在程序集中获取系统调用的返回值

Question

我正在Darwin x86_64（MacOS 10.14.6）上编写一个简单的汇编程序，该程序将创建文件（test.txt）并向其中写入“ E”。但是，由于某种原因，“ E”未写入文件。我在做什么错？

这是我的程序：

global start
section .text
    start:
        ;Create the file
        mov rax, 0x2000005
        mov rdi, fname
        mov rsi, 0x200
        mov rdx, 0o644
        syscall

        ;Write to file
        mov rdi, rax     ;As far as I know, this uses the fd returned by creating a file (previous syscall)
        mov rsi, msg
        mov rdx, 1
        mov rax, 0x2000004
        syscall

        ;Exit
        mov rax, 0x2000001
        mov rdi, 0
        syscall
section .data
    msg: db "E"     ;Message
    fname: db "test.txt"     ;File name

我也尝试过这个：

global start
section .text
    start:
        mov rax, 0x2000005
        mov rdi, fname
        mov rsi, 0x200
        mov rdx, 0o644
        syscall
        mov rdi, rax
        mov rsi, msg
        mov rdx, 2
        mov rax, 0x2000004
        syscall
        mov rax, 0x2000001
        mov rdi, 0
        syscall
section .data
    msg: db "E", -1
    fname: db "test.txt", 0

但都不起作用。

Answer 1

我找到了答案：

[我发现先创建文件，然后将其打开以进行写然后写作品。

global start
section .text
    start:
        ;Create file
        mov rax, 0x2000005
        mov rdi, fname
        mov rsi, 0x200
        mov rdx, 0o644
        syscall

        ;Open for writing
        mov rax, 0x2000005
        mov rdi, fname
        mov rsi, 1
        mov rdx, 0o644

        ;Now write
        syscall
        mov rdi, rax
        mov rsi, msg
        mov rdx, 1
        mov rax, 0x2000004
        syscall

        ;Exit
        mov rax, 0x2000001
        mov rdi, 0
        syscall
section .data
    msg: db "E"
    fname: db "test.txt"