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

Question

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.

Answer 1

There are at least three things that you need to do to successfully use libc with dynamic linking:

1. Link /usr/lib/crt1.o, which contains _start, which will be the entry point for the ELF binary;
2. Link /usr/lib/crti.o (before libc) and /usr/lib/crtn.o (after), which provide some initialisation and finalisation code;
3. Tell the linker that the binary will use the dynamic linker, /lib/ld-linux.so.

For example:

$ cat hello.s
 .text
 .globl main
main:
 push %ebp
 mov %esp, %ebp
 pushl $hw_str
 call puts
 add $4, %esp
 xor %eax, %eax
 leave
 ret

 .data
hw_str:
 .asciz "Hello world!"

$ as -o hello.o hello.s
$ ld -o hello -dynamic-linker /lib/ld-linux.so.2 /usr/lib/crt1.o /usr/lib/crti.o -lc hello.o /usr/lib/crtn.o
$ ./hello
Hello world!
$