[ovs-discuss] Enabling user defined preprocessing definitions during compilation of OVS 1.2.2

Ben Pfaff blp at nicira.com
Sat Nov 12 15:32:50 UTC 2011

>From the Automake manual:

File: automake-1.11.info,  Node: Usage of Conditionals,  Next: Limits of Conditionals,  Up: Conditionals

20.1 Usage of Conditionals

Before using a conditional, you must define it by using
`AM_CONDITIONAL' in the `configure.ac' file (*note Macros::).

     The conditional name, CONDITIONAL, should be a simple string
     starting with a letter and containing only letters, digits, and
     underscores.  It must be different from `TRUE' and `FALSE' that
     are reserved by Automake.

     The shell CONDITION (suitable for use in a shell `if' statement)
     is evaluated when `configure' is run.  Note that you must arrange
     for _every_ `AM_CONDITIONAL' to be invoked every time `configure'
     is run.  If `AM_CONDITIONAL' is run conditionally (e.g., in a
     shell `if' statement), then the result will confuse `automake'.

   Conditionals typically depend upon options that the user provides to
the `configure' script.  Here is an example of how to write a
conditional that is true if the user uses the `--enable-debug' option.

     [  --enable-debug    Turn on debugging],
     [case "${enableval}" in
       yes) debug=true ;;
       no)  debug=false ;;
       *) AC_MSG_ERROR([bad value ${enableval} for --enable-debug]) ;;
     AM_CONDITIONAL([DEBUG], [test x$debug = xtrue])

   Here is an example of how to use that conditional in `Makefile.am':

     if DEBUG
     DBG = debug
     DBG =
     noinst_PROGRAMS = $(DBG)

   This trivial example could also be handled using `EXTRA_PROGRAMS'
(*note Conditional Programs::).

   You may only test a single variable in an `if' statement, possibly
negated using `!'.  The `else' statement may be omitted.  Conditionals
may be nested to any depth.  You may specify an argument to `else' in
which case it must be the negation of the condition used for the
current `if'.  Similarly you may specify the condition that is closed
on the `endif' line:

     if DEBUG
     DBG = debug
     else !DEBUG
     DBG =
     endif !DEBUG

Unbalanced conditions are errors.  The `if', `else', and `endif'
statements should not be indented, i.e., start on column one.

   The `else' branch of the above two examples could be omitted, since
assigning the empty string to an otherwise undefined variable makes no

   In order to allow access to the condition registered by
`AM_CONDITIONAL' inside `configure.ac', and to allow conditional
`AC_CONFIG_FILES', `AM_COND_IF' may be used:

     If CONDITIONAL is fulfilled, execute IF-TRUE, otherwise execute
     IF-FALSE.  If either branch contains `AC_CONFIG_FILES', it will
     cause `automake' to output the rules for the respective files only
     for the given condition.

   `AM_COND_IF' macros may be nested when m4 quotation is used properly
(*note M4 Quotation: (autoconf)M4 Quotation.).

   Here is an example of how to define a conditional config file:

     AM_CONDITIONAL([SHELL_WRAPPER], [test "x$with_wrapper" = xtrue])

On Sat, Nov 12, 2011 at 12:01:04PM +0530, Ramana Reddy wrote:
> Thank you very much Ben for your reply. I have one more doubt regarding how
> to add our own
> files( Assume that it is in the ovs/lib directory and the files are
> dpif-mine.c and dpif-mine.h) to
> automake.mk file in ovs/lib directory without adding immediately after the
> existing ovs/lib files.
> I mean, is it possible to write some thing like below in automake.mk file,
> after adding AC_DEFINE([Ramana], [1], [Enable Ramana's extensions.]) to
> configure.ac, so that it controls both modified code in the existing files,
> as well
> as the newly added files to OVS.
> if Ramana
> lib_libopenvswitch_a_SOURCES += \
>                    lib/dpif-mine.c \
>                    lib/dpif-mine.h
> endif
> Thanks,
> Ramana Reddy.
> On Fri, Nov 11, 2011 at 10:29 PM, Ben Pfaff <blp at nicira.com> wrote:
> > On Fri, Nov 11, 2011 at 03:35:00PM +0530, Ramana Reddy wrote:
> > > I modified some code in OVS-1.2.2 both in user space and kernel
> > > space with my own preprocessing directives some thing like:
> > > #ifdef Ramana
> > > /* my code is here */
> > > #endif.
> > >
> > > Now I want to enable my code during the compilation of OVS-1.2.2.
> > > This can be done by defining #define ramana in the header file and
> > > adding this header file in the respective places can solve the problem.
> > > But some how I do not like to add this header file in all the places.
> > > How can we enable this preprocessor directive with out adding a header
> > file.
> > > Can we pass this one as a parameter to gcc like gcc -Dramana during
> > > compilation.
> > > if yes, where I should modify in the configuration file or some other
> > file.
> >
> > Add AC_DEFINE([Ramana], [1], [Enable Ramana's extensions.])
> > to configure.ac.
> >

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