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 BIO_METHOD *BIO_s_bio(void); 15 16 #define BIO_make_bio_pair(b1,b2) (int)BIO_ctrl(b1,BIO_C_MAKE_BIO_PAIR,0,b2) 17 #define BIO_destroy_bio_pair(b) (int)BIO_ctrl(b,BIO_C_DESTROY_BIO_PAIR,0,NULL) 18 19 #define BIO_shutdown_wr(b) (int)BIO_ctrl(b, BIO_C_SHUTDOWN_WR, 0, NULL) 20 21 #define BIO_set_write_buf_size(b,size) (int)BIO_ctrl(b,BIO_C_SET_WRITE_BUF_SIZE,size,NULL) 22 #define BIO_get_write_buf_size(b,size) (size_t)BIO_ctrl(b,BIO_C_GET_WRITE_BUF_SIZE,size,NULL) 23 24 int BIO_new_bio_pair(BIO **bio1, size_t writebuf1, BIO **bio2, size_t writebuf2); 25 26 #define BIO_get_write_guarantee(b) (int)BIO_ctrl(b,BIO_C_GET_WRITE_GUARANTEE,0,NULL) 27 size_t BIO_ctrl_get_write_guarantee(BIO *b); 28 29 #define BIO_get_read_request(b) (int)BIO_ctrl(b,BIO_C_GET_READ_REQUEST,0,NULL) 30 size_t BIO_ctrl_get_read_request(BIO *b); 31 32 int BIO_ctrl_reset_read_request(BIO *b); 33 34=head1 DESCRIPTION 35 36BIO_s_bio() returns the method for a BIO pair. A BIO pair is a pair of source/sink 37BIOs where data written to either half of the pair is buffered and can be read from 38the other half. Both halves must usually by handled by the same application thread 39since no locking is done on the internal data structures. 40 41Since BIO chains typically end in a source/sink BIO it is possible to make this 42one half of a BIO pair and have all the data processed by the chain under application 43control. 44 45One typical use of BIO pairs is to place TLS/SSL I/O under application control, this 46can be used when the application wishes to use a non standard transport for 47TLS/SSL or the normal socket routines are inappropriate. 48 49Calls to BIO_read() will read data from the buffer or request a retry if no 50data is available. 51 52Calls to BIO_write() will place data in the buffer or request a retry if the 53buffer is full. 54 55The standard calls BIO_ctrl_pending() and BIO_ctrl_wpending() can be used to 56determine the amount of pending data in the read or write buffer. 57 58BIO_reset() clears any data in the write buffer. 59 60BIO_make_bio_pair() joins two separate BIOs into a connected pair. 61 62BIO_destroy_pair() destroys the association between two connected BIOs. Freeing 63up any half of the pair will automatically destroy the association. 64 65BIO_shutdown_wr() is used to close down a BIO B<b>. After this call no further 66writes on BIO B<b> are allowed (they will return an error). Reads on the other 67half of the pair will return any pending data or EOF when all pending data has 68been read. 69 70BIO_set_write_buf_size() sets the write buffer size of BIO B<b> to B<size>. 71If the size is not initialized a default value is used. This is currently 7217K, sufficient for a maximum size TLS record. 73 74BIO_get_write_buf_size() returns the size of the write buffer. 75 76BIO_new_bio_pair() combines the calls to BIO_new(), BIO_make_bio_pair() and 77BIO_set_write_buf_size() to create a connected pair of BIOs B<bio1>, B<bio2> 78with write buffer sizes B<writebuf1> and B<writebuf2>. If either size is 79zero then the default size is used. BIO_new_bio_pair() does not check whether 80B<bio1> or B<bio2> do point to some other BIO, the values are overwritten, 81BIO_free() is not called. 82 83BIO_get_write_guarantee() and BIO_ctrl_get_write_guarantee() return the maximum 84length of data that can be currently written to the BIO. Writes larger than this 85value will return a value from BIO_write() less than the amount requested or if the 86buffer is full request a retry. BIO_ctrl_get_write_guarantee() is a function 87whereas BIO_get_write_guarantee() is a macro. 