crt0

<h2 id="form-and-usage">Form and usage</h2>
<p>Crt0 generally takes the form of an <a href="/facts/Object_file/sFBvosE4">object file</a> called crt0.o, often written in <a href="/facts/Assembly_language/7XuV0cla">assembly language</a>, which is automatically included by the <a href="/facts/Linker_(computing)/pn1hztYe">linker</a> into every <a href="/facts/Executable_file/VSst9eLA">executable file</a> it builds.<a class="footnote-ref" id="fnref:1" href="#fn:1"><sup>1</sup></a>
</p><p>crt0 contains the most basic parts of the <a href="/facts/Runtime_library/TJjgyw3t">runtime library</a>. As such, the exact work it performs depends on the program's compiler, operating system and <a href="/facts/C_standard_library/qdG1nW2c">C standard library</a> implementation.<a class="footnote-ref" id="fnref:2" href="#fn:2"><sup>2</sup></a> Beside the initialization work required by the environment and <a href="/facts/Toolchain/SmlWCJhY">toolchain</a>, crt0 can perform additional operations defined by the programmer, such as executing <a href="/facts/C%252B%252B/Et0F2qn9">C++</a> global <a href="/facts/Constructor_(object-oriented_programming)/8JNHsH5H">constructors</a> and C functions carrying <a href="/facts/GNU_Compiler_Collection/t07jh01k">GCC</a>'s ((constructor)) attribute.<a class="footnote-ref" id="fnref:3" href="#fn:3"><sup>3</sup></a><a class="footnote-ref" id="fnref:4" href="#fn:4"><sup>4</sup></a>
</p><p>"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 crt0 are available for special usage scenarios; for example, to enable <a href="/facts/Profiling_(computer_programming)/JbSlDHPP">profiling</a> with <a href="/facts/Gprof/cqj01RCn">gprof</a>, programs must be compiled with gcrt0 instead.<a class="footnote-ref" id="fnref:5" href="#fn:5"><sup>5</sup></a>
</p>
<h2 id="example-crt0s">Example crt0.s</h2>
<p>This example is for <a href="/facts/Linux_kernel/dlkduHuA">Linux</a> <a href="/facts/X86-64/cyWvUFlP">x86-64</a> with <a href="/facts/X86_assembly_language/GTUtvkMm">AT&T syntax</a>, without an actual C runtime.
</p>
.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

<h2 id="see-also">See also</h2>
<ul>
<li>Computer programming portal</li></ul>
<ul><li><a href="/facts/Entry_point/nN52vHKb">Entry point</a></li>
<li><a href="/facts/Runtime_system/YmCuC4xP">Runtime system</a></li></ul>

<h2 id="external-links">External links</h2>
<ul><li><a href="https://lists.uclibc.org/pipermail/uclibc/2002-December/025943.html">crt0.o vs crt1.o</a></li>
<li><a href="http://dbp-consulting.com/tutorials/debugging/linuxProgramStartup.html">Linux x86 program start-up</a></li>
<li><a href="https://blogs.oracle.com/linux/post/hello-from-a-libc-free-world-part-1">Hello from a libc-free world! (Part 1)</a>, March 16, 2010</li>
<li><a href="https://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/x86_64/start.S;hb=b5367a08ae810e3c648fb036f2e5766204f9d83f">glibc x86_64 start.S </a></li></ul>

<h2 id="references">References</h2>

<ol>
<li id="fn:1"><p>"The C Runtime Initialization, crt0.o". embecosm.com. 2010. Retrieved 2013-12-30. <a href="https://www.embecosm.com/appnotes/ean9/html/ch05s02.html" target="_blank">https://www.embecosm.com/appnotes/ean9/html/ch05s02.html</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li>
<li id="fn:2"><p>"The C Runtime Initialization, crt0.o". embecosm.com. 2010. Retrieved 2013-12-30. <a href="https://www.embecosm.com/appnotes/ean9/html/ch05s02.html" target="_blank">https://www.embecosm.com/appnotes/ean9/html/ch05s02.html</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li>
<li id="fn:3"><p>"Program initialization: Creating a C library". osdev.org. 2014-02-25. Retrieved 2014-04-21. <a href="https://wiki.osdev.org/Creating_a_C_Library#Program_Initialization" target="_blank">https://wiki.osdev.org/Creating_a_C_Library#Program_Initialization</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></p></li>
<li id="fn:4"><p>"Calling Global Constructors". osdev.org. 2014-04-08. Retrieved 2014-04-21. <a href="https://wiki.osdev.org/Calling_Global_Constructors" target="_blank">https://wiki.osdev.org/Calling_Global_Constructors</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></p></li>
<li id="fn:5"><p>"Compiling a Program for Profiling: GNU gprof". sourceware.org. Retrieved 2013-12-30. <a href="https://sourceware.org/binutils/docs-2.16/gprof/Compiling.html" target="_blank">https://sourceware.org/binutils/docs-2.16/gprof/Compiling.html</a> <a href="#fnref:5" class="footnote-back-ref">↩</a></p></li>
</ol>

crt0 open-in-new

crt0