(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. After the main function completes the control returns to crt0, which calls the library function exit with the return code of the main function to terminate the process.
Form and usage Crt0 generally takes the form of an object file called , often originally 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.
Usually, crt0 does not contain more complex initialization code like the previously mentioned calling of functions carrying GCC's ((constructor)) attribute. Most modern libc implementations use __libc_start_main, a function written in C, to handle more complex tasks.
"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
See also
Entry point Runtime system
References
External links crt0.o vs crt1.o Linux x86 program start-up Hello from a libc-free world! (Part 1), March 16, 2010 glibc x86_64 start.S