Undefined reference to `clock_gettime` although `-lrt` is given

Question

I've give -lrt as the last linker flag to the compiler. But still getting this error.

arif@khost:~/sak/sak.exosip$ gcc eXo_init.c -I/opt/osip2/include -I/opt/exosip/include  -L/opt/osip2/lib -L/opt/exosip/lib -leXosip2 -losipparser2 -losip2 -lrt
/opt/osip2/lib/libosip2.so: undefined reference to `clock_gettime'
collect2: ld returned 1 exit status

The man page says :

NAME
       clock_getres, clock_gettime, clock_settime - clock and time functions

SYNOPSIS
       #include <time.h>

       int clock_getres(clockid_t clk_id, struct timespec *res);

       int clock_gettime(clockid_t clk_id, struct timespec *tp);

       int clock_settime(clockid_t clk_id, const struct timespec *tp);

       Link with -lrt.

So i'm kind of confused where i'm doing it wrong.

I've tried to read symbols in librt.so with no luck :

arif@khost:~/sak/ortp/src/tests$ nm /lib/x86_64-linux-gnu/librt-2.15.so 
nm: /lib/x86_64-linux-gnu/librt-2.15.so: no symbols

UPDATE 1 The reason i can't read symbols out of librt.so is that they are "stripped". Where can i get the symbol names ?

arif@khost:~/sak/ortp/src/tests$ file /lib/x86_64-linux-gnu/librt-2.15.so 
/lib/x86_64-linux-gnu/librt-2.15.so: ELF 64-bit LSB shared object, x86-64, version 1 (SYSV), dynamically linked (uses shared libs), BuildID[sha1]=0x375b2c35c4e6503a5d1a88ab6f76f5b6e0ee81df, for GNU/Linux 2.6.24, stripped

UPDATE 2

Well things become very confusing because the following test code compiles and runs just fine :

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

int main(int argc, char **argv, char **arge) {
    struct timespec tps, tpe;
    if ((clock_gettime(CLOCK_REALTIME, &tps) != 0)
        || (clock_gettime(CLOCK_REALTIME, &tpe) != 0)) {
        perror("clock_gettime");
        return -1;
    }
    printf("%lu s, %lu ns\n", tpe.tv_sec-tps.tv_sec,tpe.tv_nsec-tps.tv_nsec);
    return 0;
}

Built with

arif@khost:~/sak/sak.exosip$ gcc what.c -lrt

UPDATE3 The code i'm trying compile :

#include <eXosip2/eXosip.h>
#include <netinet/in.h>
#include <unistd.h>

int ex_init(int port)
{
    struct eXosip_t *eXcontext;
    int i;
    TRACE_INITIALIZE(6, stdout);
    i = eXosip_init(eXcontext);
    if (i != 0)
        return -1;

    i = eXosip_listen_addr(eXcontext, IPPROTO_UDP, NULL, port, AF_INET, 0);
    if (i != 0) {
        eXosip_quit(eXcontext);
        fprintf (stderr, "could not initialize transport layer\n");
        return -1;
    }

    return 1;
}

int main(int argc, char **argv) {
    if(ex_init(1000))
        printf("success \n");
    return 0;
}

Answer 1

Well the problem is solved If i pass this linker flag

-Wl,--no-as-needed

Before the library list in command line.

Why this works because in my platform, linker is always passed with -Wl,--as-needed.

From ld manual :

--as-needed
       --no-as-needed
           This option affects ELF DT_NEEDED tags for dynamic libraries
           mentioned on the command line after the --as-needed option.
           Normally the linker will add a DT_NEEDED tag for each dynamic
           library mentioned on the command line, regardless of whether the
           library is actually needed or not.  --as-needed causes a DT_NEEDED
           tag to only be emitted for a library that satisfies an undefined
           symbol reference from a regular object file or, if the library is
           not found in the DT_NEEDED lists of other libraries linked up to
           that point, an undefined symbol reference from another dynamic
           library.  --no-as-needed restores the default behaviour.

So when --as-needed is given before a library , liker only links with the libraries which are given in NEEDED section of the library.

For example,

-Wl,--as-needed -llibA -llibB -llibC

Here --as-needed is given before libA. So during linking, linker will examine the NEEDED section of libA. If in NEEDED section of libA lists only libC, then the libB will not be linked.

