Where to find the source code of _IO_putc_unlocked (c, _IO_stdout); ?

I was browsing for the source code of printf() from glibc ( http://ftp.gnu.org/gnu/glibc/glibc-2.9.tar.gz ) and in the file printf.c I discovered printf uses putc to print characters on screen, well, I checked the source code of putc in putc.c and found out it uses some strange function _IO_putc_unlocked (c, _IO_stdout); but I can't seem to find the source code for this function. Could anyone please tell me where to find this so I can finally understand the ultimate inner workings of printf?