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_ex(), SSL_read(), SSL_peek_ex(), SSL_peek(), SSL_shutdown(), SSL_write_ex() 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.
Some TLS implementations do not send a close_notify alert on shutdown.
On an unexpected EOF, versions before OpenSSL 3.0 returned SSL_ERROR_SYSCALL, nothing was added to the error stack, and errno was 0. Since OpenSSL 3.0 the returned error is SSL_ERROR_SSL with a meaningful error on the error stack.
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 peer has closed the connection for writing by sending the close_notify alert. No more data can be read. Note that SSL_ERROR_ZERO_RETURN does not necessarily indicate that the underlying transport has been closed.
This error can also appear when the option SSL_OP_IGNORE_UNEXPECTED_EOF is set. See SSL_CTX_set_options(3) for more details.
- SSL_ERROR_WANT_READ, SSL_ERROR_WANT_WRITE
The operation did not complete and can be retried later.
For non-QUIC SSL objects, SSL_ERROR_WANT_READ is returned when the last operation was a read operation from a nonblocking BIO. It means that not enough data was available at this time to complete the operation. If at a later time the underlying BIO has data available for reading the same function can be called again.
SSL_read() and SSL_read_ex() can also set SSL_ERROR_WANT_READ when there is still unprocessed data available at either the SSL or the BIO layer, even for a blocking BIO. See SSL_read(3) for more information.
For non-QUIC SSL objects, SSL_ERROR_WANT_WRITE is returned when the last operation was a write to a nonblocking BIO and it was unable to sent all data to the BIO. When the BIO is writable again, the same function can be called again.
Note that the retry may again lead to an 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 QUIC SSL objects, the meaning of SSL_ERROR_WANT_READ and SSL_ERROR_WANT_WRITE have different but largely compatible semantics. Since QUIC implements its own flow control and uses UDP datagrams, backpressure conditions in terms of the underlying BIO providing network I/O are not directly relevant to the circumstances in which these errors are produced. In particular, SSL_ERROR_WANT_WRITE indicates that the OpenSSL internal send buffer for a given QUIC stream has been filled. Likewise, SSL_ERROR_WANT_READ indicates that the OpenSSL internal receive buffer for a given QUIC stream is empty.
It is safe to call SSL_read() or SSL_read_ex() when more data is available even when the call that set this error was an SSL_write() or SSL_write_ex(). However, if the call was an SSL_write() or SSL_write_ex(), it should be called again to continue sending the application data. If you get SSL_ERROR_WANT_WRITE from SSL_write() or SSL_write_ex() then you should not do any other operation that could trigger IO other than to repeat the previous SSL_write() call.
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_ex(), SSL_read(), SSL_peek_ex(), or SSL_peek() may want to write data and SSL_write() or SSL_write_ex() 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_ex(), SSL_read(), SSL_peek_ex(), SSL_peek(), SSL_write_ex(), 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(3) or SSL_set_mode(3) 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_all_async_fds(3) or SSL_get_changed_async_fds(3). 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.
The asynchronous job could not be started because there were no async jobs available in the pool (see ASYNC_init_thread(3)). This will only occur if the mode has been set to SSL_MODE_ASYNC using SSL_CTX_set_mode(3) or SSL_set_mode(3) and a maximum limit has been set on the async job pool through a call to ASYNC_init_thread(3). The application should retry the operation after a currently executing asynchronous operation for the current thread has completed.
The operation did not complete because an application callback set by SSL_CTX_set_client_hello_cb() has asked to be called again. The TLS/SSL I/O function should be called again later. Details depend on the application.
Some non-recoverable, fatal I/O error occurred. The OpenSSL error queue may contain more information on the error. For socket I/O on Unix systems, consult errno for details. If this error occurs then no further I/O operations should be performed on the connection and SSL_shutdown() must not be called.
This value can also be returned for other errors, check the error queue for details.
A non-recoverable, fatal error in the SSL library occurred, usually a protocol error. The OpenSSL error queue contains more information on the error. If this error occurs then no further I/O operations should be performed on the connection and SSL_shutdown() must not be called.
The SSL_ERROR_WANT_ASYNC error code was added in OpenSSL 1.1.0. The SSL_ERROR_WANT_CLIENT_HELLO_CB error code was added in OpenSSL 1.1.1.
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