weak and strong symbols

compile-run.sh

clang -Wall -fPIC -c ctest1.c ctest2.c
clang -shared -Wl,-soname,libctest.so.1.0 -o libctest.so.1.0 *.o
clang -fPIC prog.c -Wl,-rpath,. libctest.so.1.0 -o prog1
./prog1 > result1
clang prog.c ctest*.c -o prog2
./prog2 > result2

ctest1.c

void ctest1(char **s)
{
  *s = "from first";
}

ctest2.c

void ctest2(char **s)
{
   *s = "from second";
}

prog.c

#include <stdio.h>
__attribute((weak)) void ctest1(char** s) {
  *s = "from weak";
}
void ctest2(char**);

int main()
{
   char *x;
   ctest1(&x);
   printf("result=%s\n",x);
   ctest2(&x);
   printf("result=%s\n",x);

   return 0;
}

result1

result=from weak        
result=from second

result2

result=from first
result=from second