-I DIR
-iquote DIR
-isystem DIR
-idirafter DIR
Add the directory DIR to the list of directories to be searched for header files during preprocessing. If DIR begins with = or $SYSROOT, then the = or $SYSROOT is replaced by the sysroot prefix; see --sysroot and -isysroot.

Directories specified with -iquote apply only to the quote form of the directive, #include "FILE". Directories specified with -I, -isystem, or -idirafter apply to lookup for both the #include "FILE" and #include <FILE> directives.

You can specify any number or combination of these options on the command line to search for header files in several directories. The lookup order is as follows:

 1. For the quote form of the include directive, the directory of the current file is searched first.

 2. For the quote form of the include directive, the directories specified by -iquote options are searched in left-to-right order, as they appear on the command line.

 3. Directories specified with -I options are scanned in left-to-right order.

 4. Directories specified with -isystem options are scanned in left-to-right order.

 5. Standard system directories are scanned.

 6. Directories specified with -idirafter options are scanned in left-to-right order.

You can use -I to override a system header file, substituting your own version, since these directories are searched before the standard system header file directories. However, you should not use this option to add directories that contain vendor-supplied system header files; use -isystem for that.

The -isystem and -idirafter options also mark the directory as a system directory, so that it gets the same special treatment that is applied to the standard system directories.

If a standard system include directory, or a directory specified with -isystem, is also specified with -I, the -I option is ignored.  The directory is still searched but as a system directory at its normal position in the system include chain. This is to ensure that GCC's procedure to fix buggy system headers and the ordering for the #include_next directive are not inadvertently changed. If you really need to change the search order for system directories, use the -nostdinc and/or -isystem options.

-I-
Split the include path. This option has been deprecated.  Please use -iquote instead for -I directories before the -I- and remove the -I- option.

Any directories specified with -I options before -I- are searched only for headers requested with #include "FILE"; they are not searched for #include <FILE>.  If additional directories are specified with -I options after the -I-, those directories are searched for all #include directives.

In addition, -I- inhibits the use of the directory of the current file directory as the first search directory for #include "FILE".  There is no way to override this effect of -I-.

-iprefix PREFIX
Specify PREFIX as the prefix for subsequent -iwithprefix options.  If the prefix represents a directory, you should include the final /.

-iwithprefix DIR
-iwithprefixbefore DIR
Append DIR to the prefix specified previously with -iprefix, and add the resulting directory to the include search path.  -iwithprefixbefore puts it in the same place -I would; -iwithprefix puts it where -idirafter would.

-isysroot DIR
This option is like the --sysroot option, but applies only to header files (except for Darwin targets, where it applies to both header files and libraries).  See the --sysroot option for more information.

-imultilib DIR
Use DIR as a subdirectory of the directory containing target-specific C++ headers.

-nostdinc
Do not search the standard system directories for header files. Only the directories explicitly specified with -I, -iquote, -isystem, and/or -idirafter options (and the directory of the current file, if appropriate) are searched.

-nostdinc++
Do not search for header files in the C++-specific standard directories, but do still search the other standard directories.  (This option is used when building the C++ library.)