1=pod 2 3=head1 NAME 4 5BIO_s_bio, BIO_make_bio_pair, BIO_destroy_bio_pair, BIO_shutdown_wr, 6BIO_set_write_buf_size, BIO_get_write_buf_size, BIO_new_bio_pair, 7BIO_get_write_guarantee, BIO_ctrl_get_write_guarantee, BIO_get_read_request, 8BIO_ctrl_get_read_request, BIO_ctrl_reset_read_request - BIO pair BIO 9 10=head1 SYNOPSIS 11 12 #include <openssl/bio.h> 13 14 const BIO_METHOD *BIO_s_bio(void); 15 16 int BIO_make_bio_pair(BIO *b1, BIO *b2); 17 int BIO_destroy_bio_pair(BIO *b); 18 int BIO_shutdown_wr(BIO *b); 19 20 int BIO_set_write_buf_size(BIO *b, long size); 21 size_t BIO_get_write_buf_size(BIO *b, long size); 22 23 int BIO_new_bio_pair(BIO **bio1, size_t writebuf1, BIO **bio2, size_t writebuf2); 24 25 int BIO_get_write_guarantee(BIO *b); 26 size_t BIO_ctrl_get_write_guarantee(BIO *b); 27 int BIO_get_read_request(BIO *b); 28 size_t BIO_ctrl_get_read_request(BIO *b); 29 int BIO_ctrl_reset_read_request(BIO *b); 30 31=head1 DESCRIPTION 32 33BIO_s_bio() returns the method for a BIO pair. A BIO pair is a pair of source/sink 34BIOs where data written to either half of the pair is buffered and can be read from 35the other half. Both halves must usually by handled by the same application thread 36since no locking is done on the internal data structures. 37 38Since BIO chains typically end in a source/sink BIO it is possible to make this 39one half of a BIO pair and have all the data processed by the chain under application 40control. 41 42One typical use of BIO pairs is to place TLS/SSL I/O under application control, this 43can be used when the application wishes to use a non standard transport for 44TLS/SSL or the normal socket routines are inappropriate. 45 46Calls to BIO_read_ex() will read data from the buffer or request a retry if no 47data is available. 48 49Calls to BIO_write_ex() will place data in the buffer or request a retry if the 50buffer is full. 51 52The standard calls BIO_ctrl_pending() and BIO_ctrl_wpending() can be used to 53determine the amount of pending data in the read or write buffer. 54 55BIO_reset() clears any data in the write buffer. 56 57BIO_make_bio_pair() joins two separate BIOs into a connected pair. 58 59BIO_destroy_pair() destroys the association between two connected BIOs. Freeing 60up any half of the pair will automatically destroy the association. 61 62BIO_shutdown_wr() is used to close down a BIO B<b>. After this call no further 63writes on BIO B<b> are allowed (they will return an error). Reads on the other 64half of the pair will return any pending data or EOF when all pending data has 65been read. 66 67BIO_set_write_buf_size() sets the write buffer size of BIO B<b> to B<size>. 68If the size is not initialized a default value is used. This is currently 6917K, sufficient for a maximum size TLS record. 70 71BIO_get_write_buf_size() returns the size of the write buffer. 72 73BIO_new_bio_pair() combines the calls to BIO_new(), BIO_make_bio_pair() and 74BIO_set_write_buf_size() to create a connected pair of BIOs B<bio1>, B<bio2> 75with write buffer sizes B<writebuf1> and B<writebuf2>. If either size is 76zero then the default size is used. BIO_new_bio_pair() does not check whether 77B<bio1> or B<bio2> do point to some other BIO, the values are overwritten, 78BIO_free() is not called. 79 80BIO_get_write_guarantee() and BIO_ctrl_get_write_guarantee() return the maximum 81length of data that can be currently written to the BIO. Writes larger than this 82value will return a value from BIO_write_ex() less than the amount requested or 83if the buffer is full request a retry. BIO_ctrl_get_write_guarantee() is a 84function whereas BIO_get_write_guarantee() is a macro. 85 86BIO_get_read_request() and BIO_ctrl_get_read_request() return the 87amount of data requested, or the buffer size if it is less, if the 88last read attempt at the other half of the BIO pair failed due to an 89empty buffer. This can be used to determine how much data should be 90written to the BIO so the next read will succeed: this is most useful 91in TLS/SSL applications where the amount of data read is usually 92meaningful rather than just a buffer size. After a successful read 93this call will return zero. It also will return zero once new data 94has been written satisfying the read request or part of it. 95Note that BIO_get_read_request() never returns an amount larger 96than that returned by BIO_get_write_guarantee(). 