diff --git a/Sources/PerfectPostgreSQL/PerfectPostgreSQL.swift b/Sources/PerfectPostgreSQL/PerfectPostgreSQL.swift
index ebbf057..5487f87 100644
--- a/Sources/PerfectPostgreSQL/PerfectPostgreSQL.swift
+++ b/Sources/PerfectPostgreSQL/PerfectPostgreSQL.swift
@@ -327,74 +327,100 @@ public final class PGResult {
 
 /// connection management class
 public final class PGConnection {
-	
+
 	/// Connection Status enum
 	public enum StatusType {
 		case ok
 		case bad
 	}
-	
+
+	// Take care that conn is not thread-safe.
 	var conn = OpaquePointer(bitPattern: 0)
 	var connectInfo: String = ""
-	
+
+	// Acquire this lock before accessing conn.
+	// Need to be recursive to support transaction.
+	private var lock = NSRecursiveLock()
+
 	/// empty init
 	public init() {
-		
+
 	}
-	
+
 	deinit {
 		close()
 	}
-	
+
 	/// Makes a new connection to the database server.
 	public func connectdb(_ info: String) -> StatusType {
+		lock.lock()
+		defer { lock.unlock() }
+
 		conn = PQconnectdb(info)
 		connectInfo = info
 		return status()
 	}
-	
+
 	/// Close db connection
 	public func close() {
 		finish()
 	}
-	
+
 	/// Closes the connection to the server. Also frees memory used by the PGconn object.
 	public func finish() {
+		lock.lock()
+		defer { lock.unlock() }
+
 		if conn != nil {
 			PQfinish(conn)
 			conn = OpaquePointer(bitPattern: 0)
 		}
 	}
-	
+
 	/// Returns the status of the connection.
 	public func status() -> StatusType {
+		lock.lock()
+		defer { lock.unlock() }
+
 		let status = PQstatus(conn)
 		return status == CONNECTION_OK ? .ok : .bad
 	}
-	
+
 	/// Returns the error message most recently generated by an operation on the connection.
 	public func errorMessage() -> String {
+		lock.lock()
+		defer { lock.unlock() }
+
 		return String(validatingUTF8: PQerrorMessage(conn)) ?? ""
 	}
-	
+
 	/// Submits a command to the server and waits for the result.
 	public func exec(statement: String) -> PGResult {
+		lock.lock()
+		defer { lock.unlock() }
+
 		return PGResult(PQexec(conn, statement))
 	}
-	
+
 	/// Sends data to the server during COPY_IN state.
 	public func putCopyData(data: String) {
+		lock.lock()
+		defer { lock.unlock() }
+
 		PQputCopyData(self.conn, data, Int32(data.count))
 	}
-	
+
 	/// Sends end-of-data indication to the server during COPY_IN state.
 	/// If withError is set, the copy is forced to fail with the error description supplied.
 	public func putCopyEnd(withError: String? = nil) -> PGResult {
+		lock.lock()
+		defer { lock.unlock() }
+
 		PQputCopyEnd(self.conn, withError)
 		let result = PGResult(PQgetResult(self.conn))
 		return result
 	}
-	
+
 	// !FIX! does not handle binary data
 	/// Submits a command to the server and waits for the result, with the ability to pass parameters separately from the SQL command text.
 	public func exec(statement: String, params: [Any?]) -> PGResult {
@@ -456,10 +482,14 @@ public final class PGConnection {
 				formats[idx] = 0
 			}
 		}
+
+		lock.lock()
+		defer { lock.unlock() }
+
 		let r = PQexecParams(conn, statement, Int32(count), nil, values, lengths, formats, Int32(0))
 		return PGResult(r)
 	}
-	
+
 	/// Assert that the connection status is OK
 	///
 	/// - throws: If the connection status is bad
@@ -498,13 +528,16 @@ public final class PGConnection {
 			throw PostgreSQLError.error("Failed to execute statement. status: \(status)")
 		}
 	}
-	
+
 	/// Executes a BEGIN, calls the provided closure and executes a ROLLBACK if an exception occurs or a COMMIT if no exception occurs.
 	///
 	/// - parameter closure: Block to be executed inside transaction
 	/// - throws: If the provided closure fails
 	/// - returns: If successful then the return value from the `closure`
 	public func doWithTransaction<Result>(closure: () throws -> Result) throws -> Result {
+		lock.lock()
+		defer { lock.unlock() }
+
 		try ensureStatusIsOk()
 		try execute(statement: "BEGIN")
 		do {
@@ -516,23 +549,26 @@ public final class PGConnection {
 			throw error
 		}
 	}
-	
+
 	/// Handler for receiving a PGResult
 	public typealias ReceiverProc = (PGResult) -> Void
-	
+
 	/// Handler for processing a text message
 	public typealias ProcessorProc = (String) -> Void
-	
+
 	/// internal callback for notice receiving
 	internal var receiver: ReceiverProc = { _ in }
-	
+
 	/// internal callback for notice processing
 	internal var processor: ProcessorProc = { _ in }
-	
+
 	/// Set a new notice receiver
 	/// - parameter handler: a closure to handle the incoming notice
 	/// - returns: a C convention function pointer; would be nil if failed to set.
 	public func setReceiver(_ handler: @escaping ReceiverProc) -> PQnoticeReceiver? {
+		lock.lock()
+		defer { lock.unlock() }
+
 		guard let cn = self.conn else {
 			return nil
 		}
@@ -547,11 +583,14 @@ public final class PGConnection {
 			this.receiver(pgresult)
 		}, me)
 	}
-	
+
 	/// Set a new notice processor
 	/// - parameter handler: a closure to handle the incoming notice
 	/// - returns: a C convention function pointer; would be nil if failed to set.
 	public func setProcessor(_ handler: @escaping ProcessorProc) -> PQnoticeProcessor?{
+		lock.lock()
+		defer { lock.unlock() }
+
 		guard let cn = self.conn else {
 			return nil
 		}