gcc optimization impact on shared library operation - c

Question

I have an own shared library, compiled and linked as below:
gcc -g -Wall -fpic -c -o cont.o cont.c
gcc -g -shared -o libcont.so cont.o -lrt
I have a program (nr) which uses this library:
gcc -g -Wall -I/include_path -c -o nr.o nr.c
gcc -L/shared_lib_path nr.o -lcont -o nr
One of the shared library's functions, if invoked, calls POSIX message sending (mq_send()) possibly which results an assertion error. strace ./nr says the following about this:
) = 1 (in [5])
mq_timedreceive(5, "\17\0ryName\0stester\0\t\0op\0srate\0\22\0ro"..., 2000, 0, NULL) = 76
mq_open("tester_asdf", O_WRONLY|O_NONBLOCK) = 6
writev(2, [{"Inconsistency detected by ld.so:"..., 33}, {"dl-lookup.c", 11}, {": ", 2}, {"167", 3}, {": ", 2}, {"check_match", 11}, {": ", 2}, {"Assertion `", 11}, {"version->filename == ((void *)0)"..., 79}, {"' failed!\n", 10}], 10Inconsistency detected by ld.so: dl-lookup.c: 167: check_match: Assertion `version->filename == ((void *)0) || ! _dl_name_match_p (version->filename, map)' failed!
) = 164
The very interesting thing is, that if I compile the program with any level of optimization other than the default (e.g., -O3), everything works fine:
gcc -g -Wall -O3 -I/include_path -c -o nr.o nr.c
gcc -L/shared_lib_path nr.o -lcont -o nr
In this case strace ./nr says:
) = 1 (in [5])
mq_timedreceive(5, "\17\0ryName\0stester\0\t\0op\0srate\0\22\0ro"..., 2000, 0, NULL) = 76
mq_open("tester_asdf", O_WRONLY|O_NONBLOCK) = 6
mq_timedsend(6, "\t\0op\0srate\0\22\0root\0scall_rating\0\t"..., 59, 0, NULL) = 0
mq_timedreceive(5, "\t\0op\0srate\0\22\0root\0scall_rating\0\t"..., 2000, 0, NULL) = -1 EAGAIN (Resource temporarily unavailable)
pselect6(6, [5], NULL, NULL, NULL, {~[INT ILL BUS FPE KILL SEGV ALRM TERM STOP RTMIN], 8}
Which is exactly that I would expect independently from the optimization level.
What could be the possible reason causing this?
The corresponding code snippet:
fprintf(stderr, "mqFd: %d, msg: %p, size: %d\n", (int)mqFd, msg, (int)size);
mq_send(mqFd, msg, size, 0);
fprintf prints just before the mq_send:
mqFd: 6, msg: 0x7ffdadef1820, size: 59
which is fine, but calling mq_send results the above detailed assertion.

Answer 1

I think this part from gcc man page, might be relevant to what you see:
-shared
Produce a shared object which can then be linked with other objects to form an executable.
Not all systems support this option. For predictable results, you must also specify the
same set of options used for compilation (-fpic, -fPIC, or model suboptions) when you
specify this linker option.[1]
This means, you need to add -fpic to the command line of the library creation.