Microsoft extensions to C and C++

Microsoft Visual C++ (MSVC) extends the C and C++ language standards in several ways, detailed in this article.

The MSVC C++ compiler defaults to support for ISO C++14 with some ISO C++17 features and some Microsoft-specific language extensions. For more information on supported features, see Microsoft C/C++ language conformance by Visual Studio version. You can use the /std compiler option to enable full support for ISO C++17 and ISO C++20 language features. For more information, see /std (Specify language standard version).

Where specified, some MSVC C++ language extensions can be disabled by use of the /Za compiler option. In Visual Studio 2017 and later versions, the /permissive- compiler option disables Microsoft-specific C++ language extensions. The /permissive- compiler option is implicitly enabled by the /std:c++20 and /std:c++latest compiler options.

By default, when MSVC compiles code as C, it implements ANSI C89 with Microsoft-specific language extensions. Some of these MSVC extensions are standardized in ISO C99 and later. Most MSVC C extensions can be disabled by use of the /Za compiler option, as detailed later in this article. You can use the /std compiler option to enable support for ISO C11 and C17. For more information, see /std (Specify language standard version).

The standard C runtime library is implemented by the Universal C runtime library (UCRT) in Windows. The UCRT also implements many POSIX and Microsoft-specific library extensions. The UCRT supports the ISO C11 and C17 C runtime library standards, with certain implementation-specific caveats. It doesn't support the full ISO C99 standard C runtime library. For more information, see compatibility in the Universal C runtime library documentation.

MSVC adds several Microsoft-specific keywords to C and C++. In the list in Keywords, the keywords that have two leading underscores are MSVC extensions.

Both the C++ compiler and C compiler support these kinds of non-standard casts:

• The C compiler supports non-standard casts to produce l-values. For example:

  char *p;
  (( int * ) p )++;
  // In C with /W4, both by default and under /Ze:
  //     warning C4213: nonstandard extension used: cast on l-value
  // Under /TP or /Za:
  //     error C2105: '++' needs l-value
  

  Note

  This extension is available in the C language only. You can use the following C standard form in C++ code to modify a pointer as if it's a pointer to a different type.

  The preceding example could be rewritten as follows to conform to the C standard.

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

• Both the C and C++ compilers support non-standard casts of a function pointer to a data pointer. For example:

  int ( * pfunc ) ();
  int *pdata;
  pdata = ( int * ) pfunc;
  /* No diagnostic at any level, whether compiled with default options or under /Za */
  

Both C and C++ compilers support 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 )
{ }
// In C with /W4, either by default or under /Ze:
//     warning C4212: nonstandard extension used: function declaration used ellipsis
// In C with /W4, under /Za:
//     warning C4028: formal parameter 2 different from declaration
// In C++, no diagnostic by default or under /Za.

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

// This is a single-line comment.

Single-line comments are a C99 feature. They're unaffected by /Za and cause no diagnostic at any level.

The C compiler supports the following scope-related features.

• Redefinitions of extern items as static:

  extern int clip();
  static int clip() {}
  // In C and C++ with /W4, either by default or under /Ze:
  //     warning C4211: nonstandard extension used: redefined extern to static
  // In C and C++ under /Za:
  //     error C2375: 'clip': redefinition; different linkage
  

• Use of benign typedef redefinitions within the same scope:

  typedef int INT;
  typedef int INT; // No diagnostic at any level in C or C++
  

• Function declarators have file scope:

  void func1()
  {
       extern double func2( double );
       // In C at /W4:  warning C4210: nonstandard extension used: function given file scope
  }
  int main( void )
  {
       func2( 4 );    //  /Ze passes 4 as type double
  }                  //  /Za passes 4 as type int
  

• Use of block-scope variables that are initialized by using non-constant expressions:

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

The C compiler supports the following data declaration and definition features.

• Mixed character and string constants in an initializer:

  char arr[6] = {'a', 'b', "cde"};
  // In C with /W4, either by default or under /Ze:
  //     warning C4207: nonstandard extension used: extended initializer form
  // Under /Za:
  //     error C2078: too many initializers
  

• Bit fields that have base types other than unsigned int or signed int.

• Declarators that don't have a type:

  x;
  // By default or under /Ze, /Za, /std:c11, and /std:c17, when /W4 is specified:
  //     warning C4431: missing type specifier - int assumed. Note: C no longer supports default-int
  //     warning C4218: nonstandard extension used: must specify at least a storage class or a type
  */
  int main( void )
  {
       x = 1;
  }
  

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

  struct zero
  {
       char *c;
       int zarray[];
       // In C with /W4, either by default, under /Ze, /std:c11, and /std:c17:
       //     warning C4200: nonstandard extension used: zero-sized array in struct/union
       // Under /Za:
       //     error C2133: 'zarray': unknown size
  };
  

• Unnamed (anonymous) structures:

  struct
  {
       int i;
       char *s;
  };
  // By default or under /Ze, /std:c11, and /std:c17, when /W4 is specified:
  //     warning C4094: untagged 'struct' declared no symbols
  // Under /Za:
  //     error C2059: syntax error: 'empty declaration'
  

• Unnamed (anonymous) unions:

  union
  {
       int i;
       float fl;
  };
  // By default or under /Ze, /std:c11, and /std:c17, when /W4 is specified:
  //     warning C4094: untagged 'union' declared no symbols
  // Under /Za:
  //     error C2059: syntax error: 'empty declaration'
  

Both the x86 C++ compiler and C compiler support inline generation of the atan, atan2, cos, exp, log, log10, sin, sqrt, and tan functions when /Oi is specified. These intrinsics don't conform to the standard, because they don't set the errno variable.

Under /Ze, you have to include iso646.h if you want to use text forms of the following operators:

These text forms are available as C++ keywords under /Za or when /permissive- is specified or implied.

/Za, /Ze (Disable language extensions)
MSVC compiler options
MSVC compiler command-line syntax