SSL_write(3) OpenSSL SSL_write(3)
SSL_write - write bytes to a TLS/SSL connection.
int SSL_write(SSL *ssl, const void *buf, int num);
SSL_write() writes num bytes from the buffer buf into the specified ssl
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 dur-
ing 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 ini-
tialized 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 com-
pleted. 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
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
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.
SSL_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)
0.9.7a 2002-07-19 SSL_write(3)