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.
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 ALSO
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)
