Hello world in C inline assembly

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!

" ; 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!

" ; 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!

" . 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.

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