linking against librt doesn't provide sem_post

[Gary Duzan]

We have software that uses semaphores but not pthreads. Therefore, our code links against librt, as documented in the sem_post man page. This worked fine in 10.04 and 11.04, but not in 11.10.

> cat a.c
#include <semaphore.h>
int main()
{
    sem_t *mysem;

    int ret;

    ret = sem_post(mysem);
    return 0;
}
> gcc -g -lrt -o a a.c
/tmp/ccUadHKv.o: In function `main':
a.c:8: undefined reference to `sem_post'
collect2: ld returned 1 exit status

ProblemType: Bug
DistroRelease: Ubuntu 11.10
Package: libc6-dev 2.13-20ubuntu5
ProcVersionSignature: Ubuntu 3.0.0-12.20-generic 3.0.4
Uname: Linux 3.0.0-12-generic x86_64
NonfreeKernelModules: nvidia
ApportVersion: 1.23-0ubuntu3
Architecture: amd64
Date: Fri Oct 14 12:30:58 2011
SourcePackage: eglibc
UpgradeStatus: Upgraded to oneiric on 2011-10-07 (6 days ago)

[Matthias Klose (doko)]

see https://wiki.ubuntu.com/OneiricOcelot/ReleaseNotes?action=show&redirect=OneiricOcelot%2FTechnicalOverview#GCC_4.6_Toolchain

[Gary Duzan (gary-duzan)]

If this is going to be the case going forward, then the sem_* man pages should be updated to indicate that -lrt is no longer sufficient to use semaphores. I don't have a problem with the change since LSB says to use libpthread, anyway, but if the behavior is going to change then the documentation that suggests use of the old behavior should be changed.

[Gary Duzan (gary-duzan)]

The sem_* man pages need to be updated to reflect a "blessed" way of linking in semaphore code now that -lrt doesn't work.

[Matthias Klose (doko)]

no, the manpages doesn't tell you about the order of objects

[Gary Duzan (gary-duzan)]

True, but it does say that -lrt will work for semaphores, but now it doesn't.

[Gary Duzan (gary-duzan)]

Note that while the example I gave had the -lrt before the object, the same problem occurs if it is included at the end.

[Michael Kerrisk (mtk-manpages)]

So, it'd be good if someone had directed this to upstream man-pages. I found this bug report by chance. I've made appropriate changes for the upcoming man-pages-3.42.