According to the C Standard, subclause 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
errnoare 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.
No other identifiers are reserved. (Note that the POSIX standard extends the set of identifiers reserved by the C Standard to include an open-ended set of its own. See section 2.2, "The Compilation Environment," in IEEE Std 1003.1-2008.) The behavior of a program that declares or defines an identifier in a context in which it is reserved, or that defines a reserved identifier as a macro name, is undefined. See also undefined behavior 106 in Annex J of the C Standard.
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.
Compliant Solution (Header Guard)
This compliant solution avoids using leading 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. Because 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 in order 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 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.
Compliant Solution (File Scope Objects)
In this compliant solution, names of file-scope objects do not begin with an underscore:
Noncompliant Code Example (Reserved Macros)
In this noncompliant code example, because the C standard library header
<inttypes.h> is specified to include
<stdint.h>, the name
MAX_SIZE conflicts with a standard macro of the same name, 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 subclause 7.31.10 of the C Standard [ISO/IEC 9899:2011].)
Compliant Solution (Reserved Macros)
This compliant solution 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 others,
math_errhandling, 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
free(). Although this practice is permitted by many traditional implementations of UNIX (e.g., the Dmalloc library), it is disallowed by the C Standard because it is undefined behavior. Even on systems that allow replacing
malloc(), doing so without also replacing
realloc() is likely to cause problems.
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:
Noncompliant Code Example (
The behavior of a program is undefined when
a macro definition of
errnois suppressed in order to access an actual object, or the program defines an identifier with the name
errno. [ISO/IEC 9899:2011]
(See subclause 7.5 paragraph 2, and undefined behavior 114 in Annex J of the C Standard.)
errno identifier expands to a modifiable lvalue that has type
int but is not necessarily the identifier of an object. It might expand to a modifiable lvalue resulting from a function call, such as
*errno(). It is unspecified whether
errno is a macro or an identifier declared with external linkage. If a macro definition is suppressed to access an actual object, or if a program defines an identifier with the name
errno, the behavior is undefined.
Legacy code is apt to include an incorrect declaration, such as the following:
Compliant Solution (
The correct way to declare
errno is to include the header
Implementations conforming to C are required to declare
<errno.h>, although some historic implementations failed to do so.
DCL37-EX0: It is permissible to use reserved words in declarations when the risk of clashing with a preexisting variable is greater than the risk of clashing with a reserved word. In particular, the scope must be used in a macro that may be invoked with arbitrary preexisting variables (possibly as arguments). The following code demonstrates a
SWAP_UNSAFE() macro that exchanges two values, and uses a
__tmp variable as a temporary value. This code is permitted because the temporary variable is more likely to clash with a nonreserved variable in the current scope than with a reserved word. This code should be considered nonportable because it requires the current platform to allow the use of
Such macros should be used only with great care. See PRE31-C. Avoid side effects in arguments to unsafe macros and PRE00-C. Prefer inline or static functions to function-like macros for more information.
DCL37-EX1: Provided that a library function can be declared without reference to any type defined in a header, it is permissible to declare that function without including its header as long as that declaration is compatible with the standard declaration.
Such code is compliant because the declaration matches what
stdlib.h would provide and does not redefine the reserved identifier. It would not be acceptable to provide a definition for the
free() function in the preceding example.
Using reserved identifiers can lead to incorrect program operation.
|[IEEE Std 1003.1-2008]||Section 2.2, "The Compilation Environment"|
|[ISO/IEC 9899:2011]||Subclause 7.1.3, "Reserved Identifiers"|
Subclause 7.31.10, "Integer Types