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SSL_WRITE(3) | Library Functions Manual | SSL_WRITE(3) |
NAME
SSL_write_ex
,
SSL_write
—
write bytes to a TLS connection
SYNOPSIS
#include
<openssl/ssl.h>
int
SSL_write_ex
(SSL
*ssl, 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
() stores the number of bytes
written in *written.
In the following, SSL_write_ex
() and
SSL_write
() are called “write
functions”.
If necessary, a write function negotiates a TLS 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 done by calling
SSL_set_connect_state(3)
or
SSL_set_accept_state(3)
before the first call to a write function.
If the underlying BIO is
blocking, the write function will only return
once the write operation has been finished or an error occurred, except when a
renegotiation takes 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
SSL_CTX_set_mode(3)
call.
If the underlying BIO is
non-blocking, the write function 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
non-blocking socket, nothing is to be done, but
select(2) 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 have 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.
When calling SSL_write
() with
num=0 bytes to be sent, the behaviour is
undefined. SSL_write_ex
() can be called
with num=0, but will not send application
data to the peer.
RETURN VALUES
SSL_write_ex
() returns 1 for success or 0 for
failure. Success means that all requested application data bytes have been
written to the TLS connection or, if
SSL_MODE_ENABLE_PARTIAL_WRITE
is in use, at
least one application data byte has been written to the TLS 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 TLS 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 connection.
- 0
- The write operation was not successful. Probably the underlying connection
was closed. Call
SSL_get_error(3)
with the return value to find out whether an error occurred or the
connection was shut down cleanly
(
SSL_ERROR_ZERO_RETURN
). - <0
- The write operation was not successful, because either an error occurred or action must be taken by the calling process. Call SSL_get_error(3) with the return value to find out the reason.
SEE ALSO
BIO_new(3), ssl(3), SSL_accept(3), SSL_connect(3), SSL_CTX_new(3), SSL_CTX_set_mode(3), SSL_get_error(3), SSL_read(3), SSL_set_connect_state(3)HISTORY
SSL_write
() appeared in SSLeay 0.4 or earlier
and has been available since OpenBSD 2.4.
SSL_write_ex
() first appeared in OpenSSL
1.1.1 and has been available since OpenBSD 7.1.October 24, 2021 | Debian |