SSL_write — write bytes to a TLS/SSL connection.
SSL_write(SSL *ssl, const void *buf, int num);
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()
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.
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
The following return values can occur:
The write operation was successful, the return value is the
number of bytes actually written to the TLS/SSL connection.
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.
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)