'weak' attribute in gcc

Recently I came to know about gcc attributes which can be used in code for specific purpose. One of them I understood early was 'weak' attribute. A 'weak' attribute may be specified if you don want compiler to throw any error if it was unable to resolve external symbols. This is useful if you are planning to provide function or variable in near future and do not want to modify the main source at that time.

#include<stdio.h>

extern int k() __attribute__((weak));
extern int k2 __attribute__((weak));

int main()
{
if(k == NULL)
{
    if(&k2 == NULL)
    {
        printf("No symbol k or k2 found\n");
    }
}
else
{
    k();
}
}

This example compiles just fine and gives no linkage error even if the symbols are not found. However if we are using those symbols we need to check NULLness of those symbols else the program will result in seg fault.