Weak function

From gnu gcc document:
The weak attribute causes the declaration to be emitted as a weak symbol rather than a global. This is primarily useful in defining library functions which can be overridden in user code, though it can also be used with non-function declarations.

libweak.c

#include <stdio.h>

void weakfunc() __attribute__((weak));

void weakfunc()
{
    printf ("default weak function\n");

}

main.c

#include <stdio.h>
void weakfunc()
{
    printf ("weak function from main\n");    
}

int main ()
{
    weakfunc();
    return 0;
}

Compile the library and main function.

#gcc -shared -o libweak.so weakfunc.c
#gcc  -o weakfunc{,.c} -lweak -L.
#LD_LIBRARY_PATH=. ./main
weak function from main 

From above, it is very clear that the weak function of the library is replaced.

#objdump -t libweak.so |grep weakfunc
0000046c  w    F .text 00000014   weakfunc

In the library, this function has weak attribute.

#objdump -t main |grep weakfunc
080484e4  g    F .text 00000014   weakfunc

However, this function is presented as  global attribute.