sysdeps/x86_64/elf/start.S

This file contains ELF x86-64 ABI compliant startup code, noteably the _start entry point which is found in the object files /usr/lib/crt1.o.

Part of this _start routine is to call __libc_start_main (defined in csu/libc-start.c) which in turn calls main.

The source file (http://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/x86_64/elf/start.S;h=3c2caf9d00a0396ef2b74adb648f76c6c74ff65f;hb=cvs/glibc-2_9-branch) is well documented.