This specific problem occurred because

arif@khost:~/sak/sak.exosip$ objdump -p /opt/osip2/lib/libosip2.so.10 | grep NEEDED
  NEEDED               libosipparser2.so.10
  NEEDED               libc.so.6

libosip2 does not lists librt as NEEDED.

If i pass --no-as-needed, then all the libraries will be linked regardless of what is given in ELF's NEEDED section.

Although this should not be the case because,

arif@khost:~/sak/sak.exosip$ nm --demangle /opt/osip2/lib/libosip2.so.10 | grep clock_gettime
                 U clock_gettime

It has undefined symbol clock_gettime which is provided by librt.so.

Well its actually a fault of the libosip2 devs that their autotools is not working with --as-needed.

The link command used by osip:

libtool: link: gcc -shared  -fPIC -DPIC  .libs/ict_fsm.o .libs/ist_fsm.o .libs/nict_fsm.o .libs/nist_fsm.o .libs/ict.o .libs/ist.o .libs/nict.o .libs/nist.o .libs/fsm_misc.o .libs/osip.o .libs/osip_transaction.o .libs/osip_event.o .libs/port_fifo.o .libs/osip_dialog.o .libs/osip_time.o .libs/port_sema.o .libs/port_thread.o .libs/port_condv.o   -Wl,-rpath -Wl,/home/arif/sak/osip/src/osipparser2/.libs -Wl,-rpath -Wl,/opt/osip2-test/lib -lnsl ../osipparser2/.libs/libosipparser2.so    -Wl,-soname -Wl,libosip2.so.10 -o .libs/libosip2.so.10.0.0

So its not linking with librt and thats why its not listing librt in its NEEDED list

If configured with :

 LDFLAGS="${LDFLAGS} -lrt" ./configure --prefix=/opt/osip2-test/ 

Then the link command becomes :

libtool: link: gcc -shared  -fPIC -DPIC  .libs/ict_fsm.o .libs/ist_fsm.o .libs/nict_fsm.o .libs/nist_fsm.o .libs/ict.o .libs/ist.o .libs/nict.o .libs/nist.o .libs/fsm_misc.o .libs/osip.o .libs/osip_transaction.o .libs/osip_event.o .libs/port_fifo.o .libs/osip_dialog.o .libs/osip_time.o .libs/port_sema.o .libs/port_thread.o .libs/port_condv.o   -Wl,-rpath -Wl,/home/arif/sak/osip/src/osipparser2/.libs -Wl,-rpath -Wl,/opt/osip2-test/lib -lnsl ../osipparser2/.libs/libosipparser2.so -lrt    -Wl,-soname -Wl,libosip2.so.10 -o .libs/libosip2.so.10.0.0

So its linking with librt. Its also reflected in its ELF:

arif@khost:~/sak/osip/src/osip2/.libs$ objdump -p libosip2.so.10 | grep NEEDED
  NEEDED               libosipparser2.so.10
  NEEDED               librt.so.1
  NEEDED               libc.so.6

This patch fixes this :

diff --git a/src/osip2/Makefile.am b/src/osip2/Makefile.am
index bb0d8f3..b72c22a 100644
--- a/src/osip2/Makefile.am
+++ b/src/osip2/Makefile.am
@@ -14,7 +14,7 @@ libosip2_la_SOURCES+=port_sema.c port_thread.c port_condv.c
 endif

 libosip2_la_LDFLAGS = -version-info $(LIBOSIP_SO_VERSION) \
- $(FSM_LIB) $(EXTRA_LIB) ../osipparser2/libosipparser2.la -no-undefined
+ $(FSM_LIB) $(EXTRA_LIB) ../osipparser2/libosipparser2.la -no-undefined -lrt


 INCLUDES = -I$(top_srcdir)/includ

Relevant usenet discussion thread : https://groups.google.com/forum/#!topic/comp.unix.programmer/VKbARy6W4AY

UPDATE:

osip developer responded to my mail. He fixed it with a different patch (More general solution then mine) http://git.savannah.gnu.org/cgit/osip.git/commit/?id=bd5b1ad58381e4bfce08bad9b66ad00cd28f9b65