How can I check if my syscall failed in NASM

Question

I would like to know if anybody has a solution to my problem. I'm doing a project for school. This project is about creating a function (only in NASM) that uses a syscall (I have to create ft_read which reproduces the behaviour of the real read system call).

Here is the situation :

When everything is okay (FD, buff, count), my function works properly.

BUT I don't know how to check if the syscall was successful or if it failed.

For example : When I use a fake fd (-1 for example), the syscall read returns 9 in rax. I understood that 9 is the error code for the variable Errno.

The problem is that I don't know how to differentiate the error code for errno from the return value of read (red of 9 char from the file).

If anybody has an idea on how to do I would be glad to know !

Here is my code at the moment :

extern  ___error
SYS_READ_MAC equ 0x2000003
SYS_READ_LINUX equ 3
section .text
    global _ft_read
_ft_read:
    cmp rdi, 0
    jl _error
    cmp rsi, 0
    je _error
    cmp rdx, 0
    jl _error
    mov rax, SYS_READ_MAC
    syscall
    cmp rax, 0
    jl _error
    ret
_error:
    mov r10, rax
    call ___error
    mov qword [rax], r10
    mov rax, -1
    ret

Sorry if my english isn't perfect, i'm not a native english speaker.

Answer 1

I found my answer :

I didn't realized it, and I don't know exactly why, but I was set on unsigned mode. So I can't see if rax contains a negative value.

In unsigned mode, the carry flag (CF) is set at 1 if the last operation (or the last syscall ?) failed.

So the solution was to do : JC _error (jump to error if carry flag is set on 1).

My code now :

extern  ___error
SYS_READ_MAC equ 0x2000003
SYS_READ_LINUX equ 3
section .text
    global _ft_read
_ft_read:
    mov rax, SYS_READ_MAC
    syscall
    jc _error
    ret
_error:
    mov r10, rax
    call ___error
    mov qword [rax], r10
    mov rax, -1
    ret