CCache/debian/README.Debian

Installing ccache
-----------------

The recommended way to use this with Debian is to either create "cc"
and "gcc" symlinks to /usr/bin/ccache in your private bin directory
(which must be before the real cc and gcc in your path), or use
CC="ccache gcc" on the make command line.

Another option is to just prepend /usr/lib/ccache in your PATH
environment variable, like

  export PATH="/usr/lib/ccache:$PATH"

Note that ccache works with both native and cross compilers.

Ignoring whitespace
-------------------

If you wish to set up ccache so that it ignores blank lines, have a
look at the CCACHE_UNIFY option.  However, please note that this
option is off by default since the reported line numbers may not
match the source files anymore.


NFS Issues
----------

(from John Coiner <john.coiner@amd.com> on the ccache mailing list)

When CCache creates a hardlinked output file, it calls utime() to update
the timestamp on the object, so that Make realizes that the object has
changed.

On NFS, utime() has no coherency guarantee, AFAIK. When utime() runs on
host A, and our parallel implementation of Make is running on host B,
sometimes Make doesn't see the new timestamp soon enough -- and neglects
to relink the final binary. That's a one-way ticket to Silent Mysterious
Failure Town.

Instead of relying on the object file timestamp, we create a dummy file
with a reliable timestamp:

objs/foo.o objs/foo.o.built :
        if ( ccache gcc -o foo.o -c foo.c ) ; \
        then touch objs/foo.o.built ; \
        else exit 1; \
        fi

binary : objs/foo.o.built
        gcc -o binary objs/foo.o

NFS does make a coherency guarantee, that if a file is written and
close()d on host A, and subsequently open()ed on host B, that the second
open() will reflect all modifications and attributes from the close().
Since Make does open() when checking timestamps, and the dummy file is
close()d when it's created, the binary will always relink after the
object is recompiled.

 -- Francois Marier <francois@debian.org>  Sun, 20 May 2007 17:35:36 +1200