Accessing libc,runtime linker,shared libraries functions using pragma weak

Sometimes you are working on libc, glibc linker ,shared libraries.
You have seen in the code of these libraries as follows:
#pragma weak dlopen = _dlopen
#pragma weak dlcose= _dlclose

What is this? what is the use of these lines of code written in libc,glibc etc.
The above lines code indicates that any user applications can call/use "dlopen" and "dlclose"
functions.

How is this possible?
There is little trick/work done by compiler, which creates the
symbol as a weak.

#pragma weak symbol1 = symbol2
This pragma declares symbol1 to be a weak alias of symbol2. It is an error if symbol2 is not defined in the
current translation unit.
for more information please refers following link.
http://gcc.gnu.org/onlinedocs/gcc/Pragmas.html#Pragmas

Now we have some idea on weak symbol,so move forwards to do some R&D.
I have done this R&D on Open-Soalris. Its also applicable to Linux.

$cat mylibc.c

#pragma weak my_function = _my_function
void _my_function()
{

printf("inside _my_fnction()\n");

}

$ cat main.c

int
main()
{
my_function();
return 0;

}

Now build the shared library:
$ gcc -o mylibc.so -fpic -shared mylibc.c
$ elfdump -s mylibc.so |fgrep my_function
[1] 0x0000049c 0x0000002f FUNC WEAK D 0 .text my_function
[10] 0x0000049c 0x0000002f FUNC GLOB D 0 .text _my_function
[49] 0x0000049c 0x0000002f FUNC WEAK D 0 .text my_function
[58] 0x0000049c 0x0000002f FUNC GLOB D 0 .text _my_function

You can see here my_function as a weak symbol.

Now build the executable using mylibc.so as a dependency:
$ gcc -o main main.c ./mylibc.so
$ elfdump -s main |fgrep my_function
[27] 0x08050680 0x00000000 FUNC GLOB D 0 UNDEF my_function
[81] 0x08050680 0x00000000 FUNC GLOB D 0 UNDEF my_function

Now run the executable:
$ ./main
inside _my_function

Cool! we got the result, Now try yourself.
Happy Hacking.