Syscall - write from asm

Linux provides a set of system calls aviable for each process. Some of the most used are:

Number	Name	Description
0	read	Read file
1	write	Write file
2	open	Open file
3	close	Close file
4	stat	Get info about file
9	mmap	Map memory page to file
12	brk	Reset top of the heap
32	dup2	Copy file descriptor
33	pause	Suspend process until signal arrives
37	alarm	Schedule delivery of alarm signal
39	getpid	Get process id
57	fork	Create process
59	execve	Execute program
60	_exit	Terminate process
61	wait4	Wait for a process to terminate
62	kill	Send signal to a process

Now using the write we can perform a syscall asm-wise:

#include <stdio.h> __asm( //setup string named msg ".section .data;" "msg: .ascii \"Hello from global space!\n\";" "msg_end: .equ len, msg_end - msg;" ".section .text;" ); int main(){ __asm( //call write "movq $1,%rax;" "movq $1,%rdi;" "movq $msg,%rsi;" "movq $len,%rdx;" "syscall;" //call exit "movq $60,%rax;" "movq $0,%rdi;" "syscall;" ); }

So we have chosen the syscall write with number 1, that number goes to rax. The next thing is to man 2 write which yields:

WRITE(2) Linux Programmer's Manual NAME write - write to a file descriptor SYNOPSIS #include <unistd.h> ssize_t write(int fd, const void *buf, size_t count);

So we need arg fd - that is a file descriptor. STDIO has fd = 1. We need a buffer. That we have set up in the first __asm. Last we need a number of bytes to be written. This also has been set up in the first call to __asm. The last part of the code is a similar call to exit - let's man:

_EXIT(2) Programmer's Manual NAME _exit, _Exit - terminate the calling process SYNOPSIS #include <unistd.h> void _exit(int status); #include <stdlib.h> void _Exit(int status);

As before we man for the type signature. In this case we need rax to be 60, as that it the number of the syscall, and we need to pass the exit status as 0. Pretty low level!