Microsoft Extensions to C

Article
09/15/2006

| Overview | How Do I | Compiler Options

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.

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