gcc-4.8.2 doesn't link pthread

Question

all.

Compiling simple stuff using the gcc toolchain for several years, today I ran against a curious phenomenon.

I installed Kubuntu 14.04 to a common desktop i686 machine with gcc 4.8.2 in it. But then, trying to build some well coded stuff pulled out from my local repository, I ran against tons of 'undefined reference to' messages. The code compiles, links und runs well under Ubuntu 11.04 / gcc 4.5.2. I checked the linking process (by -Wl,--verbose to gcc), think it works. It finds all libraries I specify in the link command. An objdump -t myLib.so brings exactly the symbols I'd expect - but the linker doesn't see them.
Checking the pthread library also brings according symbols, except they are suffixed with some @GLIBC... stuff. Didn't check linker/loader tricks so far.

A sample like

#include <stdio.h>
#include <unistd.h>
#include <pthread.h>

static void *fooo (void *xxx) {
    char *txt = (char*)xxx;
    printf("My job is to print this :'%s'. Bye now!\n", txt);
    return 0;
 }
 int main (int argc, char *argv[]) {
    pthread_t thd;
    pthread_create(&thd,  NULL, fooo,  "A POSIX thread");
    sleep(1);
    return 0;
 }

runs very well on the old system just saying

gcc -l pthread fooo.c && ./a.out

but breaks at the linking step with 4.8.2.
Any idea would be very welcome.

.M

possible duplicate of [Linker order - GCC](http://stackoverflow.com/questions/45135/linker-order-gcc) — Some programmer dude, Nov 20 '14 at 13:04
@JoachimPileborg I guess not since there is only once library linked into the program. We definitely need the exact error message. — sfrehse, Nov 20 '14 at 13:13
Just try *gcc -pthread -o fooo fooo.c && ./fooo* not *gcc -l pthread* — sfrehse, Nov 20 '14 at 13:13
@sfrehse The important thing is the *order* of the dependencies, not if one library uses another library. The file `fooo.c` depends on the `pthread` library, therefore it must come *before* the `-lpthread` on the command line. — Some programmer dude, Nov 20 '14 at 13:21

score 1 · Answer 1 · answered Nov 20 '14 at 13:46

1

Thanks to sfrehse, JoachimPileborg et al!

Indeed, success depends on argument order. I knew this fact for static linking, but it is new in processing of shared objects with gcc.

Does someone know what the background of this improvement is? It breaks innumerable build processes, and I guess thousands of tomatoes are being made ready against gcc.gnu.org .....

.M

answered Nov 20 '14 at 13:46

Mikes

21
2

Save your tomatoes, this is a property of the linker not GCC, and technically it's [always](http://c-faq.com/lib/libsearch.html) been required, it just happened to work in the past for some cases. – Jonathan Wakely Nov 20 '14 at 13:54
@Jonathan Wrong. It's definitely _NOT_ always required. See my initial post, please. You will see then this effect rose up somewhere between 4.5.2 and 4.8.2. And even on ancient versions - I run 4.1.2, too - the stuff links quite well. Thanks for your hint regarding the linker job. – Mikes Nov 20 '14 at 14:20
How does "I tried it and it worked" prove me wrong when I say "it just happened to work in the past"? The documentation for the linker has always said libraries are searched once in the order they appear on the command line. e.g. as far back as [binutils 2.9.1](http://ftp.gnu.org/old-gnu/Manuals/ld-2.9.1/html_node/ld_3.html), see the `-l` option – Jonathan Wakely Nov 20 '14 at 14:35
btw I think what changed is that your distro might now default to using the `--as-needed` linker option, which can affect whether other shared libraries are pulled in to resolve missing references. I believe you'd get the same behaviour with ancient versions if you passed `--as-needed` to the linker. (But again, stating the version of GCC is meaningless since it's the linker that decides this behaviour not GCC.) – Jonathan Wakely Nov 20 '14 at 14:48

gcc-4.8.2 doesn't link pthread

1 Answers1