# SSLReadFunc A pointer to a customized read function that secure transport calls to read data from the connection. ``` typealias SSLReadFunc = (SSLConnectionRef, UnsafeMutableRawPointer, UnsafeMutablePointer) -> OSStatus ``` ## Parameters `connection` A connection reference. `data` On return, your callback should overwrite the memory at this location with the data read from the connection. `dataLength` On input, a pointer to an integer representing the length of the data in bytes. On return, your callback should overwrite that integer with the number of bytes actually transferred. ## Return Value Your callback must return an appropriate result code. See [Secure Transport Result Codes](/documentation/Security/secure-transport-result-codes). ## Discussion Before using the secure transport API, you must create a read function (conforming to the [`SSLReadFunc`](/documentation/Security/SSLReadFunc) prototype) and a write function (conforming to the [`SSLWriteFunc`](/documentation/Security/SSLWriteFunc) prototype) 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 read 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)