openssl/doc/man3/SSL_write.pod

=pod

=head1 NAME

SSL_write_ex, SSL_write - write bytes to a TLS/SSL connection

=head1 SYNOPSIS

 #include <openssl/ssl.h>

 int SSL_write_ex(SSL *s, const void *buf, size_t num, size_t *written);
 int SSL_write(SSL *ssl, const void *buf, int num);

=head1 DESCRIPTION

SSL_write_ex() and SSL_write() write B<num> bytes from the buffer B<buf> into
the specified B<ssl> connection. On success SSL_write_ex() will store the number
of bytes written in B<*written>.

=head1 NOTES

If necessary, SSL_write_ex() or SSL_write() 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 SSL_write_ex() or SSL_write() operation. The behaviour of SSL_write_ex() or
SSL_write() 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 call to an L<SSL_read_ex(3)>, L<SSL_read(3)>, SSL_write_ex() or
SSL_write() function.

If the underlying BIO is B<blocking>, SSL_write_ex() and SSL_write() will only
return, once the write 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)> call.

If the underlying BIO is B<non-blocking>, SSL_write_ex() or SSL_write() will
also return, when the underlying BIO could not satisfy the needs of
SSL_write_ex() or SSL_write() to continue the operation. In this case a call to
L<SSL_get_error(3)> with the
return value of SSL_write_ex() or SSL_write() 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_write_ex() or SSL_write() can also cause read operations! The
calling process then must repeat the call after taking appropriate action to
satisfy the needs of SSL_write_ex() or SSL_write(). 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.

SSL_write_ex() or SSL_write() will only return with success, when the complete
contents of B<buf> of length B<num> has been written. This default behaviour
can be changed with the SSL_MODE_ENABLE_PARTIAL_WRITE option of
L<SSL_CTX_set_mode(3)>. When this flag is set,
SSL_write_ex() or SSL_write() will also return with success, when a partial
write has been successfully completed. In this case the SSL_write_ex() or
SSL_write() operation is considered completed. The bytes are sent and a new
SSL_write_ex() or SSL_write() operation with a new
buffer (with the already sent bytes removed) must be started.
A partial write is performed with the size of a message block, which is
16kB for SSLv3/TLSv1.

=head1 WARNING

When an SSL_write_ex() or SSL_write() 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.

When calling SSL_write_ex() or SSL_write() with num=0 bytes to be sent the
behaviour is undefined.

=head1 RETURN VALUES

SSL_write_ex() will return 1 for success or 0 for failure. In the event of a
failure call SSL_get_error() to find out the reason.

For SSL_write() the following return values can occur:

=over 4

=item E<gt>0

The write operation was successful, the return value is the number of
bytes actually written to the TLS/SSL connection.

=item Z<>0

The write operation was not successful. Probably the underlying connection
was closed. 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 write 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_read_ex(3)>, L<SSL_read(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(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