OSDev.org

Posted: **Thu Jun 07, 2007 11:50 am**

Nowadays, I'm studying L4 kernel. It uses user-level applications on it.

As a newbie I found that I have to write init codes like crt0-ia32.S for L4 applications. I was really wondering where crt0 is from.

I found the following command in ld 2.9.1 manual.

$ ld -o output /lib/crt0.o hello.o -lc

My debian stable etch doesn't have /lib/crt0.o, instead, has /usr/lib/crt1.o

I thought my application with the next source working on Linux will be built as shown below:

Code: Select all

#include <stdio.h>

int main(void)
{
    return 0;
}

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$ ld m.o /usr/lib/crt1.o /usr/lib/libc.a
/usr/lib/libc.a(elf-init.o): In function `__libc_csu_init': undefined reference to `_init'
/usr/lib/libc.a(elf-init.o): In function `__libc_csu_fini': undefined reference to `_fini'

Now it says I can't resolve the symbols, '_init' and '_fini'. Which object file has those symbols?? I'd like to ask this out of my curiosity.

Thank you.

aeeee! one more thing I'd like to know is about ld script.

Does anyone know good tutorial how to write a ld script? ld manual isn't enough, it's too rough because I'm a beginner. I need help.

Posted: **Thu Jun 07, 2007 12:05 pm**

http://en.wikipedia.org/wiki/Crt0

Posted: **Thu Jun 07, 2007 12:25 pm**

The reference was really helpful to get to know a bit more about crt0.

The C compiler link in the object file crt0.o to provide startup capabilities and environments for program execution.

No problem.

I don't want to build easy command gcc m.c like this. I'd like to know steps how the application is built.

That's why I commanded as follows :

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$ gcc -c m.c
$ ld m.o /usr/lib/crt1.o -lc
/usr/lib/libc_nonshared.a(elf-init.oS): In function `__libc_csu_init': undefined reference to `_init'
/usr/lib/libc_nonshared.a(elf-init.oS): In function `__libc_csu_fini': undefined reference to `_fini'

ps. Today I'm happy to boot with grub and easy c++ and c tutorial 'hello-world' kernels.

Posted: **Sat Jun 09, 2007 10:37 pm**

cheeky wrote: I don't want to build easy command gcc m.c like this. I'd like to know steps how the application is built.

http://csapp.cs.cmu.edu/

Posted: **Mon Jun 11, 2007 12:07 am**

I was able to see lots of illustrative descriptions about linking.

Posted: **Thu Jun 14, 2007 10:03 am**

If you run gcc -Wl,-v m.c, ld will print the linker command. On my machine it's:

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/usr/bin/ld --eh-frame-hdr -m elf_i386 --hash-style=both
    -dynamic-linker /lib/ld-linux.so.2 usr/lib/gcc/i486-linux-gnu/4.1.2/../../../../lib/crt1.o
    /usr/lib/gcc/i486-linux-gnu/4.1.2/../../../../lib/crti.o /usr/lib/gcc/i486-linux-gnu/4.1.2/crtbegin.o
    -L/usr/lib/gcc/i486-linux-gnu/4.1.2 -L/usr/lib/gcc/i486-linux-gnu/4.1.2
    -L/usr/lib/gcc/i486-linux-gnu/4.1.2/../../../../lib -L/lib/../lib -L/usr/lib/../lib
    -v /tmp/cck1Ady4.o -lgcc --as-needed -lgcc_s --no-as-needed
    -lc -lgcc --as-needed -lgcc_s --no-as-needed
    /usr/lib/gcc/i486-linux-gnu/4.1.2/crtend.o
    /usr/lib/gcc/i486-linux-gnu/4.1.2/../../../../lib/crtn.o

If you ignore all the paths and ELF stuff, it's:

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ld crt1.o crti.o crtbegin.o m.o -lgcc -lgcc_s -lc -lgcc -lgcc_s crtend.o crtn.o

crt1.o is the startup code; crti.o and crtn.o define a few things to help with running constructors (crti defines _init and _fini), and crtbegin.o and crtend.o run C++ constructors and destructors for global variables.

Posted: **Thu Jun 14, 2007 10:11 pm**

http://www.lisha.ufsc.br/~guto/teaching ... hello.html

It was also very helpful for me to understand something more detail. Thanks a lot everbody!

OSDev.org

linking crt1.o

linking crt1.o

Thank you for your reference

Re: Thank you for your reference

Thank you

another link