How to wrap `printf` in libc properly?

[iosmanthus]

I try to wrap printf just like the following code:

extern crate libc;
use libc::{c_char, c_int};

extern "C" {
    fn printf(fmt: *const c_char, ...) -> c_int;
}
fn main() {
    unsafe {
        printf("hello world\n".as_ptr() as *const i8);
    }
}

But it prints something really strange...

However, if I append extra \0 to string I want to print, it prints properly.

extern crate libc;
use libc::{c_char, c_int};

extern "C" {
    fn printf(fmt: *const c_char, ...) -> c_int;
}
fn main() {
    unsafe {
        printf("hello world\n\0".as_ptr() as *const i8);
    }
}

Is it a bug or I didn't wrap the C function properly?

[DanielKeep]

C strings are zero-terminated. Rust strings are not. This is why the CStr and CString types exist.

[iosmanthus]

Thanks!

[kornel]

Warning in advance - assing CString to a variable before calling as_ptr() on it. Temporary value (not assigned to a variable) will give invalid, crash-inducing pointer.

[kinggoesgaming]

assing CString

• kornel, 2018

[Michael-F-Bryan]

I always like linking to your original post on the CString::as_ptr() issue.

CString/.as_ptr() is incredibly unsafe

I knew that CString::new().as_ptr() is dangerous, so I consciously avoided that convenient construct, and still, I got a use-after-free. I needed a nullable string for a C function, so I wrote: let option = Some("foo"); // Explicitly bind CString to a variable to keep it alive let c_option = option.and_then(|s| CString::new(s).ok()); // And it's not alive! This causes use-after-free, and Rust is doesn't say a word about it let p = c_option.map(|c| c.as_ptr()).unwrap_or(ptr::null()); // And…