_start fn in C

A lot of info is provided here

_start is called by most c,c++ programs before main.

It is the linker that controls the programs entry point.

_start is provided by libc. _start is often written in assembly and is provided by the object file crt0.o

C, C++ standards do not specify _start behaviour. They mostly specify what conditions must exist before main excecution begins.

TODO: What does _start do ?

Where is _start located ? Most OSes or platforms provide _start in a crt0.o file.

A series of crt0...n.o files in each platform (OS, barebone hardware etc) provide this functionality.

What does _start do ? All of the below, but not just limited to this :

  1. Configuring registers
  2. Initializing external memory
  3. enabling caches
  4. MMU
  5. stack initialization
  6. stack alignment (16 bit address at start of execution in case of x86-64)
  7. Frame Pointer initialization
  8. C/C++ runtime setup
  9. Jumping to main
  10. Exiting the program once main finishes

Note: In Linux (Ubuntu 19.10, gcc 9.2.1), initialization starts with _init that is part of the final executable (question, does it come from crt0.o. Also linux doesn’t seem to have any crt0.o file at all ). I followed the tutorial here

Linux + glibc defines _start in crt1.o under /usr/lib/x86_64

After following tutorial ^ , you will realize that crt1.o , crti.o, crtn.o define a lot of program startup stuff. WHy does it do so ? Can we directly execute our program from main ? Follow this tutorial http://www.muppetlabs.com/~breadbox/software/tiny/teensy.html

objdump -R shows relocation entries only on compiled dynanic object files (ie, files linked together)