GCC directory option -isystem

Question

From this link: http://gcc.gnu.org/onlinedocs/gcc/Directory-Options.html

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.

What is the way to run this?

[14:45:37 Wed Apr 27] ~/junkPrograms/src  $gcc hello.c -isystem -I ../include/
../include/: file not recognized: Is a directory
collect2: ld returned 1 exit status
[14:45:42 Wed Apr 27] ~/junkPrograms/src  $gcc hello.c -I isystem ../include/
../include/: file not recognized: Is a directory
collect2: ld returned 1 exit status
[14:45:57 Wed Apr 27] ~/junkPrograms/src  $

and does this mean that if -isystem is attached, the dir will be given the precedence of a normal system dir?

Answer 1

The documentation says:

-isystem dir

Search dir for header files, after all directories specified by -I but before the standard system directories. Mark it as a system directory, so that it gets the same special treatment as is applied to the standard system directories. If dir begins with =, then the = will be replaced by the sysroot prefix; see --sysroot and -isysroot.

So you're using it wrong. You need to specify a directory for the -isystem option itself, it doesn't work like a "modifier" of the -I option like you seem to be trying.

I believe your command should be:

$ gcc -isystem ../include hello.c

System headers get special treatment w.r.t. warnings (since they are read only, and some cannot be written in strictly conforming code)