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Pragmas

Contents

Predefined Pragmas
Vendor Specific 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. ArgumentList is a comma-separated list of AssignExpressions. The AssignExpressions must be parsable as expressions, but their meaning 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

Implementation Defined: An implementation may ignore these pragmas.

pragma 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.

It takes three forms:

```
pragma(inline)
```
Sets the behavior to match the implementation's default behavior.
```
pragma(inline, false)
```
Functions are never inlined.
```
pragma(inline, true)
```
Always inline the functions.

There can be only zero or one AssignExpressions. If one is there, it must be true, false, or an integer value. An integer value is implicitly converted to a bool.

If there are multiple pragma inlines in a function, the lexically last one takes effect.

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

Implementation Defined:

The default inline behavior is typically selectable with a compiler switch such as -inline.
Whether a particular function can be inlined or not is implementation defined.
What happens for pragma(inline, true) if the function cannot be inlined. An error message is typical.

pragma lib

There must be one AssignExpression and it must evaluate at compile time to a string literal.

pragma(lib, "foo.lib");

Implementation Defined: Typically, the string literal specifies the file name of a library file. This name is inserted into the generated object file, or otherwise is passed to the linker, so the linker automatically links in that library.

pragma mangle

Overrides the default mangling for a symbol.

There must be one AssignExpression and it must evaluate at compile time to a string literal.

Implementation Defined: On macOS and Win32, an extra underscore (_) is prepended to the string since 2.079, as is done by the C/C++ toolchain. This allows using the same pragma(mangle) for all compatible (POSIX in one case, win64 in another) platforms instead of having to special-case.

Implementation Defined: 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();

pragma msg

Constructs a message from the ArgumentList.

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

Implementation Defined: The arguments are typically presented to the user during compilation, such as by printing them to the standard error stream.

pragma startaddress

There must be one AssignExpression and it must evaluate at compile time to a function symbol.

Implementation Defined: The function symbol specifies the start address for the program. The symbol is inserted into the object file or is otherwise presented to the linker to set the start address. 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.

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

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_extension) { ... }

Implementations must diagnose an error for unrecognized Pragmas, even if they are vendor specific ones.

Implementation Defined: Vendor specific pragmas.

Best Practices: vendor specific pragmas should be wrapped in version statements

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

Attributes

Expressions