NAME
SSL_write
—
write bytes to a TLS/SSL
connection
SYNOPSIS
#include
<openssl/ssl.h>
int
SSL_write
(SSL
*ssl, const void
*buf, int num);
DESCRIPTION
SSL_write
()
writes num bytes from the buffer
buf into the specified ssl
connection.
If necessary,
SSL_write
()
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 SSL_write
()
operation. The behaviour of SSL_write
() 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(3) before the first call to an
SSL_read(3) or
SSL_write
()
function.
If the underlying BIO is
blocking,
SSL_write
()
will only return once the write operation has been finished or an error
occurred, except when a renegotiation take 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,
SSL_write
()
will also return when the underlying BIO could not
satisfy the needs of SSL_write
() to continue the
operation. In this case a call to
SSL_get_error(3) with the return value of
SSL_write
() will yield
SSL_ERROR_WANT_READ
or
SSL_ERROR_WANT_WRITE
. As at any time a
re-negotiation is possible, a call to SSL_write
()
can also cause read operations! The calling process then must repeat the
call after taking appropriate action to satisfy the needs of
SSL_write
(). 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.
SSL_write
()
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,
SSL_write
() will also return with success when a
partial write has been successfully completed. In this case the
SSL_write
() operation is considered completed. The
bytes are sent and a new SSL_write
() operation 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 an
SSL_write
()
operation 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.
RETURN VALUES
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. 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 before SSLeay 0.8 and
has been available since OpenBSD 2.4.