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.\" ========================================================================
.\"
.IX Title "SSL_WRITE 3ossl"
.TH SSL_WRITE 3ossl "2026-01-27" "3.5.5" "OpenSSL"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
.if n .ad l
.nh
.SH "NAME"
SSL_write_ex2, SSL_write_ex, SSL_write, SSL_sendfile, SSL_WRITE_FLAG_CONCLUDE \-
write bytes to a TLS/SSL connection
.SH "SYNOPSIS"
.IX Header "SYNOPSIS"
.Vb 1
\& #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);
.Ve
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
\&\fBSSL_write_ex()\fR and \fBSSL_write()\fR write \fBnum\fR bytes from the buffer \fBbuf\fR into
the specified \fBssl\fR connection. On success \fBSSL_write_ex()\fR will store the number
of bytes written in \fB*written\fR.
.PP
\&\fBSSL_write_ex2()\fR functions similarly to \fBSSL_write_ex()\fR but can also accept
optional flags which modify its behaviour. Calling \fBSSL_write_ex2()\fR with a
\&\fIflags\fR argument of 0 is exactly equivalent to calling \fBSSL_write_ex()\fR.
.PP
\&\fBSSL_sendfile()\fR writes \fBsize\fR bytes from offset \fBoffset\fR in the file
descriptor \fBfd\fR to the specified \s-1SSL\s0 connection \fBs\fR. This function provides
efficient zero-copy semantics. \fBSSL_sendfile()\fR is available only when
Kernel \s-1TLS\s0 is enabled, which can be checked by calling \fBBIO_get_ktls_send()\fR.
It is provided here to allow users to maintain the same interface.
The meaning of \fBflags\fR is platform dependent.
Currently, under Linux it is ignored.
.PP
The \fIflags\fR argument to \fBSSL_write_ex2()\fR can accept zero or more of the
following flags. Note that which flags are supported will depend on the kind of
\&\s-1SSL\s0 object and underlying protocol being used:
.IP "\fB\s-1SSL_WRITE_FLAG_CONCLUDE\s0\fR" 4
.IX Item "SSL_WRITE_FLAG_CONCLUDE"
This flag is only supported on \s-1QUIC\s0 stream \s-1SSL\s0 objects (or \s-1QUIC\s0 connection \s-1SSL\s0
objects with a default stream attached).
.Sp
If this flag is set, and the call to \fBSSL_write_ex2()\fR succeeds, and all of the
data passed to the call is written (meaning that \f(CW\*(C`*written == num\*(C'\fR), the
relevant \s-1QUIC\s0 stream's send part is concluded automatically as though
\&\fBSSL_stream_conclude\fR\|(3) was called (causing transmission of a \s-1FIN\s0 for the
stream).
.Sp
While using this flag is semantically equivalent to calling
\&\fBSSL_stream_conclude\fR\|(3) after a successful call to this function, using this
flag enables greater efficiency than making these two \s-1API\s0 calls separately, as
it enables the written stream data and the \s-1FIN\s0 flag indicating the end of the
stream to be scheduled as part of the same \s-1QUIC STREAM\s0 frame and \s-1QUIC\s0 packet.
.Sp
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.
.PP
A call to \fBSSL_write_ex2()\fR fails if a flag is passed which is not supported or
understood by the given \s-1SSL\s0 object. An application should determine if a flag is
supported (for example, for \fB\s-1SSL_WRITE_FLAG_CONCLUDE\s0\fR, that a \s-1QUIC\s0 stream \s-1SSL\s0
object is being used) before attempting to use it.
.SH "NOTES"
.IX Header "NOTES"
In the paragraphs below a \*(L"write function\*(R" is defined as one of either
\&\fBSSL_write_ex()\fR, or \fBSSL_write()\fR.
.PP
If necessary, a write function will negotiate a \s-1TLS/SSL\s0 session, if not already
explicitly performed by \fBSSL_connect\fR\|(3) or \fBSSL_accept\fR\|(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 \s-1BIO.\s0
.PP
For the transparent negotiation to succeed, the \fBssl\fR must have been
initialized to client or server mode. This is being done by calling
\&\fBSSL_set_connect_state\fR\|(3) or \fBSSL_set_accept_state()\fR
before the first call to a write function.
.PP
If the underlying \s-1BIO\s0 is \fBblocking\fR, the write functions will only return, once
the write operation has been finished or an error occurred.
.PP
If the underlying \s-1BIO\s0 is \fBnonblocking\fR the write functions will also return
when the underlying \s-1BIO\s0 could not satisfy the needs of the function to continue
the operation. In this case a call to \fBSSL_get_error\fR\|(3) with the
return value of the write function will yield \fB\s-1SSL_ERROR_WANT_READ\s0\fR
or \fB\s-1SSL_ERROR_WANT_WRITE\s0\fR. 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 \s-1BIO.\s0 When using a
nonblocking socket, nothing is to be done, but \fBselect()\fR can be used to check
for the required condition. When using a buffering \s-1BIO,\s0 like a \s-1BIO\s0 pair, data
must be written into or retrieved out of the \s-1BIO\s0 before being able to continue.
