4.20. Using Precompiled Headers

Often large projects have many header files that are included in every source file. The time the compiler takes to process these header files over and over again can account for nearly all of the time required to build the project. To make builds faster, GCC allows users to `precompile' a header file; then, if builds can use the precompiled header file they will be much faster.

Caution: There are a few known situations where GCC will crash when trying to use a precompiled header. If you have trouble with a precompiled header, you should remove the precompiled header and compile without it. In addition, please use GCC's on-line defect-tracking system to report any problems you encounter with precompiled headers.

To create a precompiled header file, simply compile it as you would any other file, if necessary using the -x option to make the driver treat it as a C or C++ header file. You will probably want to use a tool like make to keep the precompiled header up-to-date when the headers it contains change.

A precompiled header file will be searched for when #include is seen in the compilation. As it searches for the included file () the compiler looks for a precompiled header in each directory just before it looks for the include file in that directory. The name searched for is the name specified in the #include with .gch appended. If the precompiled header file can't be used, it is ignored.

For instance, if you have #include "all.h", and you have all.h.gch in the same directory as all.h, then the precompiled header file will be used if possible, and the original header will be used otherwise.

Alternatively, you might decide to put the precompiled header file in a directory and use -I to ensure that directory is searched before (or instead of) the directory containing the original header. Then, if you want to check that the precompiled header file is always used, you can put a file of the same name as the original header in this directory containing an #error command.

This also works with -include. So yet another way to use precompiled headers, good for projects not designed with precompiled header files in mind, is to simply take most of the header files used by a project, include them from another header file, precompile that header file, and -include the precompiled header. If the header files have guards against multiple inclusion, they will be skipped because they've already been included (in the precompiled header).

If you need to precompile the same header file for different languages, targets, or compiler options, you can instead make a directory named like all.h.gch, and put each precompiled header in the directory. (It doesn't matter what you call the files in the directory, every precompiled header in the directory will be considered.) The first precompiled header encountered in the directory that is valid for this compilation will be used; they're searched in no particular order.

There are many other possibilities, limited only by your imagination, good sense, and the constraints of your build system.

A precompiled header file can be used only when these conditions apply:

• Only one precompiled header can be used in a particular compilation.

• A precompiled header can't be used once the first C token is seen. You can have preprocessor directives before a precompiled header; you can even include a precompiled header from inside another header, so long as there are no C tokens before the #include.

• The precompiled header file must be produced for the same language as the current compilation. You can't use a C precompiled header for a C++ compilation.

• The precompiled header file must be produced by the same compiler version and configuration as the current compilation is using. The easiest way to guarantee this is to use the same compiler binary for creating and using precompiled headers.

• Any macros defined before the precompiled header (including with -D) must either be defined in the same way as when the precompiled header was generated, or must not affect the precompiled header, which usually means that the they don't appear in the precompiled header at all.

• Certain command-line options must be defined in the same way as when the precompiled header was generated. At present, it's not clear which options are safe to change and which are not; the safest choice is to use exactly the same options when generating and using the precompiled header.

For all of these but the last, the compiler will automatically ignore the precompiled header if the conditions aren't met. For the last item, some option changes will cause the precompiled header to be rejected, but not all incompatible option combinations have yet been found.