SSL_get_error - obtain result code for TLS/SSL I/O operation
#include <openssl/ssl.h> int SSL_get_error(const SSL *ssl, int ret);
SSL_get_error() returns a result code (suitable for the C "switch" statement) for a preceding call to SSL_connect(), SSL_accept(), SSL_do_handshake(), SSL_read(), SSL_peek(), or SSL_write() on ssl. The value returned by that TLS/SSL I/O function must be passed to SSL_get_error() in parameter ret.
In addition to ssl and ret, SSL_get_error() inspects the current thread's OpenSSL error queue. Thus, SSL_get_error() must be used in the same thread that performed the TLS/SSL I/O operation, and no other OpenSSL function calls should appear in between. The current thread's error queue must be empty before the TLS/SSL I/O operation is attempted, or SSL_get_error() will not work reliably.
The following return values can currently occur:
The TLS/SSL I/O operation completed. This result code is returned if and only if ret > 0.
The TLS/SSL connection has been closed. If the protocol version is SSL 3.0 or TLS 1.0, this result code is returned only if a closure alert has occurred in the protocol, i.e. if the connection has been closed cleanly. Note that in this case SSL_ERROR_ZERO_RETURN does not necessarily indicate that the underlying transport has been closed.
- SSL_ERROR_WANT_READ, SSL_ERROR_WANT_WRITE
The operation did not complete; the same TLS/SSL I/O function should be called again later. If, by then, the underlying BIO has data available for reading (if the result code is SSL_ERROR_WANT_READ) or allows writing data (SSL_ERROR_WANT_WRITE), then some TLS/SSL protocol progress will take place, i.e. at least part of an TLS/SSL record will be read or written. Note that the retry may again lead to a SSL_ERROR_WANT_READ or SSL_ERROR_WANT_WRITE condition. There is no fixed upper limit for the number of iterations that may be necessary until progress becomes visible at application protocol level.
For socket BIOs (e.g. when SSL_set_fd() was used), select() or poll() on the underlying socket can be used to find out when the TLS/SSL I/O function should be retried.
Caveat: Any TLS/SSL I/O function can lead to either of SSL_ERROR_WANT_READ and SSL_ERROR_WANT_WRITE. In particular, SSL_read() or SSL_peek() may want to write data and SSL_write() may want to read data. This is mainly because TLS/SSL handshakes may occur at any time during the protocol (initiated by either the client or the server); SSL_read(), SSL_peek(), and SSL_write() will handle any pending handshakes.
- SSL_ERROR_WANT_CONNECT, SSL_ERROR_WANT_ACCEPT
The operation did not complete; the same TLS/SSL I/O function should be called again later. The underlying BIO was not connected yet to the peer and the call would block in connect()/accept(). The SSL function should be called again when the connection is established. These messages can only appear with a BIO_s_connect() or BIO_s_accept() BIO, respectively. In order to find out, when the connection has been successfully established, on many platforms select() or poll() for writing on the socket file descriptor can be used.
The operation did not complete because an application callback set by SSL_CTX_set_client_cert_cb() has asked to be called again. The TLS/SSL I/O function should be called again later. Details depend on the application.
The operation did not complete because an asynchronous engine is still processing data. This will only occur if the mode has been set to SSL_MODE_ASYNC using SSL_CTX_set_mode or SSL_set_mode and an asynchronous capable engine is being used. An application can determine whether the engine has completed its processing using select() or poll() on the asynchronous wait file descriptor. This file descriptor is available by calling SSL_get_async_wait_fd. The TLS/SSL I/O function should be called again later. The function must be called from the same thread that the original call was made from.
Some I/O error occurred. The OpenSSL error queue may contain more information on the error. If the error queue is empty (i.e. ERR_get_error() returns 0), ret can be used to find out more about the error: If ret == 0, an EOF was observed that violates the protocol. If ret == -1, the underlying BIO reported an I/O error (for socket I/O on Unix systems, consult errno for details).
A failure in the SSL library occurred, usually a protocol error. The OpenSSL error queue contains more information on the error.
SSL_ERROR_WANT_ASYNC was added in OpenSSL 1.1.0.