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about variable ac_link and gcc

From: Changqing Li
Subject: about variable ac_link and gcc
Date: Thu, 20 May 2021 15:24:25 +0800
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0


I met a problem with variable ac_link,  it does not work in some condition.

in,   there is a line:  AC_CHECK_FUNCS(xdr_uint32 xdr_uint32_t xdr_u_int32_t)

And here is ac_link:

ac_cpp='$CPP $CPPFLAGS'
ac_compile='$CC -c $CFLAGS $CPPFLAGS conftest.$ac_ext >&5'
ac_link='$CC -o conftest$ac_exeext $CFLAGS $CPPFLAGS $LDFLAGS conftest.$ac_ext $LIBS >&5'

In my build env, that is  ""

from glibc 2.27 rpc support is dropped, so linked to -ltirpc. And I expect that xdr_uint32_t  is found,

ac_cv_func_xdr_uint32_t=yes.  but during configure, I got ac_cv_func_xdr_uint32_t=no.

According to gcc doc,, the  link order will influence

link result.  As I understand,   according to this doc,  function xdr_uint32_t in conftest.c cannot be loaded

since -ltirpc is before conftest.c.

So I tried "gcc conftest.c -o conftest  -ltirpc", then it works well, and contest can be generated.

gcc -v:

Using built-in specs.
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu 10.3.0-1ubuntu1' --with-bugurl=file:///usr/share/doc/gcc-10/README.Bugs --enable-languages=c,ada,c++,go,brig,d,fortran,objc,obj-c++,m2 --prefix=/usr --with-gcc-major-version-only --program-suffix=-10 --program-prefix=x86_64-linux-gnu- --enable-shared --enable-linker-build-id --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --libdir=/usr/lib --enable-nls --enable-bootstrap --enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes --with-default-libstdcxx-abi=new --enable-gnu-unique-object --disable-vtable-verify --enable-plugin --enable-default-pie --with-system-zlib --enable-libphobos-checking=release --with-target-system-zlib=auto --enable-objc-gc=auto --enable-multiarch --disable-werror --with-arch-32=i686 --with-abi=m64 --with-multilib-list=m32,m64,mx32 --enable-multilib --with-tune=generic --enable-offload-targets=nvptx-none=/build/gcc-10-gDeRY6/gcc-10-10.3.0/debian/tmp-nvptx/usr,amdgcn-amdhsa=/build/gcc-10-gDeRY6/gcc-10-10.3.0/debian/tmp-gcn/usr,hsa --without-cuda-driver --enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu --with-build-config=bootstrap-lto-lean --enable-link-mutex
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 10.3.0 (Ubuntu 10.3.0-1ubuntu1)

And idea how to reslove above problem?

Another confuse problem is  in my another build environment, which gcc is different,  AC_CHECK_FUNCS

works well no matter -ltirpc is before or after conftest.c.


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