trusty (3) SSL_write.3ssl.gz

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)