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::{CStr, CString, c_char};
use thiserror::Error as TeError;

// region: Error

/// Error occurs in this crate.
#[derive(Debug, TeError)]
pub enum Error {
    #[error("unexpected NUL when parsing into C/C++ string")]
    UnexpectedNul(#[from] std::ffi::NulError),

    #[error("given string pointer is nullptr when parsing from C/C++ string")]
    NullPtr,
    #[error("invalid UTF8 sequence when parsing from C/C++ string")]
    InvalidEncoding(#[from] std::str::Utf8Error),
}

/// Result type used in this crate.
type Result<T> = std::result::Result<T, Error>;

// endregion

// region: FFI String

struct FfiString {
    msg: CString,
}

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

    pub fn set_msg(&mut self, msg: &str) -> Result<()> {
        self.msg = CString::new(msg)?;
        Ok(())
    }

    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");
    }
}

// endregion

// region: Exposed Functions

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

/// Set thread local string exposed for C code.
pub fn set_ffi_string(msg: &str) -> Result<()> {
    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) -> Result<&'a str> {
    if ptr.is_null() {
        Err(Error::NullPtr)
    } else {
        let c_str = unsafe { CStr::from_ptr(ptr) };
        Ok(c_str.to_str()?)
    }
}

// endregion