26.4.3 Search Path

GCC looks in several different places for header files to be included. On the GNU system, and Unix systems, the default directories for system header files are:

libdir/gcc/target/version/include
/usr/local/include
libdir/gcc/target/version/include-fixed
libdir/target/include
/usr/include/target
/usr/include

The list may be different in some operating systems. Other directories are added for C++.

In the above, target is the canonical name of the system GCC was configured to compile code for; often but not always the same as the canonical name of the system it runs on. version is the version of GCC in use.

You can add to this list with the -Idir command-line option. All the directories named by -I are searched, in left-to-right order, before the default directories. The only exception is when dir is already searched by default. In this case, the option is ignored and the search order for system directories remains unchanged.

Duplicate directories are removed from the quote and bracket search chains before the two chains are merged to make the final search chain. Thus, it is possible for a directory to occur twice in the final search chain if it was specified in both the quote and bracket chains.

You can prevent GCC from searching any of the default directories with the -nostdinc option. This is useful when you are compiling an operating system kernel or some other program that does not use the standard C library facilities, or the standard C library itself. -I options are not ignored as described above when -nostdinc is in effect.

GCC looks for headers requested with #include "file" first in the directory containing the current file, then in the quote directories specified by -iquote options, then in the same places it looks for a system header. For example, if /usr/include/sys/stat.h contains #include "types.h", GCC looks for types.h first in /usr/include/sys, then in the quote directories and then in its usual search path.

#line (see Line Control) does not change GCC’s idea of the directory containing the current file.

The -I- is an old-fashioned, deprecated way to specify the quote directories. To look for headers in a directory named -, specify -I./-. There are several more ways to adjust the header search path. See Invoking GCC in Using the GNU Compiler Collection.