# SSLWriteFunc A pointer to a customized write function that secure transport calls to write data to the connection. ``` typealias SSLWriteFunc = (SSLConnectionRef, UnsafeRawPointer, UnsafeMutablePointer) -> OSStatus ``` ## Parameters `connection` The SSL session connection reference. `data` A pointer to the data to write to the connection.You must allocate this memory before calling this function. `dataLength` Before calling, an integer representing the length of the data in bytes. On return, this is the number of bytes actually transferred. ## Return Value A result code. See [Secure Transport Result Codes](/documentation/Security/secure-transport-result-codes). ## Discussion Before using the secure transport API, you must write the functions [`SSLReadFunc`](/documentation/Security/SSLReadFunc) and [`SSLWriteFunc`](/documentation/Security/SSLWriteFunc) and provide them to the library by calling the [`SSLSetIOFuncs(_:_:_:)`](/documentation/Security/SSLSetIOFuncs(_:_:_:)) function. You may configure the underlying connection to operate in a non-blocking manner. In that case, a write operation may well return [`errSSLWouldBlock`](/documentation/Security/errSSLWouldBlock), indicating less data than requested was transferred and nothing is wrong except that the requested I/O hasn’t completed. This result is returned to the caller from the functions [`SSLRead(_:_:_:_:)`](/documentation/Security/SSLRead(_:_:_:_:)), [`SSLWrite(_:_:_:_:)`](/documentation/Security/SSLWrite(_:_:_:_:)), or [`SSLHandshake(_:)`](/documentation/Security/SSLHandshake(_:)). --- Copyright © 2026 Apple Inc. All rights reserved. | [Terms of Use](https://www.apple.com/legal/internet-services/terms/site.html) | [Privacy Policy](https://www.apple.com/privacy/privacy-policy)