Directive (programming)

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In computer programming, a directive or pragma (from "pragmatic") is a language construct that specifies how a compiler (or other translator) should process its input. Depending on the programming language, directives may or may not be part of the grammar of the language and may vary from compiler to compiler. They can be processed by a preprocessor to specify compiler behavior, or function as a form of in-band parameterization.

Contents

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 and may be ignored, but normally they are prescriptive and must be followed. However, a directive does not perform any action in the language itself, but rather only a change in the behavior of the compiler.

This term could be used to refer to proprietary third-party tags and commands (or markup) embedded in code that result in additional executable processing that extend the existing compiler, assembler and language constructs present in the development environment. The term "directive" is also applied in a variety of ways that are similar to the term command.

The C preprocessor

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.

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 (#) with the exception of the keywords export, import and module in C++ (handled by the preprocessor prior to C++26). [1]

History

Directives date to JOVIAL. [2]

COBOL has a COPY directive.

In ALGOL 68, directives 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.

Other languages

Assembly language

PL/SQL

See also

Footnotes

  1. "P1857R1 - Modules Dependency Discovery".
  2. "Chapter 17 - Directives" (PDF). Computer Programming Manual for JOVIAL (J73) Language (PDF) (Technical report). June 1981. pp. 243–263. RADC-TR-81-143. Retrieved May 28, 2023.
  3. Steele 1990, Chapter 9: Declarations, p. 215–237.
  4. "7.20. Pragmas". GHC 7.8.3 Documentation. Retrieved 18 July 2014.
  5. dotnet-bot. "Lexical structure - C# language specification". docs.microsoft.com. Retrieved 2019-11-01.
  6. BillWagner. "#pragma - C# Reference". docs.microsoft.com. Retrieved 2019-11-01.
  7. "Pragma statements supported by SQLite". www.sqlite.org.
  8. "Layout of a Solidity Source File — Solidity 0.8.27 documentation". docs.soliditylang.org. Retrieved 2024-06-03.
  9. Feuerstein, Steven; Pribyl, Bill (23 January 2014). Oracle PL/SQL Programming (6 ed.). O'Reilly Media, Inc. (published 2014). ISBN   9781449324414 . Retrieved 2016-06-16. PL/SQL has a PRAGMA keyword with the following syntax: PRAGMA instruction_to_compiler; [...] PL/SQL offers several pragmas [...]

References