Herman J. Radtke III

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Terminal Window Size With Rust FFI

Written by Herman J. Radtke III on 12 Jan 2015

I was writing some code in Rust and wanted to get the size of my terminal. This is currently not implemented in Rust though. I decided to read up on The Foreign Function Interface Guide to figure out how to do it myself. The Foreign Function Interface (FFI) is how Rust code interfaces with native C code. I also found a great Stack Overflow post that showed me how to write native C to get the terminal size. Based on my research, I needed to do three things in order to get my terminal size:

  • Create a winsize struct in Rust.
  • Use or externalize the ioctl C function.
  • Use or externalize the STDOUT_FILENO and TIOCGWINSZ constants.

Winsize Struct

Creating the winsize struct in Rust is pretty straight forward as Rust has structs too. I first needed to find the definition of winsize in C, so I did some googling and found the sys/ioctl.h source. When defining the struct, we must tell Rust to represent the struct as a C struct using #[repr(C)]. If you read the FFI Guide, then you may be wondering about #[repr(C, packed)]. I talk about packing in more detail at the end of the post. The struct members within winsize are all unsigned short. The C unsigned short is represented in Rust as c_ushort in the libc Rust module. We now have:

use libc::c_ushort;

#[repr(C)]
struct winsize {
    ws_row: c_ushort, /* rows, in characters */
    ws_col: c_ushort, /* columns, in characters */
    ws_xpixel: c_ushort, /* horizontal size, pixels */
    ws_ypixel: c_ushort /* vertical size, pixels */
} 

ioctl

Now I need to figure out what to do about the ioctl function. Checking out the Rust docs leads me to the ioctl function signature but I notice that this signature does not look like a variadic function (no varargs). I guess I have to externalize it in my code as a variadic function. I decided to check the Rust source to see if I could find an example of a variadic function and I stumbled in the definition of ioctl. This definition is variadic, so I guess rustdoc does not show this. Strange.

I have read that ws_xpixel and ws_ypixel are not used. I also have no use for them. I still opted to include them in my struct definition as I have no idea what ioctl is doing to that struct.

I have used this word externalized a few times already, so maybe I should now define it. To externalize something is to make that somethings C representation accessible to Rust code. You normally do this with function signatures, constants and global variables. Note that we did not externalize winsize, but instead copied the definition from C to Rust. We cannot externalize winsize as Rust needs to directly manage the definition and memory related to that struct.

The Constants

Finally, I need to deal with my constants. I was pretty sure STDOUT_FILENO would already be in Rust. Sure enough, libc::STDOUT_FILENO exists. I was not so lucky with TIOCGWINSZ. The TIOCGWINSZ constant acts as a command to ioctl. If you read the source of sys/ioctl.h, you will notice the value of the commands is based on some rules that encode information to ioctl. There is a fair amount of bit twiddling going on to generate these values. Even if we do the bitwise math by hand, we should still check our work. To do that, I wrote a simple C program that would tell us the proper hex value of TIOCGWINSZ:

#include <sys/ioctl.h>
#include <stdio.h>
#include <unistd.h>

int main (int argc, char **argv)
{
    printf("0x%x", TIOCGWINSZ);
    return 0;
}

Using this value I can create the same constant in Rust:

const TIOCGWINSZ: c_ulong = 0x40087468;

Putting It All Together

My function for get_winsize now looks like this:

fn get_winsize() -> IoResult<(isize, isize)> {
    let w = winsize { ws_row: 0, ws_col: 0, ws_xpixel: 0, ws_ypixel: 0 };
    let r = unsafe { ioctl(STDOUT_FILENO, TIOCGWINSZ, &w) };
     
    match r {
        0 => Ok((w.ws_col as isize, w.ws_row as isize)),
        _ => {
            return Err(standard_error(ResourceUnavailable))
        }
    }
} 

I initialize my variable containing a winsize struct with values of zero, just like I would memset(w, 0, sizeof(winsize)) in C. In order to use the externalized ioctl function, we have to wrap the code in unsafe {} blocks. This informs Rust this code is not to be checked by the compiler for safety. The ioctl function follows the C convention of returning a 0 for success and a -1 for an error. If an error occurs, I decided to throw an existing IoResult error already in Rust. I need to spend a little more time to externalize the errno global variable in C so I can get the exact error. If the function is successful, I return the width and height as a tuple.

Here is a gist of the complete program, including a simple test. This puts all the peices discussed above together and will properly calculate the terminal window size when executed.

To Pack Or Not

If you see a struct defined with __attribute__((__packed__)) then you need to use #[repr(C, packed)]. Example:

struct __attribute__((__packed__)) foo {
    char first;
    int second;
};

A packed C struct, usually only found in kernel development, is not padded. If you are not familiar with padding in C, then you may not understand what #[repr(C, packed)] does. When defining a struct in C, the struct members are aligned to word boundaries. A word is the natural address boundary for a given architecture. For example, on a 32-bit machine a word is 4 bytes. If a struct member does not align to a word boundary, the compiler will insert padding after the variable. A struct like

struct foo {
    char first; // 1 byte
    int second; // 4 bytes
};

is not 5 bytes in size, but 8 bytes due to padding. Here is how the same struct looks after the compiler has added padding:

struct foo {
    char first; // 1 byte
    char padding[3]; // 3 bytes
    int second; // 4 bytes
};

I found a Stack Overflow post the explains it in even greater detail. Also, check out the Data Structure Alignment article on Wikipedia.