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ignacio ibarra
Asked: 2020-09-19 19:32:53 +0800 CST 2020-09-19 19:32:53 +0800 CST

How to print string with asm! in rust?

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I am creating a code to be able to print text on the screen with the rust macro (asm!) and this is the code:

fn printmsg(string: &str) {
    let length: usize = string.len();
    unsafe {
        asm!("syscall" :: "{rax}"(1), "{rdi}"(1), "{rsi}"(&string), "{rdx}"(length));
    }
}

fn main() {
    let s = "hello world!";
    printmsg(s);
}

When I run the code it prints random letters...

How do I get it to print "hello world!" and not something else?

rust

1 Answers

  1. Best Answer

    A string slice consists of a pointer to the string and a length. If you pass the output by odyou get:

    $ cargo run 2> /dev/null | od -tx1
0000000 90 48 30 8c e6 7f 00 00 0c 00 00 00
0000014

    The first 8 are a pointer, the next (0x0c = 12) are the length. Another way of looking at it:

     $ cargo run | perl -ne 'printf "ptr: %x, len: %d\n", unpack "QL"'
    Finished dev [unoptimized + debuginfo] target(s) in 0.03s
     Running `target/debug/printasm`
ptr: 7f706ab51890, len: 12

    To access the pointer to the actual string, the () method can be used as_ptr.

    asm!("syscall" :: "{rax}"(1), "{rdi}"(1), "{rsi}"(string.as_ptr()), "{rdx}"(length));

    Note that this string is not terminated by a NUL byte, so if you wanted to pass it to a library written in C or similar you would have to convert it to CStringdel module std::ffi::CString.

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