GLSL defines a subset of the [[C preprocessor]] (CPP), combined with its own special directives for specifying versions and OpenGL extensions. The parts removed from CPP are those relating to file names such as {{code|#include}} and {{code|__FILE__}}.<ref>{{cite book |title=OpenGL Programming Guide: The Official Guide to Learning OpenGL, Version 4.3, Eighth Edition |url=https://www.oreilly.com/library/view/opengl-programming-guide/9780132748445/ch02lev2sec5.html |language=en |chapter=Shader Preprocessor}}</ref>
The {{code|GL_ARB_shading_language_include}} extension<ref>{{cite web |title=ARB_shading_language_include |url=https://www.khronos.org/registry/OpenGL/extensions/ARB/ARB_shading_language_include.txt |website=Khronos.org |access-date=2020-05-31}}</ref> (implemented for example in Nvidia drivers<ref>{{cite web |title=NVIDIA driver 265.90 WHQL Quadro |url=https://forums.laptopvideo2go.com/topic/27639-nvidia-driver-26590-whql-quadro/?tab=comments#comment-134520 |websitearchive-url=LaptopVideo2Gohttps://web.archive.org/web/20210308165926/https://forums.laptopvideo2go.com/topic/27639-nvidia-driver-26590-whql-quadro/?tab=comments#comment-134520 Forums }}{{Dead|url-status=dead link|archive-date=NovemberMarch 20248, |bot=InternetArchiveBot2021 |fix-attemptedwebsite=yesLaptopVideo2Go Forums }}</ref> on Windows and Linux, and all Mesa 20.0.0<ref>{{cite web |title=Mesa 20.0.0 Release Notes / 2020-02-19 |url=https://www.mesa3d.org/relnotes/20.0.0.html |website=www.mesa3d.org |access-date=2020-05-31}}</ref> drivers on Linux, FreeBSD and Android) implements ability to use {{code|#include}} in source code, allowing easier sharing of code and definitions between many shaders without extra manual pre-processing. Similar extension {{code|GL_GOOGLE_include_directive}} and {{code|GL_GOOGLE_cpp_style_line_directive}} exist for using GLSL with Vulkan, and are supported in reference SPIR-V compiler ({{code|glslang}} aka glslangValidator).<ref>{{cite web |title=#include directive support by antiagainst · Pull Request #46 · KhronosGroup/glslang |url=https://github.com/KhronosGroup/glslang/pull/46 |website=GitHub |access-date=2020-05-31 |language=en}}</ref><ref>{{cite web |title=Preprocessing line number handling by antiagainst · Pull Request #38 · KhronosGroup/glslang |url=https://github.com/KhronosGroup/glslang/pull/38 |website=GitHub |language=en}}</ref><ref>{{cite web |title=Extend the syntax of #line and __FILE__ to support filename strings by antiagainst · Pull Request #43 · KhronosGroup/glslang |url=https://github.com/KhronosGroup/glslang/pull/43 |website=GitHub |language=en}}</ref>
