diff --git a/wfassoc-cdylib/src/cstr_ffi.rs b/wfassoc-cdylib/src/cstr_ffi.rs
index ed5b2a7..d44e72f 100644
--- a/wfassoc-cdylib/src/cstr_ffi.rs
+++ b/wfassoc-cdylib/src/cstr_ffi.rs
@@ -1,7 +1,7 @@
 //! 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.
@@ -9,21 +9,44 @@
 //! 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};
+use std::ffi::{CStr, CString, c_char};
+use thiserror::Error as TeError;
 
-struct StringCache {
+// 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 StringCache {
+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) {
-        self.msg = CString::new(msg).expect("unexpected blank in string push into string cache");
+    pub fn set_msg(&mut self, msg: &str) -> Result<()> {
+        self.msg = CString::new(msg)?;
+        Ok(())
     }
 
     pub fn get_msg(&self) -> *const c_char {
@@ -35,15 +58,19 @@ impl StringCache {
     }
 }
 
+// endregion
+
+// region: Exposed Functions
+
 thread_local! {
-    static STRING_CACHE: RefCell<StringCache> = RefCell::new(StringCache::new());
+    static STRING_CACHE: RefCell<FfiString> = RefCell::new(FfiString::new());
 }
 
 /// Set thread local string exposed for C code.
-pub fn set_ffi_string(msg: &str) {
+pub fn set_ffi_string(msg: &str) -> Result<()> {
     STRING_CACHE.with(|e| {
-        e.borrow_mut().set_msg(msg);
-    });
+        e.borrow_mut().set_msg(msg)
+    })
 }
 
 /// Get const pointer to thread local string exposed for C code.
@@ -52,7 +79,7 @@ pub fn get_ffi_string() -> *const c_char {
 }
 
 /// 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| {
@@ -61,6 +88,13 @@ pub fn clear_ffi_string() {
 }
 
 /// 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) }
+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
diff --git a/wfassoc-cdylib/src/lib.rs b/wfassoc-cdylib/src/lib.rs
index dc20722..bd01077 100644
--- a/wfassoc-cdylib/src/lib.rs
+++ b/wfassoc-cdylib/src/lib.rs
@@ -12,6 +12,9 @@ mod wrapper;
 /// Error occurs in this crate.
 #[derive(Debug, TeError)]
 enum Error {
+    /// Error when operating Schema.
+    #[error("{0}")]
+    Schema(#[from] wfassoc::highlevel::SchemaError),
     /// Error when parsing Schema into Program.
     #[error("{0}")]
     ParseProgram(#[from] wfassoc::highlevel::ParseProgramError),
@@ -19,10 +22,9 @@ enum Error {
     #[error("{0}")]
     Program(#[from] wfassoc::highlevel::ProgramError),
 
-    #[error("error occurs when parsing into C/C++ string")]
-    CastIntoCStr(#[from] std::ffi::NulError),
-    #[error("error occurs when parsing from C/C++ string")]
-    CastFromCStr(#[from] std::ffi::IntoStringError),
+    /// Error when manipulating with C-style string.
+    #[error("{0}")]
+    CStrFfi(#[from] cstr_ffi::Error),
 }
 
 /// Result type used in this crate.
@@ -37,12 +39,12 @@ type Result<T> = std::result::Result<T, Error>;
 
 #[unsafe(no_mangle)]
 pub extern "C" fn WFStartup() -> bool {
-    false
+    true
 }
 
 #[unsafe(no_mangle)]
 pub extern "C" fn WFShutdown() -> bool {
-    false
+    true
 }
 
 #[unsafe(no_mangle)]