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 in SSLeay 0.4 or
earlier and has been available since OpenBSD
2.4.