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Re: can't find ceil function in math library

From: Ingo Krabbe
Subject: Re: can't find ceil function in math library
Date: Thu, 3 Jul 2014 14:01:59 +0200

Actually it makes not much sense to ask about why autoconf a/o libtool does 
this or that, as the tools themselves are coded beyond understanding or 
readability. They just do on some systems what they are expected to do.

Of course AC_CHECK_LIB([m],[main]), shouldn't return true as main is no member 
of libm. On the other hand AC_CHECK_LIB or other AC macros aren't meant to be 
tested against transzendental usage and it should be quite clear by definition 
that main is not part of the true set of AC_CHECK_LIB([x],main) ∀ (x,main). If 
this returns true its just a trivial true signal, that is to expensive and 
transzendental to get blocked.

> I've changed AC_CHECK_LIB([m], [ceil]) to AC_CHECK_LIB([m], [main]) and it
> works. according to conftest.c, libtool generate a small peace of code to
> check whether the specific function exists or not. for AC_CHECK_LIB([m],
> [main]), the code is:
> main()
> {
>     return main();
>     ;
>     return 0;
> }
> call main itself in gcc will cause a crash, why it can works here?
> On Thu, Jul 3, 2014 at 2:46 PM, Minxuan Zhuang <address@hidden> wrote:
>> hi:
>> I have a file includes:  AC_CHECK_LIB([m], [ceil])
>> then I got this result: checking for ceil in -lm... no
>> but a .c file contains ceil function works very well  with or without -lm,
>> why is that?
>> libtool version: 2.4.2
>> system: Ubuntu 13.04

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