openssl/doc/man3/SSL_write.pod

=pod

=head1 NAME

SSL_write_ex2, SSL_write_ex, SSL_write, SSL_sendfile, SSL_WRITE_FLAG_CONCLUDE -
write bytes to a TLS/SSL connection

=head1 SYNOPSIS

 #include <openssl/ssl.h>

 #define SSL_WRITE_FLAG_CONCLUDE

 ossl_ssize_t SSL_sendfile(SSL *s, int fd, off_t offset, size_t size, int flags);
 int SSL_write_ex2(SSL *s, const void *buf, size_t num,
                   uint64_t flags,
                   size_t *written);
 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>.

SSL_write_ex2() functions similarly to SSL_write_ex() but can also accept
optional flags which modify its behaviour. Calling SSL_write_ex2() with a
I<flags> argument of 0 is exactly equivalent to calling SSL_write_ex().

SSL_sendfile() writes B<size> bytes from offset B<offset> in the file
descriptor B<fd> to the specified SSL connection B<s>. This function provides
efficient zero-copy semantics. SSL_sendfile() is available only when
Kernel TLS is enabled, which can be checked by calling BIO_get_ktls_send().
It is provided here to allow users to maintain the same interface.
The meaning of B<flags> is platform dependent.
Currently, under Linux it is ignored.

The I<flags> argument to SSL_write_ex2() can accept zero or more of the
following flags. Note that which flags are supported will depend on the kind of
SSL object and underlying protocol being used:

=over 4

=item B<SSL_WRITE_FLAG_CONCLUDE>

This flag is only supported on QUIC stream SSL objects (or QUIC connection SSL
objects with a default stream attached).

If this flag is set, and the call to SSL_write_ex2() succeeds, and all of the
data passed to the call is written (meaning that C<*written == num>), the
relevant QUIC stream's send part is concluded automatically as though
L<SSL_stream_conclude(3)> was called (causing transmission of a FIN for the
stream).

While using this flag is semantically equivalent to calling
L<SSL_stream_conclude(3)> after a successful call to this function, using this
flag enables greater efficiency than making these two API calls separately, as
it enables the written stream data and the FIN flag indicating the end of the
stream to be scheduled as part of the same QUIC STREAM frame and QUIC packet.

Setting this flag does not cause a stream's send part to be concluded if not all
of the data passed to the call was consumed.

=back

A call to SSL_write_ex2() fails if a flag is passed which is not supported or
understood by the given SSL object. An application should determine if a flag is
supported (for example, for B<SSL_WRITE_FLAG_CONCLUDE>, that a QUIC stream SSL
object is being used) before attempting to use it.

=head1 NOTES

In the paragraphs below a "write function" is defined as one of either
SSL_write_ex(), or SSL_write().

If necessary, a write 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 write function operation. The behaviour of the write 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 call to a write function.

If the underlying BIO is B<blocking>, the write functions will only return, once
the write operation has been finished or an error occurred.

If the underlying BIO is B<nonblocking> the write functions 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 write function 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 a write function can also cause read operations! The calling process
then must repeat the call after taking appropriate action to satisfy the needs
of the write function. The action depends on the underlying BIO. When using a
nonblocking 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.

The write functions 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 the write functions will also return with success when a
partial write has been successfully completed. In this case the write function
operation is considered completed. The bytes are sent and a new write call 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.

When used with a QUIC SSL object, calling an I/O function such as SSL_write()
allows internal network event processing to be performed. It is important that
this processing is performed regularly. If an application is not using thread
assisted mode, an application should ensure that an I/O function such as
SSL_write() is called regularly, or alternatively ensure that SSL_handle_events()
is called regularly. See L<openssl-quic(7)> and L<SSL_handle_events(3)> for more
information.

=head1 WARNINGS

When a write function call 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.
The data that was passed might have been partially processed.
When B<SSL_MODE_ACCEPT_MOVING_WRITE_BUFFER> was set using L<SSL_CTX_set_mode(3)>
the pointer can be different, but the data and length should still be the same.

You should not call SSL_write() with num=0, it will return an error.
SSL_write_ex() can be called with num=0, but will not send application data to
the peer.

=head1 RETURN VALUES

SSL_write_ex() and SSL_write_ex2() return 1 for success or 0 for failure.
Success means that all requested application data bytes have been written to the
SSL connection or, if SSL_MODE_ENABLE_PARTIAL_WRITE is in use, at least 1
application data byte has been written to the SSL connection. Failure means that
not all the requested bytes have been written yet (if
SSL_MODE_ENABLE_PARTIAL_WRITE is not in use) or no bytes could be written to the
SSL connection (if SSL_MODE_ENABLE_PARTIAL_WRITE is in use). Failures can be
retryable (e.g. the network write buffer has temporarily filled up) 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_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, 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.

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

For SSL_sendfile(), the following return values can occur:

=over 4

=item Z<>>= 0

The write operation was successful, the return value is the number
of bytes of the file written to the TLS/SSL connection. The return
value can be less than B<size> for a partial write.

=item E<lt> 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 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<BIO_ctrl(3)>,
L<ssl(7)>, L<bio(7)>

=head1 HISTORY

The SSL_write_ex() function was added in OpenSSL 1.1.1.
The SSL_sendfile() function was added in OpenSSL 3.0.

=head1 COPYRIGHT

Copyright 2000-2024 The OpenSSL Project Authors. All Rights Reserved.

Licensed under the Apache License 2.0 (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