=pod

=head1 NAME

SSL_get_error - obtain result code for TLS/SSL I/O operation

=head1 SYNOPSIS

 #include <openssl/ssl.h>

 int SSL_get_error(const SSL *ssl, int ret);

=head1 DESCRIPTION

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_read_early_data(), SSL_peek_ex(), SSL_peek(),
SSL_write_ex(), SSL_write(), SSL_write_early_data(), SSL_sendfile()
or SSL_shutdown() on B<ssl>. The value returned by that TLS/SSL I/O
function must be passed to SSL_get_error() in parameter B<ret>.

=head1 NOTES

Some TLS implementations do not send a close_notify alert on shutdown.

On an unexpected EOF, versions before OpenSSL 3.0 returned
B<SSL_ERROR_SYSCALL>, nothing was added to the error stack, and errno was 0.
Since OpenSSL 3.0 the returned error is B<SSL_ERROR_SSL> with a meaningful
error on the error stack (SSL_R_UNEXPECTED_EOF_WHILE_READING). This error reason
code may be used for control flow decisions (see the man page for
L<ERR_GET_REASON(3)> for further details on this).

=head1 RETURN VALUES

The following return values can currently occur:

=over 4

=item SSL_ERROR_NONE

The TLS/SSL I/O operation completed.  This result code is returned
if and only if B<ret E<gt> 0>.

=item SSL_ERROR_ZERO_RETURN

The TLS/SSL peer has closed the connection for writing by sending the
close_notify alert.
No more data can be read.
Note that B<SSL_ERROR_ZERO_RETURN> does not necessarily
indicate that the underlying transport has been closed.

This error can also appear when the option B<SSL_OP_IGNORE_UNEXPECTED_EOF>
is set. See L<SSL_CTX_set_options(3)> for more details.

=item SSL_ERROR_WANT_READ, SSL_ERROR_WANT_WRITE

The operation did not complete and can be retried later.

For non-QUIC SSL objects, B<SSL_ERROR_WANT_READ> is returned when the last
operation was a read operation from a nonblocking B<BIO>.
It means that not enough data was available at this time to complete the
operation.
If at a later time the underlying B<BIO> has data available for reading the same
function can be called again.

SSL_read() and SSL_read_ex() can also set B<SSL_ERROR_WANT_READ> when there is
still unprocessed data available at either the B<SSL> or the B<BIO> layer, even
for a blocking B<BIO>.
See L<SSL_read(3)> for more information.

For non-QUIC SSL objects, B<SSL_ERROR_WANT_WRITE> is returned when the last
operation was a write to a nonblocking B<BIO> and it was unable to send all data
to the B<BIO>. When the B<BIO> is writable again, the same function can be
called again.

Note that the retry may again lead to an B<SSL_ERROR_WANT_READ> or
B<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 B<SSL_ERROR_WANT_READ> and
B<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,
B<SSL_ERROR_WANT_WRITE> indicates that the OpenSSL internal send buffer for a
given QUIC stream has been filled. Likewise, B<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 B<SSL_ERROR_WANT_WRITE>
from SSL_write() or SSL_write_ex() then you should not do any other operation
that could trigger B<IO> other than to repeat the previous SSL_write() call.

For socket B<BIO>s (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
B<SSL_ERROR_WANT_READ> and B<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.

=item 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.

=item SSL_ERROR_WANT_X509_LOOKUP

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.

=item SSL_ERROR_WANT_ASYNC

The job did not complete because an asynchronous provider is still
processing data. This will only occur if the mode has been set to SSL_MODE_ASYNC
using L<SSL_CTX_set_mode(3)> or L<SSL_set_mode(3)> and an asynchronous capable
provider is being used. An application can determine whether the job has
completed its processing using select() or poll() on the asynchronous wait file
descriptor. This file descriptor is available by calling
L<SSL_get_all_async_fds(3)> or L<SSL_get_changed_async_fds(3)>. The TLS/SSL I/O
function should be called again later. The function B<must> be called from the
same thread that the original call was made from.

=item SSL_ERROR_WANT_ASYNC_JOB

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 L<SSL_CTX_set_mode(3)> or
L<SSL_set_mode(3)> and a maximum limit has been set on the async job pool
through a call to L<ASYNC_init_thread(3)>. The application should retry the
operation after a currently executing asynchronous operation for the current
thread has completed.

=item SSL_ERROR_WANT_CLIENT_HELLO_CB

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.

=item SSL_ERROR_SYSCALL

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
B<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.

=item SSL_ERROR_SSL

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.

=back

The OpenSSL error queue can be inspected with the B<ERR> family of functions,
such as L<ERR_print_errors(3)> and L<ERR_peek_last_error_all(3)>.

=head1 SEE ALSO

L<ssl(7)>,
L<ERR_clear_error(3)>, ERR_print_errors(3), ERR_peek_last_error_all(3)

=head1 HISTORY

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.

Since OpenSSL 4.0 SSL_get_error() no longer depends on the state of the
error stack, so it is no longer necessary to empty the error queue
before the TLS/SSL I/O operations.

=head1 COPYRIGHT

Copyright 2000-2026 The OpenSSL Project Authors. All Rights Reserved.

Licensed under the Apache License 2.0 (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
L<https://www.openssl.org/source/license.html>.

=cut
