What happens before main()

Lots of things:

1. Loaded calls preinitarray, then _start
2. That calls __libc_start_main, which calls __libc_csu_init, which calls a bunch of other stuff
3. After that, __libc_start_main calls main
4. And then exit happens

Start at _start

• often if you have %ebp, %esi, etc. and relatively small addresses, probably a 32-bit binary (stack used for argument passing)
• argc popped into %esi
• argv moved from to %ecx
• stack pointer aligned to boundary
• push arguments and call __libc_start_main
  • this calls __libc_init_first, retrieving global variable __environ

__libc_start_main

• handles security stuff for setuid/setgid
• starts threading
• registers fini and rtld_fini arguments to run via at_exit for cleanup
• calls init argument
• calls main with argc and argv
• calls exit with return value of main

__libc_csu_init

• the constructor of the program
• calls _init()
• calls array of function calls with argc, argv, and envp

_init

• does a bunch of stuff, including global constructors (e.g. constructors for static C++ objects)

exit

• runs functions registered with atexit() (in reverse order of registration)
• runs all functions in fini_array
• runs destructors