[Buildroot] AC_CHECK_LIB doesn't work for cross-compiling

[Baruch Siach <baruch@tkos.co.il>]

Hi adam,

On Thu, Dec 25, 2014 at 03:27:40PM +0900, Adam Jiang wrote:
> Here is what I got from grep in config.log.
> 
> $ grep -i FCGX_InitRequest output/build/visage-master/config.log
> configure:5626: checking for FCGX_InitRequest in -lfcgi
> | char FCGX_InitRequest ();
> | return FCGX_InitRequest ();
> configure:5689: WARNING: unable to find the FCGX_InitRequest() function
> ac_cv_lib_fcgi_FCGX_InitRequest=no
> 
> I have also attached 'config.log' as well.

The interesting part in your config.log is

	libfcgi.so: undefined reference to `frexp'

This means that libfcgi.so depends on the math library (libm), but you didn't 
add it to the compile command line (-lm). Please try adding -lm to the 
AC_CHECK_LIB macro OTHER-LIBRARIES parameter as follows:

	AC_CHECK_LIB([fcgi], [FCGX_InitRequest], [LIBS="$LIBS -lfcgi],
		[AC_MSG_WARN("unable to find the FCGX_InitRequest() function")],
		[-lm])

baruch

> On 12/25/2014 01:47 PM, Baruch Siach wrote:
> > On Thu, Dec 25, 2014 at 01:34:59PM +0900, Adam Jiang wrote:
> >> I am trying to add a new package to buildroot. the package is built upon
> >> autotools. thus, it is supposed to be very easy to make it
> >> cross-compilable. ever, i got suck at AC_CHECK_LIB macro. it doesn't
> >> work well at cross-compiling while it does work on normal compiling.
> >>
> >> AC_CHECK_LIB([fcgi], [FCGX_InitRequest], [AC_MSG_WARN("unable to find
> >> the FCGX_InitRequest() function")])
> >>
> >> always failed even though I do have libfcgi in my configuration. The
> >> error message looks like
> >>
> >> checking for FCGX_InitRequest in -lfcgi... no
> >> configure: WARNING: unable to find the FCGX_InitRequest() function
> >>
> >> I don't think AC_CHECK_LIB will not work on cross-compiling because it
> >> seems there are lots of other target packages in buildroot using the
> >> macro. Most probably, I am doing something wrong.
> >>
> >> Does AC_CHECK_LIB work in buildroot? If the macro needs some changes,
> >> how could I do it?
> > What do you have in config.log for the FCGX_InitRequest check?

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