SSL_write(3) OpenSSL SSL_write(3)NAMESSL_write - write bytes to a TLS/SSL connection.
SYNOPSIS
#include <openssl/ssl.h>
int SSL_write(SSL *ssl, const void *buf, int num);
DESCRIPTIONSSL_write() writes num bytes from the buffer buf into the
specified ssl connection.
NOTES
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() 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() 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 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, 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 for SSLv3/TLSv1.
WARNING
When an SSL_write() operation has to be repeated because
of 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()
with the return value ret to find out, whether an
error occurred or the connection was shut down cleanly
(SSL_ERROR_ZERO_RETURN).
SSLv2 (deprecated) does not support a shutdown alert
protocol, so it can only be detected, whether the
underlying connection was closed. It cannot be
checked, why the closure happened.
<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() with the return
value ret to find out the reason.
SEE ALSOSSL_get_error(3), SSL_read(3), SSL_CTX_set_mode(3),
SSL_CTX_new(3), SSL_connect(3), SSL_accept(3)SSL_set_connect_state(3), ssl(3), bio(3)19/Jul/2002 0.9.6j SSL_write(3)
