#include <openssl/ssl.h> int SSL_get_error(const SSL *ssl, int 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.
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.
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.
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.
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.
This value can also be returned for other errors, check the error queue for details.
The issue is kept unfixed in OpenSSL 1.1.1 releases because many applications which choose to ignore this protocol error depend on the existing way of reporting the error.
Licensed under the OpenSSL license (the "License"). You may not use this file except in compliance with the License. You can obtain a copy in the file LICENSE in the source distribution or at <https://www.openssl.org/source/license.html>.