Crt0

' (also known as ') is a set of execution startup routines linked into a C program that performs any initialization work required before calling the program's main function. 
Form and usage.
Crt0 generally takes the form of an object file called , often written in assembly language, which is automatically included by the linker into every executable file it builds.
 contains the most basic parts of the runtime library. As such, the exact work it performs depends on the program's compiler, operating system and C standard library implementation. Beside the initialization work required by the environment and toolchain, can perform additional operations defined by the programmer, such as executing C++ global constructors and C functions carrying GCC's attribute.
"crt" stands for "C runtime", and the zero stands for "the very beginning". However, when programs are compiled using GCC, it is also used for languages other than C. Alternative versions of are available for special usage scenarios; for example, to enable profiling with gprof, programs must be compiled with instead.
Example crt0.s.
This example is for Linux x86-64 with AT&T syntax, without an actual C runtime.
.text
.globl _start
_start: # _start is the entry point known to the linker
 xor %ebp, %ebp # effectively RBP := 0, mark the end of stack frames
 mov (%rsp), %edi # get argc from the stack (implicitly zero-extended to 64-bit)
 lea 8(%rsp), %rsi # take the address of argv from the stack
 lea 16(%rsp,%rdi,8), %rdx # take the address of envp from the stack
 xor %eax, %eax # per ABI and compatibility with icc
 call main # %edi, %rsi, %rdx are the three args (of which first two are C standard) to main
 mov %eax, %edi # transfer the return of main to the first argument of _exit
 xor %eax, %eax # per ABI and compatibility with icc
 call _exit # terminate the program