97 98BIO_ctrl_reset_read_request() can also be used to reset the value returned by 99BIO_get_read_request() to zero. 100 101=head1 NOTES 102 103Both halves of a BIO pair should be freed. That is even if one half is implicit 104freed due to a BIO_free_all() or SSL_free() call the other half needs to be freed. 105 106When used in bidirectional applications (such as TLS/SSL) care should be taken to 107flush any data in the write buffer. This can be done by calling BIO_pending() 108on the other half of the pair and, if any data is pending, reading it and sending 109it to the underlying transport. This must be done before any normal processing 110(such as calling select() ) due to a request and BIO_should_read() being true. 111 112To see why this is important consider a case where a request is sent using 113BIO_write_ex() and a response read with BIO_read_ex(), this can occur during an 114TLS/SSL handshake for example. BIO_write_ex() will succeed and place data in the 115write buffer. BIO_read_ex() will initially fail and BIO_should_read() will be 116true. If the application then waits for data to be available on the underlying 117transport before flushing the write buffer it will never succeed because the 118request was never sent! 119 120BIO_eof() is true if no data is in the peer BIO and the peer BIO has been 121shutdown. 122 123BIO_make_bio_pair(), BIO_destroy_bio_pair(), BIO_shutdown_wr(), 124BIO_set_write_buf_size(), BIO_get_write_buf_size(), 125BIO_get_write_guarantee(), and BIO_get_read_request() are implemented 126as macros. 127 128=head1 RETURN VALUES 129 130BIO_new_bio_pair() returns 1 on success, with the new BIOs available in 131B<bio1> and B<bio2>, or 0 on failure, with NULL pointers stored into the 132locations for B<bio1> and B<bio2>. Check the error stack for more information. 133 134[XXXXX: More return values need to be added here] 135 136=head1 EXAMPLES 137 138The BIO pair can be used to have full control over the network access of an 139application. The application can call select() on the socket as required 140without having to go through the SSL-interface. 141 142 BIO *internal_bio, *network_bio; 143 144 ... 145 BIO_new_bio_pair(&internal_bio, 0, &network_bio, 0); 146 SSL_set_bio(ssl, internal_bio, internal_bio); 147 SSL_operations(); /* e.g. SSL_read and SSL_write */ 148 ... 149 150 application | TLS-engine 151 | | 152 +----------> SSL_operations() 153 | /\ || 154 | || \/ 155 | BIO-pair (internal_bio) 156 | BIO-pair (network_bio) 157 | || /\ 158 | \/ || 159 +-----------< BIO_operations() 160 | | 161 | | 162 socket 163 164 ... 165 SSL_free(ssl); /* implicitly frees internal_bio */ 166 BIO_free(network_bio); 167 ... 168 169As the BIO pair will only buffer the data and never directly access the 170connection, it behaves nonblocking and will return as soon as the write 171buffer is full or the read buffer is drained. Then the application has to 172flush the write buffer and/or fill the read buffer. 173 174Use the BIO_ctrl_pending(), to find out whether data is buffered in the BIO 175and must be transferred to the network. Use BIO_ctrl_get_read_request() to 176find out, how many bytes must be written into the buffer before the 177SSL_operation() can successfully be continued. 178 179=head1 WARNINGS 180 181As the data is buffered, SSL_operation() may return with an ERROR_SSL_WANT_READ 182condition, but there is still data in the write buffer. An application must 183not rely on the error value of SSL_operation() but must assure that the 184write buffer is always flushed first. Otherwise a deadlock may occur as 185the peer might be waiting for the data before being able to continue. 186 187=head1 SEE ALSO 188 189L<SSL_set_bio(3)>, L<ssl(7)>, L<bio(7)>, 190L<BIO_should_retry(3)>, L<BIO_read_ex(3)> 191 192=head1 COPYRIGHT 193 194Copyright 2000-2020 The OpenSSL Project Authors. All Rights Reserved. 195 196Licensed under the Apache License 2.0 (the "License"). You may not use 197this file except in compliance with the License. You can obtain a copy 198in the file LICENSE in the source distribution or at 199L<https://www.openssl.org/source/license.html>. 200 201=cut 202