feat: update cdylib cstr ffi module

wfassoc-cdylib/src/cstr_ffi.rs

@@ -0,0 +1,66 @@
+//! 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) }
+}