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No other identifiers are reserved. If A the behavior of a program that declares or defines an identifier in a context in which it is reserved or defines a reserved identifier as a macro name, the behavior of that identifier is undefinedis undefined . See also undefined behavior 100 of Annex J of C99. Trying to define a reserved identifier can result in its name conflicting with that used in implementation, which may or may not be detected at compile time.

Noncompliant Code Example

In this noncompliant code example, variables are defined with names reserved for the implementation.

long _Max_Value;
int __length; 

Compliant Solution

This compliant solution uses identifiers that are not reserved.

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Noncompliant Code Example (Header Guard)

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#ifndef MY_HEADER_H
#define MY_HEADER_H

/* contents of <my_header.h> */

#endif /* MY_HEADER_H */

Noncompliant Code Example (

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File Scope Objects)

In this noncompliant code example, a variable beginning with an underscore is defined with implicit global scope.

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

size_t _limit = 100;

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

Noncompliant Code Example (Static Variable)

In this example, the variable is declared as static and, hence, has file scope.

This code might have been safe if the C file containing it includes no header files. However, it requires that the header stddef.h be included to define size_t. Including any standard header files will introduce a potential name clash. Consequently it is not safe to prepend any identifier with an underline, even if its usage is confined to a single filethe 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. 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.

#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;
}

Compliant Solution (

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File Scope Objects)

In this compliant solution, the variable name does not names of no file scope objects begin with an underscore and, hence, is 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;
}

Noncompliant Code Example

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(Reserved Macros)

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)Identifiers with external linkage include, among many others, setjmp, errno, math_errhandling, and va_end.
In the example, errno is defined. The errno value set by the function open() would not be accessible to the program because its definition is suppressed. For information regarding redefining errno, see ERR31-C. Don't redefine errno.

#include <errno<inttypes.h>

#define errno 200

int validate(unsigned int secretValue){
  
  char fname[] = "non_exist.txt";
  int fd;
  int result = -1;

  fd = open(fname, O_RDONLY); 

  if(fd == -1){
    printf("Error opening file. Error code : %d\n", errno); 
    return result;
  }

  close(fd); 

  if(errno % secretValue == 20){
    result = 0;
  }
  else{
    result = -1;
  } 
  
  return result;
  
}

Compliant Solution

In this compliant solution, the reserved identifier errno is not used.

   /* 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);
    /* ... */
}

Compliant Solution (Reserved Macros)

The compliant solution below avoids redefining reserved names or using reserved prefixes and suffixes.

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

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
      snprintf(buf, BUFSIZE, "The value is %" PRIdFAST16, val);
    /* ... */
}

Noncompliant Code Example (Identifiers With External Linkage)

In addition to symbols defined as functions in each C standard library header, identifiers with external linkage include, among many others, errno, math_errhandling, setjmp(), and va_end(), regardless of whether any of them is masked by a macro of the same name or not.

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. 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.

#include <stddef.h>

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

void free(void *ptr) {
  /* return 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;
  /* allocate storage from own pool and set ptr */
  return ptr;
}

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

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

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

#include <errno.h>

#define keyNum 200

int validate(unsigned int secretValue){
  
  char fname[] = "non_exist.txt";
  int fd;
  int result = -1;

  fd = open(fname, O_RDONLY); 

  if(fd == -1){
    printf("Error opening file. Error code : %d\n", errno); 
    return result;
  }

  close(fd); 

  if(errno % secretValue == 20){
    result = 0;
  }
  else{
    result = -1;
  } 
  
  return result;
  
}

Risk Assessment

Using reserved identifiers can lead to incorrect program operation.

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