Directive (programming)

In C and C++, the language supports a simple macro preprocessor. Source lines that should be handled by the preprocessor, such as #define and #include are referred to as preprocessor directives.

Another C construct, the #pragma directive, is used to instruct the compiler to use pragmatic or implementation-dependent features. Two notable users of this directive are OpenMP and OpenACC.

Syntactic constructs similar to C's preprocessor directives, such as C#'s #if, are also typically called "directives", although in these cases there may not be any real preprocessing phase involved.

All preprocessor commands begin with a hash symbol (#).

Directives date to ALGOL 68, where they are known as pragmats (from "pragmatic"), and denoted pragmat or pr; in newer languages, notably C, this has been abbreviated to "pragma" (no 't').

A common use of pragmats in ALGOL 68 is in specifying a stropping regime, meaning "how keywords are indicated". Various such directives follow, specifying the POINT, UPPER, RES (reserved), or quote regimes. Note the use of stropping for the pragmat keyword itself (abbreviated pr), either in the POINT or quote regimes:

.PR POINT .PR
.PR UPPER .PR
.PR RES .PR
'pr' quote 'pr'

Today directives are best known in the C language, of early 1970s vintage, and continued through the current C99 standard, where they are either instructions to the C preprocessor, or, in the form of #pragma, directives to the compiler itself. They are also used to some degree in more modern languages; see below.

• In Ada, compiler directives are called pragmas (short for "pragmatic information").
• In Common Lisp, directives are called declarations, and are specified using the declare construct (also proclaim or declaim).[1] With one exception, declarations are optional, and do not affect the semantics of the program. The one exception is special, which must be specified where appropriate.
• In Turbo Pascal, directives are called significant comments, because in the language grammar they follow the same syntax as comments. In Turbo Pascal, a significant comment is a comment whose first character is a dollar sign and whose second character is a letter; for example, the equivalent of C's #include "file" directive is the significant comment {$I "file"}.
• In Perl, the keyword "use", which imports modules, can also be used to specify directives, such as use strict; or use utf8;.
• Haskell pragmas are specified using a specialized comment syntax, e.g. {-# INLINE foo #-}.[2]
• Python has two directives – from __future__ import feature (defined in PEP 236 -- Back to the __future__), which changes language features (and uses the existing module import syntax, as in Perl), and the coding directive (in a comment) to specify the encoding of a source code file (defined in PEP 263 -- Defining Python Source Code Encodings). A more general directive statement was proposed and rejected in PEP 244 -- The `directive' statement; these all date to 2001.
• ECMAScript also adopts the use syntax for directives, with the difference that pragmas are declared as string literals (e.g. "use strict";, or "use asm";), rather than a function call.
• In Visual Basic, the keyword "Option" is used for directives:
  • Option Explicit On|Off - When on disallows implicit declaration of variables at first use requiring explicit declaration beforehand.
  • Option Compare Binary - Results in string comparisons based on a sort order derived from the internal binary representations of the characters - e.g. for the English/European code page (ANSI 1252) A < B < E < Z < a < b < e < z < À < Ê < Ø < à < ê < ø. Affects intrinsic operators (e.g. =, <>, <, >), the Select Case block, and VB runtime library string functions (e.g. InStr).
  • Option Compare Text - Results in string comparisons based on a case-insensitive text sort order determined by your system's locale - e.g. for the English/European code page (ANSI 1252) (A=a) < (À = à) < (B=b) < (E=e) < (Ê = ê) < (Z=z) < (Ø = ø). Affects intrinsic operators (e.g. =, <>, <, >), the Select Case block, and VB runtime library string functions (e.g. InStr).
  • Option Strict On|Off - When on disallows:
    • typeless programming - where declarations which lack an explicit type are implicitly typed as Object.
    • late-binding (i.e. dynamic dispatch to CLR, DLR, and COM objects) on values statically typed as Object.
    • implicit narrowing conversions - requiring all conversions to narrower types (e.g. from Long to Integer, Object to String, Control to TextBox) be explicit in code using conversion operators (e.g. CInt, DirectCast, CType).
  • Option Infer On|Off - When on enables the compiler to infer the type of local variables from their initializers.
• In Ruby, interpreter directives are referred to as pragmas and are specified by top-of-file comments that follow a key: value notation. For example, coding: UTF-8 indicates that the file is encoded via the UTF-8 character encoding.
• In C#, compiler directives are called pre-processing directives. There are a number of different compiler directives including #pragma, which is specifically used to control compiler warnings and debugger checksums.[3][4]
• The SQLite DBMS includes a PRAGMA directive that is used to introduce commands that are not compatible with other DBMS.[5]

• #pragma once – Preprocessor directive in C and C++

• OpenMP Website
• OpenACC Website
• OpenHMPP Website