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SSL_WRITE(3ossl)		     OpenSSL			SSL_WRITE(3ossl)

NAME
     SSL_write_ex2, SSL_write_ex, SSL_write, SSL_sendfile, SSL_WRITE_FLAG_CON-
     CLUDE - write bytes to a TLS/SSL connection

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);

DESCRIPTION
     SSL_write_ex() and SSL_write() write num bytes from the buffer buf into the
     specified ssl connection. On success SSL_write_ex() will store the number
     of bytes written in *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
     flags argument of 0 is exactly equivalent to calling SSL_write_ex().

     SSL_sendfile() writes size bytes from offset offset in the file descriptor
     fd to the specified SSL connection 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 pro-
     vided here to allow users to maintain the same interface.	The meaning of
     flags is platform dependent.  Currently, under Linux it is ignored.

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

     SSL_WRITE_FLAG_CONCLUDE
	 This flag is only supported on QUIC stream SSL objects (or QUIC connec-
	 tion 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 "*written ==
	 num"), the relevant QUIC stream's send part is concluded automatically
	 as though SSL_stream_conclude(3) was called (causing transmission of a
	 FIN for the stream).

	 While using this flag is semantically equivalent to calling
	 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 in-
	 dicating 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.

     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 SSL_WRITE_FLAG_CONCLUDE, that a QUIC
     stream SSL object is being used) before attempting to use it.

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 al-
     ready explicitly performed by SSL_connect(3) or 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 ssl must have been initial-
     ized to client or server mode. This is being done by calling SSL_set_con-
     nect_state(3) or SSL_set_accept_state() before the first call to a write
     function.

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

     If the underlying BIO is nonblocking the write functions will also return
     when the underlying BIO could not satisfy the needs of the function to con-
     tinue the operation. In this case a call to SSL_get_error(3) with the re-
     turn value of the write function will yield SSL_ERROR_WANT_READ or SSL_ER-
     ROR_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 con-
     tents of buf of length num has been written. This default behaviour can be
     changed with the SSL_MODE_ENABLE_PARTIAL_WRITE option of
     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 openssl-quic(7) and
     SSL_handle_events(3) for more information.

WARNINGS
     When a write function call has to be repeated because SSL_get_error(3) re-
     turned SSL_ERROR_WANT_READ or SSL_ERROR_WANT_WRITE, it must be repeated
     with the same arguments.  The data that was passed might have been par-
     tially processed.	When SSL_MODE_ACCEPT_MOVING_WRITE_BUFFER was set using
     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.

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. Fail-
     ure 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). Fail-
     ures 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 fail-
     ure call 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:

     > 0 The write operation was successful, the return value is the number of
	 bytes actually written to the TLS/SSL connection.

     <= 0The 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 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.

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

     >= 0The 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 size for a partial write.

     < 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.

SEE ALSO
     SSL_get_error(3), SSL_read_ex(3), SSL_read(3) SSL_CTX_set_mode(3),
     SSL_CTX_new(3), SSL_connect(3), SSL_accept(3) SSL_set_connect_state(3),
     BIO_ctrl(3), ssl(7), bio(7)

HISTORY
     The SSL_write_ex() function was added in OpenSSL 1.1.1.  The SSL_sendfile()
     function was added in OpenSSL 3.0.  The SSL_write_ex2() function was added
     in OpenSSL 3.3.

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
     <https://www.openssl.org/source/license.html>.

3.5.6				   2026-04-07			SSL_WRITE(3ossl)
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