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="7ac471f4-87f1-4390-93a0-bf92ec650285"><ac:parameter ac:name="">1</ac:parameter></ac:structured-macro>  Note that the POSIX Â® standard extends the set of identifiers reserved by C99 to include an open-ended set of its own. See section 2.2 Compilation Environment in [\[IEEE Std 1003.1-2008\]].
Noncompliant Code Example (Header Guard)
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 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.
Compliant Solution (Header Guard)
This compliant solution avoids using leading or trailing underscores in the name of the header guard.
Noncompliant Code Example (File Scope Objects)
In this noncompliant code example, the names of the file scope objects
_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, since 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 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.
Compliant Solution (File Scope Objects)
In this compliant solution, names of no file scope objects begin with an underscore and, hence, do not encroach on the reserved name space.
Noncompliant Code Example (Reserved Macros)
In the noncompliant code example below, since 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, since it begins with the
INT prefix and ends with the
_MAX suffix it encroaches on the reserved name space (see section 8.26.8 of C99). A typical manifestation of such a clash is a compilation error.
Compliant Solution (Reserved Macros)
The compliant solution below avoids redefining reserved names or using reserved prefixes and suffixes.
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, among many others,
va_end(), regardless of whether any of them is masked by a macro of the same name or not.
The noncompliant example below provides definitions for the C standard library functions
free(). While this practice is permitted by many traditional implementations of UNIX (see, for example, the Dmalloc library), doing so is disallowed by the C99 standard as it need 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
realloc() is likely to cause problems as well.
Compliant Solution (Identifiers With External Linkage)
The compliant, portable solution avoids redefining any C standard library identifiers with external linkage. In addition, it provides definitions for all memory allocation functions.
Using reserved identifiers can lead to incorrect program operation.
A module written in Compass/ROSE can detect violations of this rule.
[[ISO/IEC 9899:1999]] Section 7.1.3, "Reserved Identifiers"
[\[IEEE Std 1003.1-2008\]] Section 2.2 "The Compilation Environment"