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{{short description|Language construct that specifies how a compiler should process its input}}
{{Refimprove|date=December 2013}} In [[computer programming]], a '''directive''' or '''pragma''' (from "pragmatic") is a [[language construct]] that specifies how a [[compiler]] (or other [[Translator (computing)|translator]]) should process its input. In some cases directives specify global behavior, while in other cases they only affect a local section, such as a block of programming code. In some cases, such as some C programs, directives are optional compiler hints
This term could be used to refer to proprietary third
==The C preprocessor==
{{Main|C preprocessor}}
In [[C (programming language)|C]] and [[C++]], the language supports a simple [[Macro (computer science)|macro]] [[preprocessor]]. Source lines that should be handled by the preprocessor, such as <code>#define</code> and <code>#include</code> are referred to as ''preprocessor directives''.
Syntactic constructs similar to C's preprocessor directives, such as [[C Sharp (programming language)|C#]]'s <code>#if</code>, are also typically called "directives", although in these cases there may not be any real preprocessing phase involved.
All preprocessor commands, except for <code>defined</code> (when following a conditional directive), begin with a hash symbol (#). Until [[C++26]], the keywords <code>export</code>, <code>import</code> and <code>module</code> were partially handled by the preprocessor.<ref>{{cite web|url=https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p1857r1.html|title=P1857R1 - Modules Dependency Discovery}}</ref>
==History==
Directives date to [[JOVIAL]].<ref>{{cite tech report
Directives date to [[ALGOL 68]], where they are known as [[ALGOL 68#pr .26 co: Pragmats and Comments|pragmats]] (from "pragmatic"), and denoted '''pragmat''' or '''pr'''; in newer languages, notably C, this has been abbreviated to "pragma" (no 't').▼
| title = Computer Programming Manual for JOVIAL (J73) Language
| id = RADC-TR-81-143
| date = June 1981
| section = Chapter 17 - Directives
| section-url = https://apps.dtic.mil/sti/pdfs/ADA101061.pdf#page=248
| pages = 243-263
| url = https://apps.dtic.mil/sti/pdfs/ADA101061.pdf
| access-date = May 28, 2023
}}
</ref>
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[[COBOL]] has a COPY directive.
A common use of pragmats in ALGOL 68 is in specifying a [[Stropping (syntax)|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'▼
▲
▲A common use of pragmats in ALGOL 68 is in specifying a [[Stropping (syntax)|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:
{{sxhl|2=man|1=
}}
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 <code>#pragma</code>, directives to the compiler itself. They are also used to some degree in more modern languages; see below.
==Other languages==
* In [[Ada (programming language)|Ada]], compiler directives are called '''pragmas''' (short for "pragmatic information").
* In [[Common Lisp]], directives are called '''declarations''', and are specified using the <code>declare</code> construct (also <code>proclaim</code> or <code>declaim</code>).{{sfn|Steele|1990|loc=[
* In [[Turbo Pascal]], directives are called '''significant comments''', because in the language [[grammar]] they follow the same syntax as [[comment (computer programming)|comment]]s. 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 <code>#include "file"</code> directive is the significant comment <code>{$I "file"}</code>.
* In [[Perl]], the [[keyword (computer programming)|keyword]] "<code>[http://perldoc.perl.org/functions/use.html use]</code>", which imports modules, can also be used to specify directives, such as <code>use strict;</code> or <code>use utf8;</code>.
*
* [[PHP]] uses the directive <code>declare(strict_types=1)</code>.
* In [[PL/I]], directives begin with a [[Percent sign]] (<code>%</code>) and end with a semicolon (<code>;</code>), e.g., <code>%INCLUDE ''foo'';</code>, <code>%NOPRINT;</code>, <code>%PAGE;</code>, <code>%POP;</code>, <code>%SKIP;</code>, the same as with preprocessor statements.
* [[Python (programming language)|Python]] has two directives – <code>from __future__ import feature</code> (defined in [https://www.python.org/dev/peps/pep-0236/ PEP 236 -- Back to the __future__]), which changes language features (and uses the existing module import syntax, as in Perl), and the <code>coding</code> directive (in a comment) to specify the encoding of a source code file (defined in [https://www.python.org/dev/peps/pep-0263/ PEP 263 -- Defining Python Source Code Encodings]). A more general directive statement was proposed and rejected in [https://www.python.org/dev/peps/pep-0244/ PEP 244 -- The `directive' statement]; these all date to 2001.
* [[ECMAScript]] also adopts the <code>use</code> syntax for directives, with the difference that pragmas are declared as string literals (e.g. <code>"use strict";</code>, or <code>"use asm";</code>), rather than a function call.
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** <code>Option Infer On|Off</code> - When on enables the compiler to infer the type of local variables from their initializers.
* In [[Ruby (programming language)|Ruby]], interpreter directives are referred to as '''pragmas''' and are specified by top-of-file comments that follow a <code>key: value</code> notation. For example, <code>coding: UTF-8</code> indicates that the file is encoded via the [[UTF-8]] [[character encoding]].
* In [[C Sharp (programming language)|C#]], compiler directives are called pre-processing directives. C# does not technically handle these using a preprocessor, but rather directly in the code. There are a number of different compiler directives, which mostly align with those from C and C++, including <code>#pragma</code>, which is specifically used to control compiler warnings and debugger checksums.<ref>{{Cite web|url=https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/language-specification/lexical-structure|title=Lexical structure - C# language specification|last=dotnet-bot|website=docs.microsoft.com|language=en-us|access-date=2019-11-01}}</ref><ref>{{Cite web|url=https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/preprocessor-directives/preprocessor-pragma|title=#pragma - C# Reference|last=BillWagner|website=docs.microsoft.com|language=en-us|access-date=2019-11-01}}</ref> C# also features some directives not used in C or C++, including <code>#nullable</code> and <code>#region</code>. C# also does not allow function-like macros, but does allow regular macros, for purposes such as conditional compilation.
* The [[SQLite]] [[DBMS]] includes a PRAGMA directive that is used to introduce commands that are not compatible with other DBMS.<ref>{{cite web |title=Pragma statements supported by SQLite |url=https://www.sqlite.org/pragma.html |website=www.sqlite.org}}</ref>
* In [[Solidity]], compiler directives are called pragmas, and are specified using the `pragma` keyword.<ref>{{Cite web |title=Layout of a Solidity Source File — Solidity 0.8.27 documentation |url=https://docs.soliditylang.org/en/latest/layout-of-source-files.html#pragmas |access-date=2024-06-03 |website=docs.soliditylang.org}}</ref>
===Assembly language===
* In [[assembly language]], directives, also referred to as pseudo-operations or "pseudo-ops", generally specify such information as the target machine, mark separations between code sections,
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===PL/SQL===
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| first2 = Bill
| title = Oracle PL/SQL Programming
| date = 23 January 2014
| url = https://books.google.com/books?id=vQaoAgAAQBAJ
| edition = 6
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| publication-date = 2014
| isbn = 9781449324414
|
| quote = PL/SQL has a PRAGMA keyword with the following syntax: PRAGMA ''instruction_to_compiler''; [...] PL/SQL offers several pragmas [...]
}}
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==See also==
*
==
{{reflist}}
==References==
{{refbegin}}
* {{cite book | isbn = 1-55558-041-6 | title = [[Common Lisp the Language]] | last = Steele | first = Guy L. |
{{refend}}
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* [http://www.openmp.org OpenMP Website]
* [http://www.openacc.org OpenACC Website]
* [
{{DEFAULTSORT:Directive (Programming)}}
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