SSL_write.pod 3.7 KB
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=pod

=head1 NAME

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SSL_write - write bytes to a TLS/SSL connection.
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=head1 SYNOPSIS

 #include <openssl/ssl.h>

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 int SSL_write(SSL *ssl, const void *buf, int num);
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=head1 DESCRIPTION

SSL_write() writes B<num> bytes from the buffer B<buf> into the specified
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B<ssl> connection.

=head1 NOTES

If necessary, SSL_write() will negotiate a TLS/SSL session, if
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not already explicitly performed by L<SSL_connect(3)> or
L<SSL_accept(3)>. If the
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peer requests a re-negotiation, it will be performed transparently during
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the SSL_write() operation. The behaviour of SSL_write() depends on the
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underlying BIO. 

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For the transparent negotiation to succeed, the B<ssl> must have been
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initialized to client or server mode. This is being done by calling
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L<SSL_set_connect_state(3)> or SSL_set_accept_state()
before the first call to an L<SSL_read(3)> or SSL_write() function.
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If the underlying BIO is B<blocking>, SSL_write() will only return, once the
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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
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L<SSL_CTX_set_mode(3)> call.
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If the underlying BIO is B<non-blocking>, SSL_write() will also return,
when the underlying BIO could not satisfy the needs of SSL_write()
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to continue the operation. In this case a call to
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L<SSL_get_error(3)> with the
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return value of SSL_write() will yield B<SSL_ERROR_WANT_READ> or
B<SSL_ERROR_WANT_WRITE>. As at any time a re-negotiation is possible, a
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call to SSL_write() can also cause read operations! The calling process
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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.

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SSL_write() will only return with success, when the complete contents
of B<buf> of length B<num> has been written. This default behaviour
can be changed with the SSL_MODE_ENABLE_PARTIAL_WRITE option of
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L<SSL_CTX_set_mode(3)>. When this flag is set,
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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.

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=head1 WARNING
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When an SSL_write() operation has to be repeated because of
B<SSL_ERROR_WANT_READ> or B<SSL_ERROR_WANT_WRITE>, it must be repeated
with the same arguments.

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When calling SSL_write() with num=0 bytes to be sent the behaviour is
undefined.

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=head1 RETURN VALUES

The following return values can occur:

=over 4

=item E<gt>0

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The write operation was successful, the return value is the number of
bytes actually written to the TLS/SSL connection.
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=item Z<>0
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The write operation was not successful. Probably the underlying connection
was closed. Call SSL_get_error() with the return value B<ret> to find out,
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whether an error occurred or the connection was shut down cleanly
(SSL_ERROR_ZERO_RETURN).

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=item E<lt>0
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The write operation was not successful, because either an error occurred
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or action must be taken by the calling process. Call SSL_get_error() with the
return value B<ret> to find out the reason.

=back

=head1 SEE ALSO

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L<SSL_get_error(3)>, L<SSL_read(3)>,
L<SSL_CTX_set_mode(3)>, L<SSL_CTX_new(3)>,
L<SSL_connect(3)>, L<SSL_accept(3)>
L<SSL_set_connect_state(3)>,
L<ssl(3)>, L<bio(3)>
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=cut