SSL_write (3)
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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 aTLS/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 ofSSL_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/SSLconnection.
- 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.