openssl/doc/man3/SSL_read.pod

=pod

=head1 NAME

SSL_read_ex, SSL_read - read bytes from a TLS/SSL connection

=head1 SYNOPSIS

 #include <openssl/ssl.h>

 int SSL_read_ex(SSL *ssl, void *buf, size_t num, size_t *read);
 int SSL_read(SSL *ssl, void *buf, int num);

 int SSL_peek_ex(SSL *ssl, void *buf, size_t num, size_t *read);
 int SSL_peek(SSL *ssl, void *buf, int num);

=head1 DESCRIPTION

SSL_read_ex() and SSL_read() try to read B<num> bytes from the specified B<ssl>
into the buffer B<buf>. On success SSL_read_ex() will store the number of bytes
actually read in B<*read>.

SSL_peek_ex() and SSL_peek() are identical to SSL_read_ex() and SSL_read()
respectively except no bytes are actually removed from the underlying BIO during
the read, so that a subsequent call to SSL_read_ex() or SSL_read() will yield
at least the same bytes.

=head1 NOTES

In the paragraphs below a "read function" is defined as one of SSL_read_ex(),
SSL_read(), SSL_peek_ex() or SSL_peek().

If necessary, a read function will negotiate a TLS/SSL session, if not already
explicitly performed by L<SSL_connect(3)> or L<SSL_accept(3)>. If the
peer requests a re-negotiation, it will be performed transparently during
the read function operation. The behaviour of the read functions depends on the
underlying BIO.

For the transparent negotiation to succeed, the B<ssl> must have been
initialized to client or server mode. This is being done by calling
L<SSL_set_connect_state(3)> or SSL_set_accept_state() before the first
invocation of a read function.

The read functions work based on the SSL/TLS records. The data are received in
records (with a maximum record size of 16kB). Only when a record has been
completely received, can it be processed (decryption and check of integrity).
Therefore data that was not retrieved at the last read call can still be
buffered inside the SSL layer and will be retrieved on the next read
call. If B<num> is higher than the number of bytes buffered then the read
functions will return with the bytes buffered. If no more bytes are in the
buffer, the read functions will trigger the processing of the next record.
Only when the record has been received and processed completely will the read
functions return reporting success. At most the contents of the record will
be returned. As the size of an SSL/TLS record may exceed the maximum packet size
of the underlying transport (e.g. TCP), it may be necessary to read several
packets from the transport layer before the record is complete and the read call
can succeed.

If the underlying BIO is B<blocking>, a read function will only return once the
read operation has been finished or an error occurred, except when a
renegotiation takes place, in which case a SSL_ERROR_WANT_READ may occur. This
behaviour can be controlled with the SSL_MODE_AUTO_RETRY flag of the
L<SSL_CTX_set_mode(3)> call.

If the underlying BIO is B<non-blocking>, a read function will also return when
the underlying BIO could not satisfy the needs of the function to continue the
operation. In this case a call to L<SSL_get_error(3)> with the
return value of the read function will yield B<SSL_ERROR_WANT_READ> or
B<SSL_ERROR_WANT_WRITE>. As at any time a re-negotiation is possible, a
a read function can also cause write operations! The calling process then must
repeat the call after taking appropriate action to satisfy the needs of the read
function. The action depends on the underlying BIO. When using a non-blocking
socket, nothing is to be done, but select() can be used to check for the
required condition. When using a buffering BIO, like a BIO pair, data must be
written into or retrieved out of the BIO before being able to continue.

L<SSL_pending(3)> can be used to find out whether there
are buffered bytes available for immediate retrieval. In this case
the read function can be called without blocking or actually receiving
new data from the underlying socket.

=head1 WARNING

When a read function operation has to be repeated because L<SSL_get_error(3)>
returned B<SSL_ERROR_WANT_READ> or B<SSL_ERROR_WANT_WRITE>, it must be repeated
with the same arguments.

=head1 RETURN VALUES

SSL_read_ex() and SSL_peek_ex() will return 1 for success or 0 for failure.
Success means that 1 or more application data bytes have been read from the SSL
connection. Failure means that no bytes could be read from the SSL connection.
Failures can be retryable (e.g. we are waiting for more bytes to
be delivered by the network) or non-retryable (e.g. a fatal network error). In
the event of a failure call L<SSL_get_error(3)> to find out the reason and if
the call is retryable or not.

For SSL_read() and SSL_peek() the following return values can occur:

=over 4

=item E<gt>0

The read operation was successful; the return value is the number of
bytes actually read from the TLS/SSL connection.

=item Z<>0

The read operation was not successful. The reason may either be a clean
shutdown due to a "close notify" alert sent by the peer (in which case
the SSL_RECEIVED_SHUTDOWN flag in the ssl shutdown state is set
(see L<SSL_shutdown(3)>,
L<SSL_set_shutdown(3)>). It is also possible, that
the peer simply shut down the underlying transport and the shutdown is
incomplete. Call SSL_get_error() with the return value B<ret> to find out,
whether an error occurred or the connection was shut down cleanly
(SSL_ERROR_ZERO_RETURN).

=item E<lt>0

The read operation was not successful, because either an error occurred
or action must be taken by the calling process. Call SSL_get_error() with the
return value B<ret> to find out the reason.

=back

=head1 SEE ALSO

L<SSL_get_error(3)>, L<SSL_write_ex(3)>,
L<SSL_CTX_set_mode(3)>, L<SSL_CTX_new(3)>,
L<SSL_connect(3)>, L<SSL_accept(3)>
L<SSL_set_connect_state(3)>,
L<SSL_pending(3)>,
L<SSL_shutdown(3)>, L<SSL_set_shutdown(3)>,
L<ssl(3)>, L<bio(3)>

=head1 COPYRIGHT

Copyright 2000-2016 The OpenSSL Project Authors. All Rights Reserved.

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
L<https://www.openssl.org/source/license.html>.

=cut