C compiler include file searching (was: per-target vpath alternatives)

[Philip Guenther]

[The topic has shifted, so I've changed the subject]

On 9/10/07, Eli Zaretskii <address@hidden> wrote:
> > Date: Sun, 9 Sep 2007 21:40:38 -0600
> > From: "Philip Guenther" <address@hidden>
> > Cc: address@hidden
> >
> > In my experience, C
> > compilers in UNIX search the directory of the source file, whether or
> > not that directory is named in a -I option.
>
> Only if you say `#include "foo.h"', not `#include <foo.h>'.

Agreed.


> The C standard specifies that "foo.h" is looked for in the current directory
> first.

I don't believe the C standard states that, especially given that
there's no formal concept of "directory", much less "current
directory" in the C standard.  The draft of the C99 I'm looking at
says, in section 6.10.2p3:

    A preprocessing directive of the form
         # include "q-char-sequence" new-line
    causes the replacement of that directive by the entire contents of
    the source file identified by the specified sequence between the "
    delimiters. The named source file is searched for in an
    implementation-defined manner. If this search is not supported,
    or if the search fails, the directive is reprocessed as if it read
        # include <h-char-sequence> new-line
    with the identical contained sequence (including > characters, if any)
    from the original directive.


The Single Unix Specification *does* specify the behavior I described.
 To quote http://www.opengroup.org/onlinepubs/000095399/utilities/c99.html

    -I  directory
        Change the algorithm for searching for headers whose names are
        not absolute pathnames to look in the directory named by the
        directory pathname before looking in the usual places.  Thus,
        headers whose names are enclosed in double-quotes ("") shall
        be searched for first in the directory of the file with the
        #include line, then in directories named in -I options, and
        last in the usual places. For headers whose names are enclosed
        in angle brackets ("<>"), the header shall be searched for
        only in directories named in -I options and then in the usual
        places. Directories named in -I options shall be searched in
        the order specified. Implementations shall support at least
        ten instances of this option in a single c99 command invocation.

Note that the first place looked is *NOT* the current directory but
rather the directory containing the source file.  That distinction is
important to many makefiles that build source from multiple
directories and should not be overlooked.


Philip Guenther