where to find exit(int) function implementation

[Vitalio D]

Hello,

How and where void _exit(int) is implemented? I've started
with exit(int) defined in exit.c un stdlib, but I've got stuck at
unistd.h where void _exit(int) __dead2;
After that I couldn't find anything by running
find /usr/src -name "*.c" -exec grep " _exit(int" /dev/null {} \;

So I guess there is some unix convention magic involved but
I can't point which one. Can someone be kind and explain or point
out a resource where this trick is explained?

Regards,
Vitalio