Libraries

Libraries#

Assembly programs link against the same C runtime everyone else uses. “Libraries” in the assembly sense usually means the ABI for calling into libc, the syscall interface to the kernel, and a handful of helper macros distributed with the assembler.

libc#

Linking against libc lets the program call printf, malloc, and friends directly from assembly.

global main
extern printf

section .data
fmt: db "n = %d", 10, 0

section .text
main:
    sub  rsp, 8
    mov  rdi, fmt
    mov  rsi, 42
    xor  eax, eax
    call printf
    add  rsp, 8
    xor  eax, eax
    ret

$ nasm -felf64 demo.asm -o demo.o
$ gcc demo.o -o demo

Linux syscalls#

Without libc, talk directly to the kernel via the syscall instruction. Numbers are in <asm/unistd.h> (e.g. read=0, write=1, open=2, execve=59, exit=60 on x86-64).

syscall #

Name

Args (rdi, rsi, rdx, r10, r8, r9)

0

read

fd, buf, count

1

write

fd, buf, count

2

open

pathname, flags, mode

3

close

fd

59

execve

pathname, argv, envp

60

exit

status

231

exit_group

status

Macro libraries#

  • NASM %include, text-substitution macros, %define, %macro.

  • GAS .macro / .endm, macros and conditional assembly.

  • nasm-stdlib and similar community macro packs.

References#

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