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No other identifiers are reserved. (Note that the The POSIX standard extends the set of identifiers reserved by the C Standard to include an open-ended set of its own. 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 See undefined behavior 106.).

Noncompliant Code Example (Header Guard)

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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 See the C Standard, 7.31.10.).

#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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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. 

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