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#ifndef MY_HEADER_H
#define MY_HEADER_H

/* contents of <my_header.h> */

#endif /* MY_HEADER_H */

Noncompliant Code Example (Global Variable)

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

size_t _limit = 100;

unsigned int getValue(unsigned int count){

  size_t i;
  unsigned int result = 0;

  for(i = 0; i < _limit; i++){    
    result++;        
    if(i == count){ 
      break;
    }
  }

}

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Noncompliant Code Example (Static Variable)

In this compliant solutionexample, 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 stdlib.h in order to define size_t. And loading 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 file.

static size_t _limit = 100;

unsigned int getValue(unsigned int count){

  size_t i;
  unsigned int result = 0;

  for(i = 0; i < _limit; i++){    
    result++;        
    if(i == count){ 
      break;
    }
  }

}

Compliant

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Solution

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(Global Variable)

In this compliant solution, the variable name does not begin with an underscore and, hence, is not reserved.

size_t limit = 100;

unsigned int getValue(unsigned int count){

  size_t i;
  unsigned int result = 0;

  for(i = 0; i < limit; i++){    
    result++;        
    if(i == count){ 
      break;
    }
  }

}

Noncompliant Code Example

Identifiers with external linkage include 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.

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