88 89BIO_get_read_request() and BIO_ctrl_get_read_request() return the 90amount of data requested, or the buffer size if it is less, if the 91last read attempt at the other half of the BIO pair failed due to an 92empty buffer. This can be used to determine how much data should be 93written to the BIO so the next read will succeed: this is most useful 94in TLS/SSL applications where the amount of data read is usually 95meaningful rather than just a buffer size. After a successful read 96this call will return zero. It also will return zero once new data 97has been written satisfying the read request or part of it. 98Note that BIO_get_read_request() never returns an amount larger 99than that returned by BIO_get_write_guarantee(). 100 101BIO_ctrl_reset_read_request() can also be used to reset the value returned by 102BIO_get_read_request() to zero. 103 104=head1 NOTES 105 106Both halves of a BIO pair should be freed. That is even if one half is implicit 107freed due to a BIO_free_all() or SSL_free() call the other half needs to be freed. 108 109When used in bidirectional applications (such as TLS/SSL) care should be taken to 110flush any data in the write buffer. This can be done by calling BIO_pending() 111on the other half of the pair and, if any data is pending, reading it and sending 112it to the underlying transport. This must be done before any normal processing 113(such as calling select() ) due to a request and BIO_should_read() being true. 114 115To see why this is important consider a case where a request is sent using 116BIO_write() and a response read with BIO_read(), this can occur during an 117TLS/SSL handshake for example. BIO_write() will succeed and place data in the write 118buffer. BIO_read() will initially fail and BIO_should_read() will be true. If 119the application then waits for data to be available on the underlying transport 120before flushing the write buffer it will never succeed because the request was 121never sent! 122 123=head1 RETURN VALUES 124 125BIO_new_bio_pair() returns 1 on success, with the new BIOs available in 126B<bio1> and B<bio2>, or 0 on failure, with NULL pointers stored into the 127locations for B<bio1> and B<bio2>. Check the error stack for more information. 128 129[XXXXX: More return values need to be added here] 130 131=head1 EXAMPLE 132 133The BIO pair can be used to have full control over the network access of an 134application. The application can call select() on the socket as required 135without having to go through the SSL-interface. 136 137 BIO *internal_bio, *network_bio; 138 ... 139 BIO_new_bio_pair(internal_bio, 0, network_bio, 0); 140 SSL_set_bio(ssl, internal_bio, internal_bio); 141 SSL_operations(); 142 ... 143 144 application | TLS-engine 145 | | 146 +----------> SSL_operations() 147 | /\ || 148 | || \/ 149 | BIO-pair (internal_bio) 150 +----------< BIO-pair (network_bio) 151 | | 152 socket | 153 154 ... 155 SSL_free(ssl); /* implicitly frees internal_bio */ 156 BIO_free(network_bio); 157 ... 158 159As the BIO pair will only buffer the data and never directly access the 160connection, it behaves non-blocking and will return as soon as the write 161buffer is full or the read buffer is drained. Then the application has to 162flush the write buffer and/or fill the read buffer. 163 164Use the BIO_ctrl_pending(), to find out whether data is buffered in the BIO 165and must be transfered to the network. Use BIO_ctrl_get_read_request() to 166find out, how many bytes must be written into the buffer before the 167SSL_operation() can successfully be continued. 168 169=head1 WARNING 170 171As the data is buffered, SSL_operation() may return with a ERROR_SSL_WANT_READ 172condition, but there is still data in the write buffer. An application must 173not rely on the error value of SSL_operation() but must assure that the 174write buffer is always flushed first. Otherwise a deadlock may occur as 175the peer might be waiting for the data before being able to continue. 176 177=head1 SEE ALSO 178 179L<SSL_set_bio(3)|SSL_set_bio(3)>, L<ssl(3)|ssl(3)>, L<bio(3)|bio(3)>, 180L<BIO_should_retry(3)|BIO_should_retry(3)>, L<BIO_read(3)|BIO_read(3)> 181 182=cut 183