openssl/doc/ssl/SSL_read.pod

=pod

=head1 NAME

SSL_read - read bytes from a TLS/SSL connection.

=head1 SYNOPSIS

 #include <openssl/ssl.h>

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

=head1 DESCRIPTION

SSL_read() tries to read B<num> bytes from the specified B<ssl> into the
buffer B<buf>.

=head1 NOTES

If necessary, SSL_read() will negotiate a TLS/SSL session, if
not already explicitly performed by L<SSL_connect(3)|SSL_connect(3)> or
L<SSL_accept(3)|SSL_accept(3)>. If the
peer requests a re-negotiation, it will be performed transparently during
the SSL_read() operation. The behaviour of SSL_read() 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 not the case if a generic
method is being used (see L<SSL_CTX_new(3)|SSL_CTX_new(3)>, so that
L<SSL_set_connect_state(3)|SSL_set_connect_state(3)> or SSL_set_accept_state()
must be used before the first call to an SSL_read() or
L<SSL_write(3)|SSL_write(3)> function.

SSL_read() works based on the SSL/TLS records. The data are received in
records (with a maximum record size of 16kB for SSLv3/TLSv1). Only when a
record has been completely received, it can be processed (decryption and
check of integrity). Therefore data that was not retrieved at the last
call of SSL_read() can still be buffered inside the SSL layer and will be
retrieved on the next call to SSL_read(). If B<num> is higher than the
number of bytes buffered, SSL_read() will return with the bytes buffered.
If no more bytes are in the buffer, SSL_read() will trigger the processing
of the next record. Only when the record has been received and processed
completely, SSL_read() will return reporting success. At most the contents
of the record will be returned.

If the underlying BIO is B<blocking>, SSL_read() will only return, once the
read operation has been finished or an error occurred, except when a
renegotiation take 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)|SSL_CTX_set_mode(3)> call.

If the underlying BIO is B<non-blocking>, SSL_read() will also return
when the underlying BIO could not satisfy the needs of SSL_read()
to continue the operation. In this case a call to
L<SSL_get_error(3)|SSL_get_error(3)> with the
return value of SSL_read() will yield B<SSL_ERROR_WANT_READ> or
B<SSL_ERROR_WANT_WRITE>. As at any time a re-negotiation is possible, a
call to SSL_read() can also cause write operations! The calling process
then must repeat the call after taking appropriate action to satisfy the
needs of SSL_read(). 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.

=head1 WARNING

When an SSL_read() operation has to be repeated because of
B<SSL_ERROR_WANT_READ> or B<SSL_ERROR_WANT_WRITE>, it must be repeated
with the same arguments.

=head1 RETURN VALUES

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 0

The read operation was not successful, probably because no data was
available. Call SSL_get_error() with the return value B<ret> to find out,
whether an error occurred.

=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)|SSL_get_error(3)>, L<SSL_write(3)|SSL_write(3)>,
L<SSL_CTX_set_mode(3)|SSL_CTX_set_mode(3)>, L<SSL_CTX_new(3)|SSL_CTX_new(3)>,
L<SSL_connect(3)|SSL_connect(3)>, L<SSL_accept(3)|SSL_accept(3)>
L<SSL_set_connect_state(3)|SSL_set_connect_state(3)>,
L<ssl(3)|ssl(3)>, L<bio(3)|bio(3)>

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