Weak characters and weaknesses

I am trying to learn how these gcc functions work.

I created a library function which should cause either sigprocmask

or pthread_sigmask

depending on whether the bond is linked libpthread

.

My weak code version works as expected, but my weak character based version doesn't seem to be overridden by the real character pthread_sigmask

as I would expect.

Could you please point out what I am doing wrong?

Weakref version:

#!/bin/sh
cat > lib.c <<EOF
#include <unistd.h>
#include <sys/types.h>
#include <stdio.h>
#include <fcntl.h>
#include <signal.h>
#include <pthread.h>
#include <stdlib.h>

__attribute__((weakref("pthread_sigmask")))
static int pthread_sigmask_ref(int How,  sigset_t const *Set, sigset_t *Oldset);

int call_pthread_sigmask(int How,  sigset_t const *Set, sigset_t *Oldset)
{
    puts("calling pthread_sigmask");
    if(pthread_sigmask_ref){
        return pthread_sigmask_ref(How, Set, Oldset);
    }else{
        puts("SIGPROCMASK");
        return sigprocmask(How, Set, Oldset);
    }
}
EOF

cat > main.c <<EOF
#include <unistd.h>
#include <sys/types.h>
#include <stdio.h>
#include <fcntl.h>
#include <signal.h>
#include <pthread.h>
#include <stdlib.h>

int call_pthread_sigmask(int How,  sigset_t const *Set, sigset_t *Oldset);

int main()
{
    sigset_t all_sigs, old_mask;
    sigemptyset(&all_sigs);
    sigaddset(&all_sigs, SIGTERM);
    call_pthread_sigmask(SIG_SETMASK, &all_sigs, &old_mask);

#if 0
    pthread_mutex_t mx;
    pthread_mutex_init(&mx,0);
#endif
}

EOF

gcc -c -fpic lib.c
gcc lib.o -o lib.so -shared

gcc main.c $PWD/lib.so -lpthread -o wrap
gcc main.c $PWD/lib.so -Wl,--no-as-needed  -lpthread -o real

echo wrap
ldd ./wrap
./wrap

echo real
ldd ./real
./real

      

        
        
        
      

    

Output example:

wrap
    linux-vdso.so.1 =>  (0x00007ffd5dda0000)
    /home/user/tmp/lib.so (0x00007faa4f8cf000)
    libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007faa4f507000)
    /lib64/ld-linux-x86-64.so.2 (0x00007faa4fad1000)
calling pthread_sigmask
SIGPROCMASK
real
    linux-vdso.so.1 =>  (0x00007ffc7f51e000)
    /home/user/tmp/lib.so (0x00007f6ff23b2000)
    libpthread.so.0 => /lib/x86_64-linux-gnu/libpthread.so.0 (0x00007f6ff2194000)
    libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f6ff1dcc000)
    /lib64/ld-linux-x86-64.so.2 (0x00007f6ff25b4000)
calling pthread_sigmask

      

        
        
        
      

    

Weak character version:

#!/bin/sh
cat > lib.c <<EOF
#include <unistd.h>
#include <sys/types.h>
#include <stdio.h>
#include <fcntl.h>
#include <signal.h>
#include <pthread.h>
#include <stdlib.h>

__attribute__((weak,noinline))
int pthread_sigmask(int How,  sigset_t const *Set, sigset_t *Oldset)
{
    puts("SIGPROCMASK");
    return sigprocmask(How, Set, Oldset);
}

int call_pthread_sigmask(int How,  sigset_t const *Set, sigset_t *Oldset)
{
    puts("calling pthread_sigmask");
    return pthread_sigmask(How, Set, Oldset);
}
EOF

cat > main.c <<EOF
#include <unistd.h>
#include <sys/types.h>
#include <stdio.h>
#include <fcntl.h>
#include <signal.h>
#include <pthread.h>
#include <stdlib.h>

int call_pthread_sigmask(int How,  sigset_t const *Set, sigset_t *Oldset);

int main()
{
    sigset_t all_sigs, old_mask;
    sigemptyset(&all_sigs);
    sigaddset(&all_sigs, SIGTERM);
    call_pthread_sigmask(SIG_SETMASK, &all_sigs, &old_mask);

#if 0
    pthread_mutex_t mx;
    pthread_mutex_init(&mx,0);
#endif
}

EOF

gcc -c -fpic lib.c
gcc lib.o -o lib.so -shared

gcc main.c $PWD/lib.so -lpthread -o wrap
gcc main.c $PWD/lib.so -Wl,--no-as-needed  -lpthread -o real

echo wrap
ldd ./wrap
./wrap

echo real
ldd ./real
./real

      

        
        
        
      

    

Output example:

wrap
    linux-vdso.so.1 =>  (0x00007ffd19d23000)
    /home/user/tmp/lib.so (0x00007fdd24b46000)
    libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007fdd2477e000)
    /lib64/ld-linux-x86-64.so.2 (0x00007fdd24d48000)
calling pthread_sigmask
SIGPROCMASK
real
    linux-vdso.so.1 =>  (0x00007fff115f6000)
    /home/user/tmp/lib.so (0x00007fb22be95000)
    libpthread.so.0 => /lib/x86_64-linux-gnu/libpthread.so.0 (0x00007fb22bc77000)
    libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007fb22b8af000)
    /lib64/ld-linux-x86-64.so.2 (0x00007fb22c097000)
calling pthread_sigmask
SIGPROCMASK