SSL_write_ex, SSL_write, SSL_sendfile - write bytes to a TLS/SSL connection
#include <openssl/ssl.h>
ossl_ssize_t SSL_sendfile(SSL *s, int fd, off_t offset, size_t size, int flags);
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);
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_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 provided here to allow users to maintain the
same interface. The meaning of
flags is platform dependent. Currently,
under Linux it is ignored.
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
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
initialized to client or server mode. This is being done by calling
SSL_set_connect_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
continue the operation. In this case a call to
SSL_get_error(3) with
the return value of the write function will yield
SSL_ERROR_WANT_READ
or
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
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 a write function call has to be repeated because
SSL_get_error(3)
returned
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
partially 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.
SSL_write_ex() will 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
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.
- <= 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
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:
- >= 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 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.
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)
The
SSL_write_ex() function was added in OpenSSL 1.1.1. The
SSL_sendfile() function was added in OpenSSL 3.0.
Copyright 2000-2021 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>.