Read a File via syscalls

Code

; cat_file(path)   rdi = path (C string)
section .rodata
    buf_len: equ 4096
section .bss
    buf: resb buf_len

section .text
global cat_file
cat_file:
    ; open(path, O_RDONLY, 0)  syscall #2
    mov     rax, 2
    mov     rsi, 0           ; O_RDONLY
    xor     rdx, rdx
    syscall
    test    eax, eax
    js      .fail            ; negative -> error
    mov     r12, rax         ; save fd in callee-saved r12

    push    r12              ; preserve r12 across our own calls
.read_loop:
    ; read(fd, buf, buf_len)  syscall #0
    mov     rax, 0
    mov     rdi, r12
    lea     rsi, [rel buf]
    mov     rdx, buf_len
    syscall
    test    rax, rax
    jle     .close           ; 0 -> EOF, <0 -> error
    mov     r13, rax         ; save bytes read

    ; write(1, buf, n)        syscall #1
    mov     rax, 1
    mov     rdi, 1
    lea     rsi, [rel buf]
    mov     rdx, r13
    syscall
    jmp     .read_loop

.close:
    ; close(fd)  syscall #3
    mov     rax, 3
    mov     rdi, r12
    syscall
    pop     r12
    xor     eax, eax
    ret
.fail:
    mov     rax, -1
    ret

Explanation

This implements a tiny 'cat' using only Linux syscalls: open (2) returns a file descriptor, read (0) fills a buffer and returns bytes read (0 at EOF), write (1) sends it to stdout, close (3) releases the fd. The fd is kept in r12 because read and write both clobber rax (their return value). 'test eax,eax; js' checks for the negative return that signals an error.

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