isn't defined by the C standard library, because it begins with the INT prefix and ends with the _MAX suffix, it encroaches on the reserved name space. (see See section 8.26.8 of C99.) . A typical manifestation of such a clash is a compilation error.

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The noncompliant example below provides definitions for the C standard library functions malloc() and free(). While this practice is permitted by many traditional implementations of UNIX (see, for example, e.g., the Dmalloc library), doing so is disallowed by the C99 standard as because it need is not generally portable and may lead to undefined behavior. Common effects range from compiler errors, to linker errors, to abnormal program behavior at runtime. In addition, even on systems where replacing malloc() is allowed, doing so without also replacing calloc() and realloc() is likely to cause problems, as well.

#include <stddef.h>

void* malloc(size_t nbytes) {
  void *ptr;
  /* allocate storage from own pool and set ptr */
  return ptr;
}

void free(void *ptr) {
  /* return storage to own pool */
}

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Using reserved identifiers can lead to incorrect program operation.

Automated Detection

A module written in Compass/ROSE can detect violations of this rule.

Other Languages

Tool	Version	Checker	Description
Section Compass/ROSE

Related Guidelines

This rule appears in the C++ Secure Coding Standard as : DCL32-CPP. Do not use names reserved for the implementation.

Bibliography

\[[ISO/IEC 9899:1999|AA. Bibliography#ISO/IEC 9899-1999]\] Section 7.1.3, "Reserved Identifiers"
[\[IEEE Std 1003.1-2008\]|AA. References#IEEE Std 1003.1-2008] Section 2.2 "The Compilation Environment"

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