How to link a gas assembly program that uses the C standard library with ld without using gcc?

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As an exercise to learn more precisely how c programs work and what minimum level of content must exist for a program to be able to use libc, I've taken it upon myself to attempt to program primarily in x86 assembly using gas and ld.

As a fun little challenge, I've successfully assembled and linked several programs linked to different self-made dynamic libraries, but I have failed to be able to code a program from scratch to use libc function calls without directly using gcc.

I understand the calling conventions of individual c library functions, and have thoroughly inspected programs compiled out of gcc through use of objdump and readelf, but haven't gotten anywhere as far as what information to include in a gas assembly file and what parameters to invoke in ld to successfully link to libc. Anyone have any insight to this?

I'm running Linux, on an x86 machine.

binutils gnu-assembler ld assembly
4 Answers

If you do use _start instead of main (as mentioned in some of the comments above), you'll also need to change the way the program exits, or you'll get a seg fault:

            .text
            .globl    _start
_start:     
            mov       $hw_str, %rdi
            call      puts
            movl      $0,%ebx   # first argument: exit code.
            movl      $1,%eax   # system call number: sys_exit.
            int       $0x80     # call kernel.

            .data
hw_str:     .asciz "Hello world!"

On Kubuntu 18.04.2 (gcc (Ubuntu 7.3.0-27ubuntu1~18.04) 7.3.0):

$ as -o hello.o hello.s
$ ld -dynamic-linker /lib64/ld-linux-x86-64.so.2 -o hello hello.o -lc

Also, one easy way to find out what the dynamic linker is on your system is to compile a small C program and then run ldd on the binary:

test.c:

int main() { return 0; }

Compile and run ldd against executable:

$ gcc -o test test.c
$ ldd test
    linux-vdso.so.1 (0x00007ffd0a182000)
    libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007ff24d8e6000)
    /lib64/ld-linux-x86-64.so.2 (0x00007ff24ded9000)
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