xref: /freebsd-11.0-release/crypto/openssl/doc/ssl/SSL_write.pod

=pod

=head1 NAME

SSL_write - write bytes to a TLS/SSL connection.

=head1 SYNOPSIS

 #include <openssl/ssl.h>

 int SSL_write(SSL *ssl, const void *buf, int num);

=head1 DESCRIPTION

SSL_write() writes B<num> bytes from the buffer B<buf> into the specified
B<ssl> connection.

=head1 NOTES

If necessary, SSL_write() 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_write() operation. The behaviour of 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)|SSL_set_connect_state(3)> or SSL_set_accept_state()
before the first call to an L<SSL_read(3)|SSL_read(3)> or SSL_write() function.

If the underlying BIO is B<blocking>, 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)|SSL_CTX_set_mode(3)> call.

If the underlying BIO is B<non-blocking>, SSL_write() will also return,
when the underlying BIO could not satisfy the needs of SSL_write()
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_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() can also cause read operations! The calling process
then must repeat the call after taking appropriate action to satisfy the
needs of 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() 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)|SSL_CTX_set_mode(3)>. When this flag is set,
SSL_write() will also return with success, when a partial write has been
successfully completed. In this case the SSL_write() operation is considered
completed. The bytes are sent and a new 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() 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() with num=0 bytes to be sent the behaviour is
undefined.

=head1 RETURN VALUES

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, because either the connection was
closed, 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.

SSLv2 (deprecated) does not support a shutdown alert protocol, so it can
only be detected, whether the underlying connection was closed. It cannot
be checked, why the closure happened.

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)|SSL_get_error(3)>, L<SSL_read(3)|SSL_read(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)>

=cut