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BIO_ctrl_get_write_guarantee.3
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A \s-1BIO\s0 pair is a pair of source/sink BIOs where data written to either half of the pair is buffered and can be read from the other half. Both halves must usually by handled by the same application thread since no locking is done on the internal data structures. .PP Since \s-1BIO\s0 chains typically end in a source/sink \s-1BIO\s0 it is possible to make this one half of a \s-1BIO\s0 pair and have all the data processed by the chain under application control. .PP One typical use of \s-1BIO\s0 pairs is to place \s-1TLS/SSL\s0 I/O under application control, this can be used when the application wishes to use a non standard transport for \&\s-1TLS/SSL\s0 or the normal socket routines are inappropriate. .PP Calls to \fIBIO_read()\fR will read data from the buffer or request a retry if no data is available. .PP Calls to \fIBIO_write()\fR will place data in the buffer or request a retry if the buffer is full. .PP The standard calls \fIBIO_ctrl_pending()\fR and \fIBIO_ctrl_wpending()\fR can be used to determine the amount of pending data in the read or write buffer. .PP \&\fIBIO_reset()\fR clears any data in the write buffer. .PP \&\fIBIO_make_bio_pair()\fR joins two separate BIOs into a connected pair. .PP \&\fIBIO_destroy_pair()\fR destroys the association between two connected BIOs. Freeing up any half of the pair will automatically destroy the association. .PP \&\fIBIO_shutdown_wr()\fR is used to close down a \s-1BIO\s0 \fBb\fR. After this call no further writes on \s-1BIO\s0 \fBb\fR are allowed (they will return an error). Reads on the other half of the pair will return any pending data or \s-1EOF\s0 when all pending data has been read. .PP \&\fIBIO_set_write_buf_size()\fR sets the write buffer size of \s-1BIO\s0 \fBb\fR to \fBsize\fR. If the size is not initialized a default value is used. This is currently 17K, sufficient for a maximum size \s-1TLS\s0 record. .PP \&\fIBIO_get_write_buf_size()\fR returns the size of the write buffer. .PP \&\fIBIO_new_bio_pair()\fR combines the calls to \fIBIO_new()\fR, \fIBIO_make_bio_pair()\fR and \&\fIBIO_set_write_buf_size()\fR to create a connected pair of BIOs \fBbio1\fR, \fBbio2\fR with write buffer sizes \fBwritebuf1\fR and \fBwritebuf2\fR. If either size is zero then the default size is used. \fIBIO_new_bio_pair()\fR does not check whether \&\fBbio1\fR or \fBbio2\fR do point to some other \s-1BIO\s0, the values are overwritten, \&\fIBIO_free()\fR is not called. .PP \&\fIBIO_get_write_guarantee()\fR and \fIBIO_ctrl_get_write_guarantee()\fR return the maximum length of data that can be currently written to the \s-1BIO\s0. Writes larger than this value will return a value from \fIBIO_write()\fR less than the amount requested or if the buffer is full request a retry. \fIBIO_ctrl_get_write_guarantee()\fR is a function whereas \fIBIO_get_write_guarantee()\fR is a macro. .PP \&\fIBIO_get_read_request()\fR and \fIBIO_ctrl_get_read_request()\fR return the amount of data requested, or the buffer size if it is less, if the last read attempt at the other half of the \s-1BIO\s0 pair failed due to an empty buffer. This can be used to determine how much data should be written to the \s-1BIO\s0 so the next read will succeed: this is most useful in \s-1TLS/SSL\s0 applications where the amount of data read is usually meaningful rather than just a buffer size. After a successful read this call will return zero. It also will return zero once new data has been written satisfying the read request or part of it. Note that \fIBIO_get_read_request()\fR never returns an amount larger than that returned by \fIBIO_get_write_guarantee()\fR. .PP \&\fIBIO_ctrl_reset_read_request()\fR can also be used to reset the value returned by \&\fIBIO_get_read_request()\fR to zero. .SH "NOTES" .IX Header "NOTES" Both halves of a \s-1BIO\s0 pair should be freed. That is even if one half is implicit freed due to a \fIBIO_free_all()\fR or \fISSL_free()\fR call the other half needs to be freed. .PP When used in bidirectional applications (such as \s-1TLS/SSL\s0) care should be taken to flush any data in the write buffer. This can be done by calling \fIBIO_pending()\fR on the other half of the pair and, if any data is pending, reading it and sending it to the underlying transport. This must be done before any normal processing (such as calling \fIselect()\fR ) due to a request and \fIBIO_should_read()\fR being true. .PP To see why this is important consider a case where a request is sent using \&\fIBIO_write()\fR and a response read with \fIBIO_read()\fR, this can occur during an \&\s-1TLS/SSL\s0 handshake for example. \fIBIO_write()\fR will succeed and place data in the write buffer. \fIBIO_read()\fR will initially fail and \fIBIO_should_read()\fR will be true. If the application then waits for data to be available on the underlying transport before flushing the write buffer it will never succeed because the request was never sent! .PP \&\fIBIO_eof()\fR is true if no data is in the peer \s-1BIO\s0 and the peer \s-1BIO\s0 has been shutdown. .SH "RETURN VALUES" .IX Header "RETURN VALUES" \&\fIBIO_new_bio_pair()\fR returns 1 on success, with the new BIOs available in \&\fBbio1\fR and \fBbio2\fR, or 0 on failure, with \s-1NULL\s0 pointers stored into the locations for \fBbio1\fR and \fBbio2\fR. Check the error stack for more information. .PP [\s-1XXXXX:\s0 More return values need to be added here] .SH "EXAMPLE" .IX Header "EXAMPLE" The \s-1BIO\s0 pair can be used to have full control over the network access of an application. The application can call \fIselect()\fR on the socket as required without having to go through the SSL\-interface. .PP .Vb 6 \& BIO *internal_bio, *network_bio; \& ... \& BIO_new_bio_pair(internal_bio, 0, network_bio, 0); \& SSL_set_bio(ssl, internal_bio, internal_bio); \& SSL_operations(); \& ... .Ve .PP .Vb 9 \& application | TLS-engine \& | | \& +----------> SSL_operations() \& | /\e || \& | || \e/ \& | BIO-pair (internal_bio) \& +----------< BIO-pair (network_bio) \& | | \& socket | .Ve .PP .Vb 4 \& ... \& SSL_free(ssl); /* implicitly frees internal_bio */ \& BIO_free(network_bio); \& ... .Ve .PP As the \s-1BIO\s0 pair will only buffer the data and never directly access the connection, it behaves non-blocking and will return as soon as the write buffer is full or the read buffer is drained. Then the application has to flush the write buffer and/or fill the read buffer. .PP Use the \fIBIO_ctrl_pending()\fR, to find out whether data is buffered in the \s-1BIO\s0 and must be transfered to the network. Use \fIBIO_ctrl_get_read_request()\fR to find out, how many bytes must be written into the buffer before the \&\fISSL_operation()\fR can successfully be continued. .SH "WARNING" .IX Header "WARNING" As the data is buffered, \fISSL_operation()\fR may return with a \s-1ERROR_SSL_WANT_READ\s0 condition, but there is still data in the write buffer. An application must not rely on the error value of \fISSL_operation()\fR but must assure that the write buffer is always flushed first. Otherwise a deadlock may occur as the peer might be waiting for the data before being able to continue. .SH "SEE ALSO" .IX Header "SEE ALSO" \&\fISSL_set_bio\fR\|(3), \fIssl\fR\|(3), \fIbio\fR\|(3), \&\fIBIO_should_retry\fR\|(3), \fIBIO_read\fR\|(3)