Using the GNU Compiler Collection (GCC): Fixed Headers

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### 14.4 Fixed Header Files

GCC needs to install corrected versions of some system header files. This is because most target systems have some header files that won’t work with GCC unless they are changed. Some have bugs, some are incompatible with ISO C, and some depend on special features of other compilers.

Installing GCC automatically creates and installs the fixed header files, by running a program called `fixincludes`. Normally, you don’t need to pay attention to this. But there are cases where it doesn’t do the right thing automatically.

- If you update the system’s header files, such as by installing a new system version, the fixed header files of GCC are not automatically updated. They can be updated using the `mkheaders` script installed in `libexecdir/gcc/target/version/install-tools/`.
- On some systems, header file directories contain machine-specific symbolic links in certain places. This makes it possible to share most of the header files among hosts running the same version of the system on different machine models.

  The programs that fix the header files do not understand this special way of using symbolic links; therefore, the directory of fixed header files is good only for the machine model used to build it.

  It is possible to make separate sets of fixed header files for the different machine models, and arrange a structure of symbolic links so as to use the proper set, but you’ll have to do this by hand.