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According to the C standard, Section 7.1.3 Standard, 6.4.2.1 paragraph 7 [ISO/IEC 9899:20112024],

Some identifiers are reserved.

    —  All identifiers that begin with a double underscore (__) or

All identifiers that

begin with an underscore

and either

(_) followed by an uppercase letter

or another underscore

are

always

reserved for any use, except those identifiers which are lexically identical to keywords.

All

    —  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.

Other identifiers may be reserved, see 7.1.3.

C Standard, 7.1.3 paragraph 1 [ISO/IEC 9899:2024],

Each header declares or defines all identifiers listed in its associated subclause, and optionally declares or defines identifiers listed in its associated future library directions subclause and identifiers which are always reserved either for any use or for use as file scope identifiers.

    — All potentially reserved identifiers (including ones listed in the future library directions) that are provided by an implementation with an external definition are reserved for any use. An implementation shall not provide an external definition of a potentially reserved identifier unless that identifier is reserved for a use where it would have external linkage. All other potentially reserved identifiers that are provided by an implementation (including in the form of a macro) are reserved for any use when the associated header is included. No other potentially reserved identifiers are reserved.

    — 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 (see 7.1.4).

    — All identifiers with external linkage in any of the following subclauses (including the 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 7.33 defines many other reserved identifiers for future library directions.

No other identifiers are reserved. (

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The POSIX

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standard extends the set of identifiers reserved by the C

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Standard to include an open-ended set of its own. See

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Portable Operating System Interface [POSIX^®], Base Specifications, Issue 7, Section 2.2, "The Compilation Environment

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" [IEEE Std 1003.1-

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2013].) 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

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undefined behavior 106

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.)

Noncompliant Code Example (

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Include Guard)

A common, but noncompliant, practice is to choose a reserved name for a macro used in a preprocessor conditional guarding against multiple inclusion inclusions of a header file. (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. A typical manifestation of such a clash is a compilation error.

#ifndef _MY_HEADER_H_
#define _MY_HEADER_H_

/* contentsContents of <my_header.h> */

#endif /* _MY_HEADER_H_ */

Compliant Solution (

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Include Guard)

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

#ifndef MY_HEADER_H
#define MY_HEADER_H

/* contentsContents 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. Since it Because _max_limit is static, the this 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. (Note, however, that a conforming compiler may implicitly declare reserved names regardless of whether or not any C standard library header has been 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 unsafe 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. Common effects of such clashes range from compiler errors to linker errors to abnormal program behavior at runtime.

#include <stddef.h>   /* for size_t */

static const size_t _max_limit = 1024;
size_t _limit = 100;

unsigned int getValue(unsigned int count) {
  return count < _limit ? count : _limit;
}

...

In this compliant solution, names of no- file - scope objects do not begin with an underscore and hence do not encroach on the reserved name space.:

#include <stddef.h>   /* for size_t */

static const size_t max_limit = 1024;
size_t limit = 100;

unsigned int getValue(unsigned int count) {
  return count < limit ? count : limit;
}

...

In this noncompliant code example, because the C standard library header <inttypes.h> is specified to include <stdint.h>, the name SIZE_MAX_SIZE conflicts with a standard macro of the name of the <stdint.h> header macro used same name, 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 encroaches on the reserved name space is a reserved identifier because it begins with the INT prefix and ends with the _MAX suffix. (See Section the C Standard, 7.31.10 of the C standard.) A typical manifestation of such a clash is a compilation error.33.14.)

#include <inttypes.h>   /* for int_fast16_t and PRIdFAST16 */
#include <stdio.h>

static const int_fast16_t INTFAST16_LIMIT_MAX = 12000;

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

Compliant Solution (Reserved Macros)

This compliant solution avoids redefining reserved names or using reserved prefixes and suffixes.:

#include <inttypes.h>   /* for int_fast16_t and PRIdFAST16 */

#include <stdio.h>
 
static const int_fast16_t MY_INTFAST16_UPPER_LIMIT = 12000;

void print_fast16(int_fast16_t val) {
    enum { BUFSIZE = 80 };
    char buf [BUFSIZE];
    if (MY_INTFAST16_UPPER_LIMIT < val) {
      sprintf(buf, "The value is too large");
  } else else{
      snprintf(buf, BUFSIZE, "The value is %" PRIdFAST16, val);
    /* ... */}
}

Noncompliant Code Example (Identifiers with External Linkage)

...

)

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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 (e.g.for example, the Dmalloc library), it is disallowed by the C standard because it 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 according to the C Standard. Even on systems where that allow replacing malloc() is allowed, doing so without also replacing aligned_alloc(), calloc(), and realloc() is likely to cause problems as well.

Code Block
bgColor #ffcccc lang c
#include <stddef.h>   void * malloc(size_t nbytes) { void *ptr; /* allocateAllocate storage from own pool and set ptr */ return ptr; } void free(void *ptr) { /* returnReturn storage to own pool */ }

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.:

#include <stddef.h>

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

void *my_aligned_alloc(size_t alignment, size_t size) {
  void *ptr;
  /* Allocate storage from own pool, align properly, set ptr */
  return ptr;
}

void *my_calloc(size_t nelems, size_t elsize) {
  void *ptr;
  /* allocateAllocate storage from own pool, zero memory, and set ptr */
  return ptr;
}

void * my_realloc(void *ptr, size_t nbytes) {
  /* reallocateReallocate storage from own pool and set ptr */
  return ptr;
}

void my_free(void *ptr) {
  /* returnReturn storage to own pool */
}

Noncompliant Code Example (errno)

The In addition to symbols defined as functions in each C standard library header, identifiers with external linkage include errno and math_errhandling.  According 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. [ISO/IEC 9899:2011] 

(See undefined behavior 114  of Annex J114.)

The 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.:

extern int errno;

...

The correct way to declare errno is to include the header <errno.h>.:

#include <errno.h>

Implementations conforming to C are required to declare errno in <errno.h>, although some historic implementations failed to do so.

Exceptions

DCL37-C-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 provided that declaration is compatible with the standard declaration.

/* Not including stdlib.h */
void free(void *);
 
void func(void *ptr) {
  free(ptr);
}

Such code is compliant because the declaration matches what stdlib.h would provide and does not redefine the reserved identifier. However, it would not be acceptable to provide a definition for the free() function in this example.

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

/* Sometimes generated by configuration tools such as autoconf */
#define const const
 
/* Allowed compilers with semantically equivalent extension behavior */
#define inline __inline

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:

/*
  The following declarations of reserved identifiers exist in the glibc implementation of
  <stdio.h>. The original source code may be found at:
  https://sourceware.org/git/?p=glibc.git;a=blob_plain;f=include/stdio.h;hb=HEAD
*/
 
#  define __need_size_t
#  include <stddef.h>
/* Generate a unique file name (and possibly open it).  */
extern int __path_search (char *__tmpl, size_t __tmpl_len,
			  const char *__dir, const char *__pfx,
			  int __try_tempdir);

Risk Assessment

Using reserved identifiers can lead to incorrect program operation.

Automated Detection

Related Guidelines

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Related Guidelines

Key here (explains table format and definitions)

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Bibliography

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