gcc - Use the first occurred implementation when linking C programs -

i have a.h , a.c gives interface , implementation of function

//a.h #ifndef a_h #define a_h int op(); #endif  //a.c #include "a.h" int op(){     return 1; } 

similarly, have b.h , b.c gives interface of function of same name different implementation

//b.h #ifndef b_h #define b_h int op(); #endif  //b.c #include "b.h" int op(){     return 2; } 

now, want link them main program

#include <stdio.h>  int op();  int main(){ printf( "returned = %d\n ", op()); } 

for purpose, first compile a.c , b.c a.o , b.o respectively.

gcc -c a.c; gcc -c b.c 

then trying link them together. usual objective here let main.c choose implementation of 'op' using implementation appears first in linking command. so, ideally:

gcc a.o b.o main.c should give me executable returns 1 when executed, , gcc b.o a.o main.c should give me executable returns 2 when executed. error message

ld: 1 duplicate symbol architecture x86_64 

to not surprised. question is: how can let first occurred implementation used main.c? thanks.

one option:

1. make 2 static libraries -- 1 a.o , 1 b.o.

   ar liba.a a.o ar libb.a b.o 

2. in link line, use library want given higher priority before other one.

   gcc main.c -l. -la -lb