Writing a putchar in Assembly for x86_64 with 64 bit Linux?

Question

I am trying to use the write syscall in order to reproduce the putchar function behavior which prints a single character. My code is as follows,

asm_putchar:
  push    rbp
  mov     rbp, rsp

  mov     r8, rdi

call:
  mov     rax, 1
  mov     rdi, 1
  mov     rsi, r8
  mov     rdx, 1
  syscall

return:
  mov     rsp, rbp
  pop     rbp
  ret

Answer 1

From man 2 write, you can see the signature of write is,

ssize_t write(int fd, const void *buf, size_t count);

It takes a pointer (const void *buf) to a buffer in memory. You can't pass it a char by value, so you have to store it to memory and pass a pointer.

(Don't print one char at a time unless you only have one to print, that's really inefficient. Construct a buffer in memory and print that. e.g. this x86-64 Linux NASM function: How do I print an integer in Assembly Level Programming without printf from the c library?)

A NASM version of GCC: putchar(char) in inline assembly:

; x86-64 System V calling convention: input = byte in DIL
; clobbers: RDI, RSI, RDX,  RCX, R11 (last 2 by syscall itself)
; returns:  RAX = write return value: 1 for success, -1..-4095 for error
writechar:
    mov    byte [rsp-4], dil      ; store the char from RDI

    mov     edi, 1                ; EDI = fd=1 = stdout
    lea     rsi, [rsp-4]          ; RSI = buf
    mov     edx, edi              ; RDX = len = 1
    syscall                    ; rax = write(1, buf, 1)
    ret

---

If you do pass an invalid pointer in RSI, such as '2' (integer 50), the system call will return -EFAULT (-14) in RAX. (The kernel returns error codes on bad pointers to system calls, instead of delivering a SIGSEGV like it would if you deref in user-space).

See also What are the return values of system calls in Assembly?

Instead of writing code to check return values, in toy programs / experiments you should just run them under strace ./a.out, especially if you're writing your own _start without libc there won't be any other system calls during startup that you don't make yourself, so it's very easy to read the output. How should strace be used?