(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
In computer programming, an entry point is the place in a program where the execution of a program begins, and where the program has access to command line arguments.
To start a program's execution, the loader or operating system passes contro ...
. After the main function completes the control returns to crt0, which calls the library function exit(0) to terminate the process.
Form and usage
Crt0 generally takes the form of an
object file
An object file is a file that contains machine code or bytecode, as well as other data and metadata, generated by a compiler or assembler from source code during the compilation or assembly process. The machine code that is generated is kno ...
called , often written in
assembly language
In computing, assembly language (alternatively assembler language or symbolic machine code), often referred to simply as assembly and commonly abbreviated as ASM or asm, is any low-level programming language with a very strong correspondence bet ...
, which is automatically included by the
linker
Linker or linkers may refer to:
Computing
* Linker (computing), a computer program that takes one or more object files generated by a compiler or generated by an assembler and links them with libraries, generating an executable program or shar ...
into every
executable file
In computer science, executable code, an executable file, or an executable program, sometimes simply referred to as an executable or binary, causes a computer "to perform indicated tasks according to encoded instructions", as opposed to a da ...
it builds.
contains the most basic parts of the
runtime library
A runtime library is a library that provides access to the runtime environment that is available to a computer program tailored to the host platform. A runtime environment implements the execution model as required for a development environme ...
. As such, the exact work it performs depends on the program's compiler, operating system and
C standard library
The C standard library, sometimes referred to as libc, is the standard library for the C (programming language), C programming language, as specified in the ISO C standard.International Organization for Standardization, ISO/International Electrote ...
implementation.
Beside the initialization work required by the environment and
toolchain
A toolchain is a set of software development tools used to build and otherwise develop software. Often, the tools are executed sequentially and form a pipeline such that the output of one tool is the input for the next. Sometimes the term is us ...
, 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
Gprof is a performance analysis tool for Unix applications. It used a hybrid of instrumentation and samplingSusan L. Graham, Peter B. Kessler, and Marshall K. Mckusick''gprof: a Call Graph Execution Profiler''// Proceedings of the SIGPLAN '82 Sy ...
, programs must be compiled with instead.
Example crt0.s
This example is for
Linux
Linux ( ) is a family of open source Unix-like operating systems based on the Linux kernel, an kernel (operating system), operating system kernel first released on September 17, 1991, by Linus Torvalds. Linux is typically package manager, pac ...
x86-64
x86-64 (also known as x64, x86_64, AMD64, and Intel 64) is a 64-bit extension of the x86 instruction set architecture, instruction set. It was announced in 1999 and first available in the AMD Opteron family in 2003. It introduces two new ope ...
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
In computer programming, an entry point is the place in a program where the execution of a program begins, and where the program has access to command line arguments.
To start a program's execution, the loader or operating system passes co ...
*
Runtime system
In computer programming, a runtime system or runtime environment is a sub-system that exists in the computer where a program is created, as well as in the computers where the program is intended to be run. The name comes from the compile time ...
References
{{Reflist
External links
crt0.o vs crt1.oHello from a libc-free world! (Part 1) March 16, 2010
glibc x86_64 start.S
C standard library
C programming language family