Can't link against libquadmath with g++

Question

This is similar to Link libquadmath with c++ on linux but the extern "C" answer appears outdated based on comments. I did try the answer, but it didn't work.

Code

#include <iostream>
#include <quadmath.h>

int main()
{
    char buffer[1024];
    quadmath_snprintf(buffer, 1023, "%Qe", 5.099q);
    std::cout << buffer << std::endl;
    return 0;
}

Invocation

$ g++ -lquadmath -o float_formatting float_formatting.cpp
/tmp/cc1juozE.o: In function `main':
float_formatting.cpp:(.text+0x3e): undefined reference to `quadmath_snprintf'
collect2: error: ld returned 1 exit status

Based on the output, it's finding libquadmath.so fine, but it's missing the symbol.

g++ Verbose

$ g++ --verbose -lquadmath -o float_formatting float_formatting.cpp
Using built-in specs.
COLLECT_GCC=g++
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/7/lto-wrapper
OFFLOAD_TARGET_NAMES=nvptx-none
OFFLOAD_TARGET_DEFAULT=1
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu 7.4.0-1ubuntu1~18.04.1' --with-bugurl=file:///usr/share/doc/gcc-7/README.Bugs --enable-languages=c,ada,c++,go,brig,d,fortran,objc,obj-c++ --prefix=/usr --with-gcc-major-version-only --program-suffix=-7 --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 --with-sysroot=/ --enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes --with-default-libstdcxx-abi=new --enable-gnu-unique-object --disable-vtable-verify --enable-libmpx --enable-plugin --enable-default-pie --with-system-zlib --with-target-system-zlib --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 --without-cuda-driver --enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu
Thread model: posix
gcc version 7.4.0 (Ubuntu 7.4.0-1ubuntu1~18.04.1)
COLLECT_GCC_OPTIONS='-v' '-o' 'float_formatting' '-shared-libgcc' '-mtune=generic' '-march=x86-64'
 /usr/lib/gcc/x86_64-linux-gnu/7/cc1plus -quiet -v -imultiarch x86_64-linux-gnu -D_GNU_SOURCE float_formatting.cpp -quiet -dumpbase float_formatting.cpp -mtune=generic -march=x86-64 -auxbase float_formatting -version -fstack-protector-strong -Wformat -Wformat-security -o /tmp/ccAU2pi6.s
GNU C++14 (Ubuntu 7.4.0-1ubuntu1~18.04.1) version 7.4.0 (x86_64-linux-gnu)
        compiled by GNU C version 7.4.0, GMP version 6.1.2, MPFR version 4.0.1, MPC version 1.1.0, isl version isl-0.19-GMP

GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
ignoring duplicate directory "/usr/include/x86_64-linux-gnu/c++/7"
ignoring nonexistent directory "/usr/local/include/x86_64-linux-gnu"
ignoring nonexistent directory "/usr/lib/gcc/x86_64-linux-gnu/7/../../../../x86_64-linux-gnu/include"
#include "..." search starts here:
#include <...> search starts here:
 /usr/include/c++/7
 /usr/include/x86_64-linux-gnu/c++/7
 /usr/include/c++/7/backward
 /usr/lib/gcc/x86_64-linux-gnu/7/include
 /usr/local/include
 /usr/lib/gcc/x86_64-linux-gnu/7/include-fixed
 /usr/include/x86_64-linux-gnu
 /usr/include
End of search list.
GNU C++14 (Ubuntu 7.4.0-1ubuntu1~18.04.1) version 7.4.0 (x86_64-linux-gnu)
        compiled by GNU C version 7.4.0, GMP version 6.1.2, MPFR version 4.0.1, MPC version 1.1.0, isl version isl-0.19-GMP

GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
Compiler executable checksum: 38816e3807cdcb3c59571e251bd6c090
COLLECT_GCC_OPTIONS='-v' '-o' 'float_formatting' '-shared-libgcc' '-mtune=generic' '-march=x86-64'
 as -v --64 -o /tmp/ccFFf6xp.o /tmp/ccAU2pi6.s
GNU assembler version 2.30 (x86_64-linux-gnu) using BFD version (GNU Binutils for Ubuntu) 2.30
COMPILER_PATH=/usr/lib/gcc/x86_64-linux-gnu/7/:/usr/lib/gcc/x86_64-linux-gnu/7/:/usr/lib/gcc/x86_64-linux-gnu/:/usr/lib/gcc/x86_64-linux-gnu/7/:/usr/lib/gcc/x86_64-linux-gnu/
LIBRARY_PATH=/usr/lib/gcc/x86_64-linux-gnu/7/:/usr/lib/gcc/x86_64-linux-gnu/7/../../../x86_64-linux-gnu/:/usr/lib/gcc/x86_64-linux-gnu/7/../../../../lib/:/lib/x86_64-linux-gnu/:/lib/../lib/:/usr/lib/x86_64-linux-gnu/:/usr/lib/../lib/:/usr/lib/gcc/x86_64-linux-gnu/7/../../../:/lib/:/usr/lib/
COLLECT_GCC_OPTIONS='-v' '-o' 'float_formatting' '-shared-libgcc' '-mtune=generic' '-march=x86-64'
 /usr/lib/gcc/x86_64-linux-gnu/7/collect2 -plugin /usr/lib/gcc/x86_64-linux-gnu/7/liblto_plugin.so -plugin-opt=/usr/lib/gcc/x86_64-linux-gnu/7/lto-wrapper -plugin-opt=-fresolution=/tmp/cco3ADOI.res -plugin-opt=-pass-through=-lgcc_s -plugin-opt=-pass-through=-lgcc -plugin-opt=-pass-through=-lc -plugin-opt=-pass-through=-lgcc_s -plugin-opt=-pass-through=-lgcc --sysroot=/ --build-id --eh-frame-hdr -m elf_x86_64 --hash-style=gnu --as-needed -dynamic-linker /lib64/ld-linux-x86-64.so.2 -pie -z now -z relro -o float_formatting /usr/lib/gcc/x86_64-linux-gnu/7/../../../x86_64-linux-gnu/Scrt1.o /usr/lib/gcc/x86_64-linux-gnu/7/../../../x86_64-linux-gnu/crti.o /usr/lib/gcc/x86_64-linux-gnu/7/crtbeginS.o -L/usr/lib/gcc/x86_64-linux-gnu/7 -L/usr/lib/gcc/x86_64-linux-gnu/7/../../../x86_64-linux-gnu -L/usr/lib/gcc/x86_64-linux-gnu/7/../../../../lib -L/lib/x86_64-linux-gnu -L/lib/../lib -L/usr/lib/x86_64-linux-gnu -L/usr/lib/../lib -L/usr/lib/gcc/x86_64-linux-gnu/7/../../.. -lquadmath /tmp/ccFFf6xp.o -lstdc++ -lm -lgcc_s -lgcc -lc -lgcc_s -lgcc /usr/lib/gcc/x86_64-linux-gnu/7/crtendS.o /usr/lib/gcc/x86_64-linux-gnu/7/../../../x86_64-linux-gnu/crtn.o
/tmp/ccFFf6xp.o: In function `main':
float_formatting.cpp:(.text+0x3e): undefined reference to `quadmath_snprintf'
collect2: error: ld returned 1 exit status

ld Verbose

I copy/pasted the LIBRARY_PATH variable and -L arguments from the g++ verbose output to check which file is picked up for -lquadmath

$ LIBRARY_PATH=/usr/lib/gcc/x86_64-linux-gnu/7/:/usr/lib/gcc/x86_64-linux-gnu/7/../../../x86_64-linux-gnu/:/usr/lib/gcc/x86_64-linux-gnu/7/../../../../lib/:/lib/x86_64-linux-gnu/:/lib/../lib/:/usr/lib/x86_64-linux-gnu/:/usr/lib/../lib/:/usr/lib/gcc/x86_64-linux-gnu/7/../../../:/lib/:/usr/lib/ ld --verbose -L/usr/lib/gcc/x86_64-linux-gnu/7 -L/usr/lib/gcc/x86_64-linux-gnu/7/../../../x86_64-linux-gnu -L/usr/lib/gcc/x86_64-linux-gnu/7/../../../../lib -L/lib/x86_64-linux-gnu -L/lib/../lib -L/usr/lib/x86_64-linux-gnu -L/usr/lib/../lib -L/usr/lib/gcc/x86_64-linux-gnu/7/../../.. -lquadmath | grep libquadmath
ld: warning: cannot find entry symbol _start; not setting start address
attempt to open /usr/lib/gcc/x86_64-linux-gnu/7/libquadmath.so succeeded

Symbol

Checking that file for the symbol, it seems to be there.

$ nm -gD /usr/lib/gcc/x86_64-linux-gnu/7/libquadmath.so | grep quadmath_snprintf
0000000000020f20 T quadmath_snprintf

Alas, it's not found during link time. What am I doing wrong? How do I link in libquadmath using g++?

Order matters. See what happens if you put the `-lquadmath` on the end of the invocation. — user4581301, Mar 10 '20 at 00:05
Does this answer your question? [Why does the order in which libraries are linked sometimes cause errors in GCC?](https://stackoverflow.com/questions/45135/why-does-the-order-in-which-libraries-are-linked-sometimes-cause-errors-in-gcc) — phuclv, Sep 21 '21 at 02:47
the number of libraries being linked is irrelevant. The order is important. That's all — phuclv, Sep 22 '21 at 06:23

Employed Russian · Accepted Answer · 2020-03-10T02:29:25.697

2

Your problem is that this link line is incorrect:

g++ -lquadmath -o float_formatting float_formatting.cpp

It should be:

g++ -o float_formatting float_formatting.cpp -lquadmath

To understand why the order matters, read this post.

edited Mar 10 '20 at 02:29

answered Mar 10 '20 at 02:21

Employed Russian

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I was aware of the need for an ordering for libraries. It didn't realize that it also impacted .o files. I guess it makes sense though, since archives are basically just object files anyway. – Brent Mar 11 '20 at 04:06
@Brent The ordering does *not* impact `.o` files (but the libraries must *follow* `.o` files). – Employed Russian Mar 11 '20 at 04:22