These options specify directories to search for header files, for libraries and for parts of the compiler:
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.
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 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 the -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 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 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.
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.
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 that is current when the compiler is 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.