https://jameshfisher.com/2018/02/20/c-inline-assembly-hello-world/

 

Hello world in C inline assembly

Here's an unusual "hello, world" in C:

int main(void) {
  register int    syscall_no  asm("rax") = 1;
  register int    arg1        asm("rdi") = 1;
  register char*  arg2        asm("rsi") = "hello, world!\n";
  register int    arg3        asm("rdx") = 14;
  asm("syscall");
  return 0;
}

$ cc hello.c
$ ./a.out
hello, world!

This C program doesn't use any C standard library functions. An
ordinary "hello world" program might use printf, ultimately making
the write system call. The above program bypasses this and makes the
system call more directly using "inline assembly". This program works
on Linux x86-64, but because assembly is so non-portable, probably
doesn't work on most other systems.

Inline assembly is written using the syntax form asm(...). The above
program uses this in a couple of ways. First, the statement asm
("syscall"); injects a syscall assembly instruction at that point in
the program. The syscall instruction expects a system call number in
register rax. The program arranges for rax to hold the value 1, the
system call number for write. It arranges this by declaring the local
variable syscall_no as register, and specifically marking this
register variable to be stored in register rax using asm("rax"). The
arguments for the write system call are put in rdi, rsi and rdx using
the same feature.

In the statement asm("syscall"), the string "syscall" is assembly in
"gas" (GNU ASsembler) syntax. We can write longer partions of
assembly here. For example, we can prepare the registers rax, rdi and
rdx using mov instructions instead of C variables:

int main(void) {
  register char* arg2 asm("rsi") = "hello, world!\n";
  asm("mov $1, %rax; mov $1, %rdi; mov $14, %rdx; syscall;");
  return 0;
}

Can we convert the rsi value to a mov instruction, too? This is
trickier, because we don't know its exact value. The value of rsi is
the second argument to write, a pointer to the bytes to be written.
So rsi should be given the pointer to the statically allocated string
"hello, world!\n". How do we get that pointer value into our assembly
code? To do so, we'll need to switch to "extended asm", a more
advanced feature which I'll cover another time.

Tagged #programming, #c, #assembly.

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