I have looked at similar questions, but cannot seem to find what is wrong with my code.

I am attempting to make the "write" syscall on MacOS to print a string to standard output.

I am able to do it with printf perfectly, and am familiar with calling other functions in x64 assembly.

This is, however, my first attempt at a syscall.

I am using GCC's GAS assembler.

This is my code:

.section __TEXT,__text

.globl _main

_main:
    pushq %rbp
    movq %rsp, %rbp

    subq $32, %rsp

    movq $0x20000004, %rax
    movq $1, %rdi
    leaq syscall_str(%rip), %rsi
    movq $25, %rdx
    syscall

    jc error

    xorq %rax, %rax

    leave
    ret

error:
    movq $1, %rax
    leave
    ret

.section __DATA,__data
syscall_str:
    .asciz "Printed with a syscall.\n"

There does not seem to be any error; there is simply nothing written to stdout.

I know that start is usually used as the starting point for an executable on MacOS, but it does not compile with GCC.

CodePudding user response：

You are using the incorrect SYSCALL number for MacOS. The base for the user system calls is 0x2000000. You are incorrectly using 0x2000000. As a result you have encoded the write SYSCALL as $0x20000004 when it should have been $0x2000004 (one less zero)

As a rule of thumb, make sure you are using the correct value for the the SYSCALL number in the %rax register; ensure you are using the correct arguments for the write SYSCALL. The write SYSCALL expects the following arguments:

• %rdi: file descriptor to write to (e.g. 1 for standard output)
• %rsi: pointer to the buffer containing the data to be written
• %rdx: number of bytes to be written
• On macOS, you need to use the syscall instruction to invoke a system call.