c++ - Why symbols of a shared library are not resolved at link time?

Question

This is my 2nd post on this site in my effort to understand the compilation/linking process with gcc. When I try to make an executable, symbols need to be resolved at link time, but when I try to make a shared library, symbols are not resolved at link time of this library. They will perhaps be resolved when I am trying to make an executable using this shared library. Hands-on:

bash$ cat printhello.c
#include <stdio.h>
//#include "look.h"

void PrintHello()
{
look();
printf("Hello World
");
}

bash$ cat printbye.c
#include <stdio.h>
//#include "look.h"

void PrintBye()
{
look();
printf("Bye bye
");
}

bash$  cat look.h
void look();

bash$ cat look.c
#include <stdio.h>

void look()
{
printf("Looking
");
}

bash$ gcc printhello.c printbye.c
/usr/lib/gcc/i386-redhat-linux/4.1.2/../../../crt1.o: In function `_start':
(.text+0x18): undefined reference to `main'
/tmp/cck21S0u.o: In function `PrintHello':
printhello.c:(.text+0x7): undefined reference to `look'
/tmp/ccNWbCnd.o: In function `PrintBye':
printbye.c:(.text+0x7): undefined reference to `look'
collect2: ld returned 1 exit status

bash$ gcc -Wall -shared -o libgreet printhello.c printbye.c
printhello.c: In function 'PrintHello':
printhello.c:6: warning: implicit declaration of function 'look'
printbye.c: In function 'PrintBye':
printbye.c:5: warning: implicit declaration of function 'look'

So my question is why are symbols not resolved when I am linking a shared library. This work(Resolving symbols of its downstream) will need to be done when I will use this library to make an executable, but that means we need to know what this library depends on when using this library, but isn't it not undesirable?

Thanks, Jagrati

Answer 1

Does adding -z defs when building the library do what you want? If not, check the ld man pages, there are quite a few options on the handling of undefined symbols.