SSL_WRITE(3) BSD Library Functions Manual SSL_WRITE(3)NAMESSL_write — write bytes to a TLS/SSL connection
SYNOPSIS
#include <openssl/ssl.h>
int
SSL_write(SSL *ssl, const void *buf, int num);
DESCRIPTIONSSL_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 ini‐
tialized to client or server mode. This is being done by calling
SSL_set_connect_state(3) or SSL_set_accept_state(3) 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(2) 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 have 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 per‐
formed 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 num‐
ber 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(3) with the return
value 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(3) with the return value to find out the reason.
SEE ALSObio(3), ssl(3), SSL_accept(3), SSL_connect(3), SSL_CTX_new(3),
SSL_CTX_set_mode(3), SSL_get_error(3), SSL_read(3),
SSL_set_connect_state(3)BSD April 28, 2024 BSD
