gcc linking fails: undefined reference to sqrt

[Tim Tei]

The code I try to compile and link:

$ cat s.c

#include <stdio.h>
#include <math.h>

int main()
{
        double n = 5.0;
        printf("%f\n", sqrt(n));
}

Linking fails with gcc, gcc-7, gcc-8, gcc-9:

$ gcc -lm s.c
/usr/bin/ld: /tmp/ccBhhS0F.o: in function `main':
s.c:(.text+0x23): undefined reference to `sqrt'
collect2: error: ld returned 1 exit status

Linking succeeds with g++, clang:

$ g++ -lm s.c
$ clang -lm s.c

Tested with Ubuntu 18.04 and 19.10.

On Debian linking with gcc works fine.

The parameters passed to ld differ in Debian and Ubuntu
(see ld-parameters-ubuntu.txt vs ld-parameters-debian.txt).

The first "--as-needed" is causing the problems.

ProblemType: Bug
DistroRelease: Ubuntu 19.10
Package: gcc 4:9.2.1-3.1ubuntu1
ProcVersionSignature: Ubuntu 5.3.0-19.20-generic 5.3.1
Uname: Linux 5.3.0-19-generic x86_64
ApportVersion: 2.20.11-0ubuntu8.2
Architecture: amd64
CurrentDesktop: ubuntu:GNOME
Date: Tue Nov 5 08:46:14 2019
ProcEnviron:
 LANGUAGE=de_DE:
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=de_DE.UTF-8
 SHELL=/bin/bash
SourcePackage: gcc-defaults
UpgradeStatus: Upgraded to eoan on 2019-10-29 (6 days ago)

[Matthias Klose (doko)]

GCC in Ubuntu passes --as-needed by default.

[Tim Tei (timtei)]

Matthias can you elaborate on that?

[Tim Tei (timtei)]

Seams to be a feature:
https://stackoverflow.com/questions/8140494/why-am-i-getting-a-gcc-undefined-reference-error-trying-to-create-shared-objec

Compiling it like this works:
gcc s.c -lm