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<ac:structured-macro ac:name="anchor" ac:schema-version="1" ac:macro-id="fb9545f0dd5ade73-d9c66d81-47a84cf0-94f5b374-399323b05fd392aab5623994"><ac:parameter ac:name="">1</ac:parameter></ac:structured-macro> \[1\] Note that the POSIX ^®^ standard extends the set of identifiers reserved by C99 to include an open-ended set of its own. See section [2.2 Compilation Environment|http://www.opengroup.org/onlinepubs/9699919799/functions/V2_chap02.html#tag_15_02] in [\[IEEE Std 1003.1-2008\]|AA. References#IEEE Std 1003.1-2008].

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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. A typical manifestation of such a clash is a compilation error.

#ifndef _MY_HEADER_H_
#define _MY_HEADER_H_

/* contents of <my_header.h> */

#endif /* _MY_HEADER_H_ */

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In this noncompliant code example, the names of the file scope objects _max_limit and _limit both begin with an underscore. Since it is static, the declaration of _max_limit might seem to be impervious to clashes with names defined by the implementation. However, since 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 or not any C standard library header has been explicitly included). In addition, since _limit has extern 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. Thus 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. 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;
}

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#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;
}

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In the noncompliant code example below, since the C standard library header <inttypes.h> is specified to include <stdint.h>, the name MAX_SIZE conflicts with the name of the <stdint.h> header macro used to denote the upper limit of size_t. In addition, while the name INTFAST16_LIMIT_MAX isn't defined by the C standard library, since it begins with the INT prefix and ends with the _MAX suffix it encroaches on the reserved name space (see section 8.26.8 of C99). A typical manifestation of such a clash is a compilation error.

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

static const int_fast16_t INTFAST16_LIMIT_MAX = 12000;

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

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The noncompliant example below provides definitions for the C standard library functions malloc() and free(). While this practice is permitted by many traditional implementations of UNIX (see, for example, the Dmalloc library), doing so is disallowed by the C99 standard as it need not generally portable and may lead to undefined behavior. Common effects range from compiler errors, to linker errors, to abnormal program behavior at runtime. In addition, even on systems where replacing malloc() is allowed, doing so without also replacing calloc() and realloc() is likely to cause problems as well.

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