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### 3.15 Options for Directory Search

These options specify directories to search for header files, for libraries and for parts of the compiler:

`-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 ‘`=`’, then the ‘`=`’ 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.)

`-iplugindir=dir`

  Set the directory to search for plugins that are passed by `-fplugin=name` instead of `-fplugin=path/name.so`. This option is not meant to be used by the user, but only passed by the driver.

`-Ldir`

  Add directory dir to the list of directories to be searched for `-l`.

`-Bprefix`

  This option specifies where to find the executables, libraries, include files, and data files of the compiler itself.

  The compiler driver program runs one or more of the subprograms `cpp`, `cc1`, `as` and `ld`. It tries prefix as a prefix for each program it tries to run, both with and without ‘`machine/version/`’ for the corresponding target machine and compiler version.

  For each subprogram to be run, the compiler driver first tries the `-B` prefix, if any. If that name is not found, or if `-B` is not specified, the driver tries two standard prefixes, `/usr/lib/gcc/` and `/usr/local/lib/gcc/`. If neither of those results in a file name that is found, the unmodified program name is searched for using the directories specified in your `PATH` environment variable.

  The compiler checks to see if the path provided by `-B` refers to a directory, and if necessary it adds a directory separator character at the end of the path.

  `-B` prefixes that effectively specify directory names also apply to libraries in the linker, because the compiler translates these options into `-L` options for the linker. They also apply to include files in the preprocessor, because the compiler translates these options into `-isystem` options for the preprocessor. In this case, the compiler appends ‘`include`’ to the prefix.

  The runtime support file `libgcc.a` can also be searched for using the `-B` prefix, if needed. If it is not found there, the two standard prefixes above are tried, and that is all. The file is left out of the link if it is not found by those means.

  Another way to specify a prefix much like the `-B` prefix is to use the environment variable `GCC_EXEC_PREFIX`. See Environment Variables.

  As a special kludge, if the path provided by `-B` is `[dir/]stageN/`, where N is a number in the range 0 to 9, then it is replaced by `[dir/]include`. This is to help with boot-strapping the compiler.

`-no-canonical-prefixes`

  Do not expand any symbolic links, resolve references to ‘`/../`’ or ‘`/./`’, or make the path absolute when generating a relative prefix.

`--sysroot=dir`

  Use dir as the logical root directory for headers and libraries. For example, if the compiler normally searches for headers in `/usr/include` and libraries in `/usr/lib`, it instead searches `dir/usr/include` and `dir/usr/lib`.

  If you use both this option and the `-isysroot` option, then the `--sysroot` option applies to libraries, but the `-isysroot` option applies to header files.

  The GNU linker (beginning with version 2.16) has the necessary support for this option. If your linker does not support this option, the header file aspect of `--sysroot` still works, but the library aspect does not.

`--no-sysroot-suffix`

  For some targets, a suffix is added to the root directory specified with `--sysroot`, depending on the other options used, so that headers may for example be found in `dir/suffix/usr/include` instead of `dir/usr/include`. This option disables the addition of such a suffix.

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