GCC looks in several different places for headers. On a normal Unix
system, if you do not instruct it otherwise, it will look for headers
/usr/local/include libdir/gcc/target/version/include /usr/target/include /usr/include
For C++ programs, it will also look in libdir/../include/c++/version, first. 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
first in the directory containing the current file, then in the
directories as specified by -iquote options, then in the same
places it would have looked for a header requested with angle
brackets. For example, if /usr/include/sys/stat.h contains
#include "types.h", GCC looks for types.h first in
/usr/include/sys, then in its usual search path.
‘#line’ (see Line Control) does not change GCC's idea of the directory containing the current file.
You may put -I- at any point in your list of -I options. This has two effects. First, directories appearing before the -I- in the list are searched only for headers requested with quote marks. Directories after -I- are searched for all headers. Second, the directory containing the current file is not searched for anything, unless it happens to be one of the directories named by an -I switch. -I- is deprecated, -iquote should be used instead.
-I. -I- is not the same as no -I options at all, and does not cause the same behavior for ‘<>’ includes that ‘""’ includes get with no special options. -I. searches the compiler's current working directory for header files. That may or may not be the same as the directory containing the current file.
If you need to look for headers in a directory named -, write -I./-.
There are several more ways to adjust the header search path. They are generally less useful. See Invocation.