While working on wallers, I came across the need to set the desktop background for Windows machines. For Linux machines, the answer was simple: delegate to feh. For Windows, I needed to delegate to a Windows API call.

The Windows API call of interest is SystemParametersInfo, specifically with the action as SPI_SETDESKWALLPAPER (20 in decimal). For SPI_SETDESKWALLPAPER specifically, the second and fourth parameters are not used, so they can be set to 0. Thus, in order to change the desktop wallpaper, one would call

SystemParametersInfo(20, 0, "path/to/wallpaper.png", 0)

In particular, this function is found in user32.dll. Luckily, retep998 has created a Rust binding for it and it is available as a crate. With this knowledge, changing the desktop wallpaper becomes quite trivial. The only complication is that there are two forms of SystemParametersInfo : SystemParametersInfoA and SystemParametersInfoW. From what I’ve gathered, the former should be called on 32 bit systems while the latter is called on 64 bit systems. This works on my desktop machine; however, on my 64bit Surface Book, SystemParametersInfoW sets the wallpaper to black while SystemParametersInfoA works as intended.

The final bit of difficulty comes in that these functions accept a *mut c_void as a parameter rather than a string (making the function unsafe.. sad). Thus, conversion must also take place.

extern crate user32;

use std::ffi::CString;
use std::os::raw::c_void;

fn set_wallpaper(path: String, force_32bit: bool) -> Result<(), ()> {
 let wallpaper_func = if force_32bit || cfg!(target_pointer_width = "32") {
        user32::SystemParametersInfoA
    }
    else {
        user32::SystemParametersInfoW
    };

    // do work to get the pointer of our owned string
    let path_ptr = CString::new(path).unwrap();
    let path_ptr_c = path_ptr.into_raw();
    let result = unsafe {
        match path_ptr_c.is_null() {
            false => wallpaper_func(20, 0, path_ptr_c as *mut c_void, 0),
            true => 0
        }
    };

    // rust documentation says we must return the pointer this way
    unsafe {
        CString::from_raw(path_ptr_c)
    };

    match result {
        0 => Err( () )
        _ => Ok( () )
    }
}

As you notice, we also insert a way for the user to call SystemParamtersInfoA on a 64bit system, just in-case…. Of course, since this uses OS-specific libraries, it’s best to use macros to prevent this code from running on non-Windows machines.

#[cfg(windows)] extern crate user32;

#[cfg(windows)] use std::ffi::CString;
#[cfg(windows)] use std::os::raw::c_void;
#[cfg(not(windows))] use std::process::Command;

#[cfg(windows)]
fn set_wallpaper(path: String, force_32bit: bool) -> Result<(), ()> {
 let wallpaper_func = if force_32bit || cfg!(target_pointer_width = "32") {
        user32::SystemParametersInfoA
    }
    else {
        user32::SystemParametersInfoW
    };

    // do work to get the pointer of our owned string
    let path_ptr = CString::new(path).unwrap();
    let path_ptr_c = path_ptr.into_raw();
    let result = unsafe {
        match path_ptr_c.is_null() {
            false => wallpaper_func(20, 0, path_ptr_c as *mut c_void, 0),
            true => 0
        }
    };

    // rust documentation says we must return the pointer this way
    unsafe {
        CString::from_raw(path_ptr_c)
    };

    match result {
        0 => Err( () )
        _ => Ok( () )
    }
}

#[cfg(not(windows))]
fn set_wallpaper(path: String, _: bool) -> Result<(), ()> {
let result = Command::new("feh")
        .arg("--bg-fill")
        .arg(path)
        .status()?.success();

    match result {
        true => Ok( () ),
        false => Err( () )
    }
}