Re: Usage of "eval" inside if

[Garrett Cooper]

On Mon, Dec 1, 2008 at 9:55 PM, Paul Smith <address@hidden> wrote:
> On Tue, 2008-12-02 at 11:12 +0530, kalyan wrote:
>> I am trying to *conditionally* define/change  some makefile variables
>> using eval. Is there a way to acheive this?
>> The condition here was trivial ( if [ 1 = 0 ] ), but in reality
>> ofcourse it would be non-trivial.
>
> Before you can do anything remotely complex with make you have to
> completely understand the distinction between makefile code and shell
> code.  Until you truly internalize this critical difference you will not
> have success.
>
> Make will fully evaluate every recipe.  Then it will invoke a shell and
> pass the fully-evaluated recipe as a string of commands to it.  The
> shell will operate on that string of commands and at the end it will
> return an exit code.  If that code is 0, the recipe succeeded and make
> continues with the next recipe.  If the code is non-0 the recipe failed
> and make will stop (modulo various ways to avoid this).
>
> That's it.  That's the only interaction there is.
>
>
> Nothing the shell does can EVER change ANYTHING inside make.  You cannot
> mix and match shell if-statements with makefile $(eval ...), for
> example.  The entire string, including ALL variables and $(eval ...)
> functions (and any other functions) will be fully expanded before the
> shell gets anywhere near the set of commands, so putting an $(eval ...)
> inside a shell if-statement is completely useless, from the standpoint
> of only expanding the $(eval ...) conditionally.
>
> If you want to do something in a makefile conditionally, you have to use
> a makefile conditional, such as $(if ...), $(or ...), $(and ...), etc.

If he's worried about conditionally executing something if a shell
command works, then he should use $(shell && echo "msg") combined with
simple conditionals via ifneq or ifeq looking for `msg'.

Longterm, if you're concerned about conditionally compiling things
though, or dealing with conditional compilation / definitions for
things, I could definitely look into autoconf.

Cheers,
-Garrett

-Garrett