computer programming Computer programming or coding is the composition of sequences of instructions, called computer program, programs, that computers can follow to perform tasks. It involves designing and implementing algorithms, step-by-step specifications of proc ...

, a prefix header is a feature found in some C or

C++ C++ (, pronounced "C plus plus" and sometimes abbreviated as CPP or CXX) is a high-level, general-purpose programming language created by Danish computer scientist Bjarne Stroustrup. First released in 1985 as an extension of the C programmin ...

compilers In computing, a compiler is a computer program that translates computer code written in one programming language (the ''source'' language) into another language (the ''target'' language). The name "compiler" is primarily used for programs tha ...

used to ensure that a certain snippet of code is inserted at the beginning of every file.

Overview

In the C and

programming languages, a

header file An include directive instructs a text file processor to replace the directive text with the content of a specified file. The act of including may be logical in nature. The processor may simply process the include file content at the location of ...

is a

file File or filing may refer to: Mechanical tools and processes * File (tool), a tool used to remove fine amounts of material from a workpiece. ** Filing (metalworking), a material removal process in manufacturing ** Nail file, a tool used to gen ...

whose text is included in another source file by the

compiler In computing, a compiler is a computer program that Translator (computing), translates computer code written in one programming language (the ''source'' language) into another language (the ''target'' language). The name "compiler" is primaril ...

, usually by the use of

compiler directive 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 ...

s at the beginning of the source file. A prefix header differs from a normal header file in that it is ''automatically'' included at the beginning of every source file by the compiler, without the use of any compiler directives. Prefix headers are usually pre-compiled in order to reduce compilation times. Use of prefix headers outside of this purpose can make your code more difficult to maintain & less re-usable. Prefix headers can also be used for cross-platform support. On *NIX systems, it is common to have a config.h header file generated at build time (via something like

autoconf GNU Autoconf is a software development tool for generating a configure script that in turn generates files for building a codebase and for packaging or installing the resulting files. Autoconf is part of the GNU Build System along with Autom ...

) that describes the capabilities of the system. However, when using certain build systems such as Visual Studio or Xcode, this config.h may be unavailable. One technique to solve this is to have HAVE_CONFIG_H be a pre-defined macro in the build-system that generates a config.h so that code knows whether it needs to #include config.h (& is safe for use by build systems that do not have it). An alternative, would be for the build system to add config.h as a prefix header instead of defining HAVE_CONFIG_H. Of course the downside is that this header will be added to every compilation unit, not just the ones that include it explicitly.

Example

macOS macOS, previously OS X and originally Mac OS X, is a Unix, Unix-based operating system developed and marketed by Apple Inc., Apple since 2001. It is the current operating system for Apple's Mac (computer), Mac computers. With ...

, the

Xcode Xcode is a suite of developer tools for building apps on Apple devices. It includes an integrated development environment (IDE) of the same name for macOS, used to develop software for macOS, iOS, iPadOS, watchOS, tvOS, and visionOS. It w ...

build system generates prefix headers automatically for new projects. A new

Cocoa Cocoa may refer to: Chocolate * Chocolate * ''Theobroma cacao'', the cocoa tree * Cocoa bean, seed of ''Theobroma cacao'' * Chocolate liquor, or cocoa liquor, pure, liquid chocolate extracted from the cocoa bean, including both cocoa butter and ...

project, for instance, gets a prefix header that looks like this: #ifdef __OBJC__ #import #endif As a result, explicit includes of the above header files in any

Objective-C Objective-C is a high-level general-purpose, object-oriented programming language that adds Smalltalk-style message passing (messaging) to the C programming language. Originally developed by Brad Cox and Tom Love in the early 1980s, it was ...

code file do not imply a second inclusion because of the #import directive of Objective-C, or more generally with #include because of the use of

include guard In the C and C++ programming languages, an #include guard, sometimes called a macro guard, header guard or file guard, is a way to avoid the problem of ''double inclusion'' when dealing with the include directive. The C preprocessor processe ...

s; hence, these includes can be forgotten, but it is advocated to have them explicitly written in order to keep the source code autonomous and reusable, and make the library dependencies clear. Similar prefix headers are generated for other types of project.

References

{{Reflist

External links

Xcode User Guide: Using a Precompiled Prefix Header
Source code C (programming language) headers C++ Articles with example Objective-C code

Overview

Example

See also

References

External links