Microsoft Extensions to C

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The following are Microsoft extensions to the ANSI C standard:

Keywords

The keywords __based, __cdecl, __declspec, __except, __fastcall, __finally, __leave, __stdcall, and __try are Microsoft-specific.

Casts

The Microsoft compiler supports the following two non-ANSI casts:

• Use of non-ANSI casts to produce l-values:

  char *p;
  (( int * ) p )++;

  The preceding example could be rewritten to conform with the ANSI C standard as follows:

  p = ( char * )(( int * )p + 1 );

• Non-ANSI casting of a function pointer to a data pointer:

  int ( * pfunc ) (); 
  int *pdata;
  pdata = ( int * ) pfunc;

  To perform the same cast while maintaining ANSI compatibility, you must cast the function pointer to an int before casting it to a data pointer:

  pdata = ( int * ) (int) pfunc;

Variable-Length Argument Lists

The Microsoft compiler supports use of a function declarator that specifies a variable number of arguments, followed by a function definition that provides a type instead:

void myfunc( int x, ... );
void myfunc( int x, char * c )
{ }

Single-Line Comments

The Microsoft C compiler supports single-line comments, which are introduced with two forward slash (//) characters:

// This is a single-line comment.

Scope

The Microsoft C compiler supports the following scope-related features:

• Redefinitions of extern items as static:

  extern int clip();
  static int clip()
  {}

• Use of benign typedef redefinitions within the same scope:

  typedef int INT;
  typedef int INT;

• Function declarators have file scope:

  void func1()
  {
      extern int func2( double );
  }
  void main( void )
  {
      func2( 4 );    //  /Ze passes 4 as type double
  }                  //  /Za passes 4 as type int

• Use of block-scope variables initialized with nonconstant expressions:

  int clip( int );
  int bar( int );
  void main( void )
  {
      int array[2] = { clip( 2 ), bar( 4 ) };
  }
  int clip( int x )
  {
      return x;
  }
  int bar( int x )
  {
      return x;
  }

Data Declarations and Definitions

The Microsoft C compiler supports the following data declaration and definition features:

• Mixed character and string constants in an initializer:

  char arr[5] = {'a', 'b', "cde"};

• Bit fields with base types other than unsigned int or signed int.
  
  
• Declarators without either a storage class or a type:

  x;
  void main( void )
  {
      x = 1;
  }

• Unsized arrays as the last field in structures and unions:

  struct zero
  {
      char *c;
      int zarray[];
  };

• Unnamed (anonymous) structures:

  struct
  {
      int i;
      char *s;
  };

• Unnamed (anonymous) unions:

  union
  {
      int i;
      float fl;
  };

  • Unnamed members:

  struct s
  {
  unsigned int flag : 1;
  unsigned int : 31;
  }

Intrinsic Floating-Point Functions

The Microsoft compiler supports inline generation of the x86 Specific —> atan, atan2, cos, exp, log, log10, sin, sqrt, and tan functions END x86 Specific when the Generate Intrinsic Functions (/Oi) option is specified. For C, ANSI conformance is lost when these intrinsics are used, because they do not set the errno variable.