wfassoc/wfassoc-cdylib/src/cstr_ffi.rs

//! When calling this dynamic library with outside programs,
//! outer programs may usually need to fetch string resource produced by Rust code.
//! However it is impossible pass Rust string directly to outer program.
//! 
//! This module provide **thread independent** string cache for resolving this issue.
//! When we need pass string to outer programs, we push that string into this string as C-like format,
//! then return its pointer to outer program.
//! So that outside program can utilize it like calling C/C++ library.
//! The only thing that outer programs should note is that this string is volatile,
//! once they get it, they must dupliate it immediately before any futher calling to this dynamic library.
use std::cell::RefCell;
use std::ffi::{CString, CStr, c_char};

struct StringCache {
    msg: CString,
}

impl StringCache {
    fn new() -> Self {
        Self {
            msg: CString::new("").expect("empty string must be valid for CString"),
        }
    }

    pub fn set_msg(&mut self, msg: &str) {
        self.msg = CString::new(msg).expect("unexpected blank in string push into string cache");
    }

    pub fn get_msg(&self) -> *const c_char {
        self.msg.as_ptr()
    }

    pub fn clear_msg(&mut self) {
        self.msg = CString::new("").expect("empty string must be valid for CString");
    }
}

thread_local! {
    static STRING_CACHE: RefCell<StringCache> = RefCell::new(StringCache::new());
}

/// Set thread local string exposed for C code.
pub fn set_ffi_string(msg: &str) {
    STRING_CACHE.with(|e| {
        e.borrow_mut().set_msg(msg);
    });
}

/// Get const pointer to thread local string exposed for C code.
pub fn get_ffi_string() -> *const c_char {
    STRING_CACHE.with(|e| e.borrow().get_msg())
}

/// Clear thread local string exposed for C code.
/// 
/// This function usually should be called at the beginning of every exposed C functions.
pub fn clear_ffi_string() {
    STRING_CACHE.with(|e| {
        e.borrow_mut().clear_msg();
    });
}

/// Parse string given by C code into Rust string.
pub fn parse_ffi_string<'a>(ptr: *const c_char) -> &'a CStr {
    unsafe { CStr::from_ptr(ptr) }
}