Anidentifier is an arbitrarily long sequence of digits, underscores, lowercase and uppercase Latin letters, and Unicode characters specified using\u and\Uescape notation(since C99), of classXID_Continue(since C23). A valid identifier must begin with a non-digit character (Latin letter, underscore, or Unicode non-digit character(since C99)(until C23), or Unicode character of classXID_Start)(since C23)). Identifiers are case-sensitive (lowercase and uppercase letters are distinct).Every identifier must conform toNormalization Form C.(since C23)

char*\U0001f431="cat";// supportedchar*🐱="cat";// implementation-defined// (e.g. works with Clang, but not GCC prior to version 10)// both are ill formed in C23. Emoji are not XID_Start characters

Identifiers can denote the following types of entities:

• objects
• functions
• tags (struct,union, orenumerations)
• structure or union members
• enumeration constants
• typedef names
• label names
• macro names
• macro parameter names

Every identifier other than macro names or macro parameter names hasscope, belongs to aname space, and may havelinkage. The same identifier can denote different entities at different points in the program, or may denote different entities at the same point if the entities are in different name spaces.

[edit]Reserved identifiers

The following identifiers arereserved and may not be declared in a program (doing so invokes undefined behavior):

1. The identifiers that arekeywords cannot be used for other purposes. In particular #define or #undef of an identifier that is identical to a keyword is not allowed.
2. All external identifiers that begin with an underscore.
3. All identifiers that begin with an underscore followed by a capital letter or by another underscore (these reserved identifiers allow the library to use numerous behind-the-scenes non-external macros and functions).
4. All external identifiers defined by the standard library (in hosted environment). This means that no user-supplied external names are allowed to match any library names, not even if declaring a function that is identical to a library function.
5. Identifiers declared as reserved for the implementation or future use by the standard library (see below).
6. Identifiers declared as potentially reserved and provided by the implementation (see below).(since C23)

All other identifiers are available. Identifiers that are not reservedor potentially reserved(since C23) can be used with no fear of unexpected collisions when moving programs from one compiler and library to another.

Note: in C++, identifiers with a double underscore anywhere are reserved everywhere; in C, only the ones that begin with a double underscore are reserved.

[edit]Reserved and potentially reserved identifiers in the library

The standard library reserves every identifier it provides. Reserved identifiers that haveexternal linkage (e.g. name of every standard function) are reserved regardless which header is included. Other reserved identifiers are reserved when any of its associated headers is included.

Following identifiers are reservedor potentially reserved(since C23) for the implementation or future use by the standard library.

• function names, all of which are potentially reserved(since C23)
  • cerf,cerfc,cexp2,cexpm1,clog10,clog1p,clog2,clgamma,ctgamma,csinpi,ccospi,ctanpi,casinpi,cacospi,catanpi,ccompoundn,cpown,cpowr,crootn,crsqrt,cexp10m1,cexp10,cexp2m1,clog10p1,clog2p1,clogp1(since C23) and their -f and -l suffixed variants, in<complex.h>(since C99)
  • beginning withis orto followed by a lowercase letter, in<ctype.h> and<wctype.h>(since C95)
  • beginning withstrorwcs(since C23) followed by a lowercase letter, in<stdlib.h>and<inttypes.h>(since C23)
  • beginning withcr_, in<math.h>(since C23)
  • beginning withwcs followed by a lowercase letter, in<wchar.h>(since C95)
  • beginning withatomic_ followed by a lowercase letter, in<stdatomic.h>(since C11)
  • beginning withcnd_,mtx_,thrd_ ortss_ followed by a lowercase letter, in<threads.h>(since C11)
• typedef names, all of which are potentially reserved(since C23)
  • beginning withint oruint and ending with_t, in<stdint.h>(since C99)
  • beginning withatomic_ ormemory_ followed by a lowercase letter, in<stdatomic.h>(since C11)
  • beginning withcnd_,mtx_,thrd_ ortss_ followed by a lowercase letter, in<threads.h>(since C11)
• macro names
  • beginning withE followed by a digit or an uppercase letter, in<errno.h>
  • beginning withFE_ followed by an uppercase letter, in<fenv.h>(since C99)
  • beginning withDBL_,DEC32_,DEC64_,DEC128_,DEC_,FLT_, orLDBL_ followed by an uppercase letter, in<float.h>; these identifiers are potentially reserved(since C23)
  • beginning withINT orUINT and ending with_MAX,_MIN,_WIDTH(since C23), or_C, in<stdint.h>; these identifiers are potentially reserved(since C23)(since C99)
  • beginning withPRI orSCN followed by lowercase letter or the letterX, in<inttypes.h>; these identifiers are potentially reserved(since C23)(since C99)
  • beginning withLC_ followed by an uppercase letter, in<locale.h>
  • beginning withFP_ followed by an uppercase letter, in<math.h>(since C23)
  • beginning withMATH_ followed by an uppercase letter, in<math.h>; these identifiers are potentially reserved(since C23)
  • beginning withSIG orSIG_ followed by an uppercase letter, in<signal.h>
  • beginning withTIME_ followed by an uppercase letter, in<time.h>(since C11)
  • beginning withATOMIC_ followed by an uppercase letter, in<stdatomic.h>; these identifiers are potentially reserved(since C23)(since C11)
• enumeration constants, all of which are potentially reserved(since C23)
  • beginning withmemory_order_ followed by a lowercase letter, in<stdatomic.h>(since C11)
  • beginning withcnd_,mtx_,thrd_ ortss_ followed by a lowercase letter, in<threads.h>(since C11)

[edit]Translation limits

Even though there is no specific limit on the length of identifiers, early compilers had limits on the number of significant initial characters in identifiers and the linkers imposed stricter limits on the names withexternal linkage. C requires that at least the following limits are supported by any standard-compliant implementation:

[edit]References

[edit]See also

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