Pragmas - D Programming Language
Description: D Programming Language
Keywords: D programming language

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# Pragmas

    Pragma:
        pragma ( Identifier )
        pragma ( Identifier , ArgumentList )

Pragmas are a way to pass special information to the compiler and to add vendor specific extensions to D. Pragmas can be used by themselves terminated with a ‘;’, they can influence a statement, a block of statements, a declaration, or a block of declarations.

Pragmas can appear as either declarations, Pragma DeclarationBlock, or as statements, PragmaStatement.

    pragma(ident);        // just by itself

    pragma(ident) declaration; // influence one declaration

    pragma(ident): // influence subsequent declarations
        declaration;
        declaration;

    pragma(ident)   // influence block of declarations
    {
        declaration;
        declaration;
    }

    pragma(ident) statement; // influence one statement

    pragma(ident)   // influence block of statements
    {
        statement;
        statement;
    }

The kind of pragma it is determined by the Identifier. ExpressionList is a comma-separated list of AssignExpressions. The AssignExpressions must be parsable as expressions, but what they mean semantically is up to the individual pragma semantics.

### Predefined Pragmas

All implementations must support these, even if by just ignoring them:

- pragma inline
- pragma lib
- pragma mangle
- pragma msg
- pragma startaddress

inline

  Affects whether functions are inlined or not. If at the declaration level, it affects the functions declared in the block it controls. If inside a function, it affects the function it is enclosed by. If there are multiple pragma inlines in a function, the lexically last one takes effect.

  It takes three forms:

  1.     pragma(inline)

     Sets the behavior to match the default behavior set by the compiler switch -inline.

  2.     pragma(inline, false)

     Functions are never inlined.

  3.     pragma(inline, true)

     If a function cannot be inlined with the -inline switch, an error message is issued. This is expected to be improved in the future to causing functions to always be inlined regardless of compiler switch settings. Whether a compiler can inline a particular function or not is implementation defined.

      pragma(inline):
      int foo(int x) // foo() is never inlined
      {
          pragma(inline, true);
          ++x;
          pragma(inline, false); // supercedes the others
          return x + 3;
      }

lib
  Inserts a directive in the object file to link in the library specified by the AssignExpression. The AssignExpressions must be a string literal:

      pragma(lib, "foo.lib");

mangle
  Overrides the default mangling for a symbol. It's only effective when the symbol is a function declaration or a variable declaration. For example this allows linking to a symbol which is a D keyword, which would normally be disallowed as a symbol name:

      pragma(mangle, "body")
      extern(C) void body_func();

msg
  Constructs a message from the arguments and prints to the standard error stream while compiling:

      pragma(msg, "compiling...", 1, 1.0);

startaddress
  Puts a directive into the object file saying that the function specified in the first argument will be the start address for the program:

      void foo() { ... }
      pragma(startaddress, foo);

  This is not normally used for application level programming, but is for specialized systems work. For applications code, the start address is taken care of by the runtime library.

### Vendor Specific Pragmas

Vendor specific pragma Identifiers can be defined if they are prefixed by the vendor's trademarked name, in a similar manner to version identifiers:

    pragma(DigitalMars_funky_extension) { ... }

Compilers must diagnose an error for unrecognized Pragmas, even if they are vendor specific ones. This implies that vendor specific pragmas should be wrapped in version statements:

    version (DigitalMars)
    {
        pragma(DigitalMars_funky_extension)
        { ... }
    }

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