SSL_write
Section: OpenSSL (3)
Updated: 2017-05-25
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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 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, because either the connection was
closed, 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.
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.
Old documentation indicated a difference between 0 and -1, and that -1 was
retryable.
You should instead call SSL_get_error() to find out if it's retryable.
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)
Index
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- NOTES
-
- WARNING
-
- RETURN VALUES
-
- SEE ALSO
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