=pod

=head1 NAME

SSL_read_ex, SSL_read, SSL_peek_ex, SSL_peek

- 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 *readbytes);

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

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

 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<*readbytes>.

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 one 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 B<SSL_MODE_AUTO_RETRY> has been switched off and a non-application data

record has been processed, the read function can return and set the error to

B<SSL_ERROR_WANT_READ>.

In this case there might still be unprocessed data available in the B<BIO>.

If read ahead was set using L<SSL_CTX_set_read_ahead(3)>, there might also still

be unprocessed data available in the B<SSL>.

This behaviour can be controlled using the L<SSL_CTX_set_mode(3)> call.

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

non-application data record has been processed and B<SSL_MODE_AUTO_RETRY> is

not set.

Note that if B<SSL_MODE_AUTO_RETRY> is set and only non-application data is

available the call will hang.

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 it's possible that non-application data needs to be sent,

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 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 which

indicates whether 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, because either the connection was closed,

an error occurred or action must be taken by the calling process.

Call L<SSL_get_error(3)> with the return value B<ret> to find out the reason.

Old documentation indicated a difference between 0 and -1, and that -1 was

retryable.

You should instead call SSL_get_error() to find out if it's retryable.

=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(7)>, L<bio(7)>

=head1 HISTORY

The SSL_read_ex() and SSL_peek_ex() functions were added in OpenSSL 1.1.1.

=head1 COPYRIGHT

Copyright 2000-2019 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