.PP
The write functions will only return with success when the complete contents of
\&\fBbuf\fR of length \fBnum\fR has been written. This default behaviour can be changed
with the \s-1SSL_MODE_ENABLE_PARTIAL_WRITE\s0 option of \fBSSL_CTX_set_mode\fR\|(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.
.PP
When used with a \s-1QUIC SSL\s0 object, calling an I/O function such as \fBSSL_write()\fR
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
\&\fBSSL_write()\fR is called regularly, or alternatively ensure that \fBSSL_handle_events()\fR
is called regularly. See \fBopenssl\-quic\fR\|(7) and \fBSSL_handle_events\fR\|(3) for more
information.
.SH "WARNINGS"
.IX Header "WARNINGS"
When a write function call has to be repeated because \fBSSL_get_error\fR\|(3)
returned \fB\s-1SSL_ERROR_WANT_READ\s0\fR or \fB\s-1SSL_ERROR_WANT_WRITE\s0\fR, it must be repeated
with the same arguments.
The data that was passed might have been partially processed.
When \fB\s-1SSL_MODE_ACCEPT_MOVING_WRITE_BUFFER\s0\fR was set using \fBSSL_CTX_set_mode\fR\|(3)
the pointer can be different, but the data and length should still be the same.
.PP
You should not call \fBSSL_write()\fR with num=0, it will return an error.
\&\fBSSL_write_ex()\fR can be called with num=0, but will not send application data to
the peer.
.SH "RETURN VALUES"
.IX Header "RETURN VALUES"
\&\fBSSL_write_ex()\fR and \fBSSL_write_ex2()\fR return 1 for success or 0 for failure.
Success means that all requested application data bytes have been written to the
\&\s-1SSL\s0 connection or, if \s-1SSL_MODE_ENABLE_PARTIAL_WRITE\s0 is in use, at least 1
application data byte has been written to the \s-1SSL\s0 connection. Failure means that
not all the requested bytes have been written yet (if
\&\s-1SSL_MODE_ENABLE_PARTIAL_WRITE\s0 is not in use) or no bytes could be written to the
\&\s-1SSL\s0 connection (if \s-1SSL_MODE_ENABLE_PARTIAL_WRITE\s0 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
\&\fBSSL_get_error\fR\|(3) to find out the reason which indicates whether the call is
retryable or not.
.PP
For \fBSSL_write()\fR the following return values can occur:
.IP "> 0" 4
.IX Item "> 0"
The write operation was successful, the return value is the number of
bytes actually written to the \s-1TLS/SSL\s0 connection.
.IP "<= 0" 4
.IX Item "<= 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 \fBSSL_get_error()\fR with the return value \fBret\fR to find out the reason.
.Sp
Old documentation indicated a difference between 0 and \-1, and that \-1 was
retryable.
You should instead call \fBSSL_get_error()\fR to find out if it's retryable.
.PP
For \fBSSL_sendfile()\fR, the following return values can occur:
.IP ">= 0" 4
.IX Item ">= 0"
The write operation was successful, the return value is the number
of bytes of the file written to the \s-1TLS/SSL\s0 connection. The return
value can be less than \fBsize\fR for a partial write.
.IP "< 0" 4
.IX Item "< 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 \fBSSL_get_error()\fR with the return value to find out the reason.
.SH "SEE ALSO"
.IX Header "SEE ALSO"
\&\fBSSL_get_error\fR\|(3), \fBSSL_read_ex\fR\|(3), \fBSSL_read\fR\|(3)
\&\fBSSL_CTX_set_mode\fR\|(3), \fBSSL_CTX_new\fR\|(3),
\&\fBSSL_connect\fR\|(3), \fBSSL_accept\fR\|(3)
\&\fBSSL_set_connect_state\fR\|(3), \fBBIO_ctrl\fR\|(3),
\&\fBssl\fR\|(7), \fBbio\fR\|(7)
.SH "HISTORY"
.IX Header "HISTORY"
The \fBSSL_write_ex()\fR function was added in OpenSSL 1.1.1.
The \fBSSL_sendfile()\fR function was added in OpenSSL 3.0.
The \fBSSL_write_ex2()\fR function was added in OpenSSL 3.3.
.SH "COPYRIGHT"
.IX Header "COPYRIGHT"
Copyright 2000\-2024 The OpenSSL Project Authors. All Rights Reserved.
.PP
Licensed under the Apache License 2.0 (the \*(L"License\*(R").  You may not use
this file except in compliance with the License.  You can obtain a copy
in the file \s-1LICENSE\s0 in the source distribution or at
<https://www.openssl.org/source/license.html>.
