SSL_write(3)

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 dur-
       ing 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() 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 com-
       pleted. 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 connec-
           tion 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)

1.0.2t                            2019-09-10                      SSL_write(3)