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According to the C Standard, 76.4.2.1 .3 paragraph 7 [ISO/IEC 9899:20112024],

Some identifiers are reserved.

    —  All All identifiers that begin with an underscore and either a double underscore (__) or begin with an underscore (_) 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.

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33 defines many other reserved identifiers for future library directions.

No other identifiers are reserved. (The POSIX standard extends the set of identifiers reserved by the C Standard to include an open-ended set of its own. See See Portable Operating System Interface [POSIX^®], Base Specifications, Issue 7, Section 2.2, "The Compilation Environment" [IEEE Std 1003.1-2013].) 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 undefined behavior 106.)

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In this noncompliant code example, because the C standard library header <inttypes.h> is specified to include <stdint.h>, the name SIZE_MAX conflicts with a standard macro of the 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 is a reserved identifier because it begins with the INT prefix and ends with the _MAX suffix. (See the C Standard, 7.3133.1014.)

#include <inttypes.h>
#include <stdio.h>

static const int_fast16_t INTFAST16_LIMIT_MAX = 12000;

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

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Related Guidelines

Key here (explains table format and definitions)

Bibliography

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