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### 3.14 Options for Directory Search

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

`-I` dir
  Add the directory dir to the head of the list of directories to be searched for header files. This can be used to override a system header file, substituting your own version, since these directories are searched before the 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). If you use more than one `-I` option, the directories are scanned in left-to-right order; the standard system directories come after.

  If a standard system include directory, or a directory specified with `-isystem`, is also specified with `-I`, the `-I` option will be ignored. The directory will still be 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.

`-iplugindir=`dir
  Set the directory to search for plugins which 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.
`-iquote` dir
  Add the directory dir to the head of the list of directories to be searched for header files only for the case of ‘`#include "file"`’; they are not searched for ‘`#include <file>`’, otherwise just like `-I`.
`-L` dir
  Add directory dir to the list of directories to be searched for `-l`.
`-B` prefix
  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/`’ (see Target Options).

  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` was not specified, the driver tries two standard prefixes, which are `/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 will check to see if the path provided by the `-B` refers to a directory, and if necessary it will add 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 includes 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 run-time 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 will be replaced by `[dir/]include`. This is to help with boot-strapping the compiler.

`-specs=`file
  Process file after the compiler reads in the standard `specs` file, in order to override the defaults that the `gcc` driver program uses when determining what switches to pass to `cc1`, `cc1plus`, `as`, `ld`, etc. More than one `-specs=file` can be specified on the command line, and they are processed in order, from left to right.
`--sysroot=`dir
  Use dir as the logical root directory for headers and libraries. For example, if the compiler would normally search for headers in `/usr/include` and libraries in `/usr/lib`, it will instead search `dir/usr/include` and `dir/usr/lib`.

  If you use both this option and the `-isysroot` option, then the `--sysroot` option will apply to libraries, but the `-isysroot` option will apply 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` will still work, but the library aspect will not.

`-I-`
  This option has been deprecated. Please use `-iquote` instead for `-I` directories before the `-I-` and remove the `-I-`. Any directories you specify with `-I` options before the `-I-` option are searched only for the case of ‘`#include "file"`’; they are not searched for ‘`#include <file>`’.

  If additional directories are specified with `-I` options after the `-I-`, these directories are searched for all ‘`#include`’ directives. (Ordinarily all `-I` directories are used this way.)

  In addition, the `-I-` option inhibits the use of the current directory (where the current input file came from) as the first search directory for ‘`#include "file"`’. There is no way to override this effect of `-I-`. With `-I.` you can specify searching the directory which was current when the compiler was invoked. That is not exactly the same as what the preprocessor does by default, but it is often satisfactory.

  `-I-` does not inhibit the use of the standard system directories for header files. Thus, `-I-` and `-nostdinc` are independent.