BIO_s_bio(3) OpenSSL BIO_s_bio(3) NNAAMMEE BIO_s_bio, BIO_make_bio_pair, BIO_destroy_bio_pair, BIO_shutdown_wr, BIO_set_write_buf_size, BIO_get_write_buf_size, BIO_new_bio_pair, BIO_get_write_guarantee, BIO_ctrl_get_write_guarantee, BIO_get_read_request, BIO_ctrl_get_read_request, BIO_ctrl_reset_read_request - BIO pair BIO SSYYNNOOPPSSIISS #include BIO_METHOD *BIO_s_bio(void); #define BIO_make_bio_pair(b1,b2) (int)BIO_ctrl(b1,BIO_C_MAKE_BIO_PAIR,0,b2) #define BIO_destroy_bio_pair(b) (int)BIO_ctrl(b,BIO_C_DESTROY_BIO_PAIR,0,NULL) #define BIO_shutdown_wr(b) (int)BIO_ctrl(b, BIO_C_SHUTDOWN_WR, 0, NULL) #define BIO_set_write_buf_size(b,size) (int)BIO_ctrl(b,BIO_C_SET_WRITE_BUF_SIZE,size,NULL) #define BIO_get_write_buf_size(b,size) (size_t)BIO_ctrl(b,BIO_C_GET_WRITE_BUF_SIZE,size,NULL) int BIO_new_bio_pair(BIO **bio1, size_t writebuf1, BIO **bio2, size_t writebuf2); #define BIO_get_write_guarantee(b) (int)BIO_ctrl(b,BIO_C_GET_WRITE_GUARANTEE,0,NULL) size_t BIO_ctrl_get_write_guarantee(BIO *b); #define BIO_get_read_request(b) (int)BIO_ctrl(b,BIO_C_GET_READ_REQUEST,0,NULL) size_t BIO_ctrl_get_read_request(BIO *b); int BIO_ctrl_reset_read_request(BIO *b); DDEESSCCRRIIPPTTIIOONN _B_I_O___s___b_i_o_(_) returns the method for a BIO pair. A BIO 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. Since BIO chains typically end in a source/sink BIO it is possible to make this one half of a BIO pair and have all the data processed by the chain under application control. One typical use of BIO pairs is to place TLS/SSL I/O under application control, this can be used when the application wishes to use a non standard transport for TLS/SSL or the normal socket routines are inap- propriate. Calls to _B_I_O___r_e_a_d_(_) will read data from the buffer or request a retry if no data is available. Calls to _B_I_O___w_r_i_t_e_(_) will place data in the buffer or request a retry if the buffer is full. The standard calls _B_I_O___c_t_r_l___p_e_n_d_i_n_g_(_) and _B_I_O___c_t_r_l___w_p_e_n_d_i_n_g_(_) can be used to determine the amount of pending data in the read or write buf- fer. _B_I_O___r_e_s_e_t_(_) clears any data in the write buffer. _B_I_O___m_a_k_e___b_i_o___p_a_i_r_(_) joins two separate BIOs into a connected pair. _B_I_O___d_e_s_t_r_o_y___p_a_i_r_(_) destroys the association between two connected BIOs. Freeing up any half of the pair will automatically destroy the associa- tion. _B_I_O___s_h_u_t_d_o_w_n___w_r_(_) is used to close down a BIO bb. After this call no further writes on BIO bb are allowed (they will return an error). Reads on the other half of the pair will return any pending data or EOF when all pending data has been read. _B_I_O___s_e_t___w_r_i_t_e___b_u_f___s_i_z_e_(_) sets the write buffer size of BIO bb to ssiizzee. If the size is not initialized a default value is used. This is cur- rently 17K, sufficient for a maximum size TLS record. _B_I_O___g_e_t___w_r_i_t_e___b_u_f___s_i_z_e_(_) returns the size of the write buffer. _B_I_O___n_e_w___b_i_o___p_a_i_r_(_) combines the calls to _B_I_O___n_e_w_(_), _B_I_O___m_a_k_e___b_i_o___p_a_i_r_(_) and _B_I_O___s_e_t___w_r_i_t_e___b_u_f___s_i_z_e_(_) to create a connected pair of BIOs bbiioo11, bbiioo22 with write buffer sizes wwrriitteebbuuff11 and wwrriitteebbuuff22. If either size is zero then the default size is used. _B_I_O___n_e_w___b_i_o___p_a_i_r_(_) does not check whether bbiioo11 or bbiioo22 do point to some other BIO, the values are over- written, _B_I_O___f_r_e_e_(_) is not called. _B_I_O___g_e_t___w_r_i_t_e___g_u_a_r_a_n_t_e_e_(_) and _B_I_O___c_t_r_l___g_e_t___w_r_i_t_e___g_u_a_r_a_n_t_e_e_(_) return the maximum length of data that can be currently written to the BIO. Writes larger than this value will return a value from _B_I_O___w_r_i_t_e_(_) less than the amount requested or if the buffer is full request a retry. _B_I_O___c_t_r_l___g_e_t___w_r_i_t_e___g_u_a_r_a_n_t_e_e_(_) is a function whereas _B_I_O___g_e_t___w_r_i_t_e___g_u_a_r_a_n_t_e_e_(_) is a macro. _B_I_O___g_e_t___r_e_a_d___r_e_q_u_e_s_t_(_) and _B_I_O___c_t_r_l___g_e_t___r_e_a_d___r_e_q_u_e_s_t_(_) 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 BIO pair failed due to an empty buffer. This can be used to determine how much data should be written to the BIO so the next read will succeed: this is most useful in TLS/SSL applications where the amount of data read is usually meaning- ful 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 _B_I_O___g_e_t___r_e_a_d___r_e_q_u_e_s_t_(_) never returns an amount larger than that returned by _B_I_O___g_e_t___w_r_i_t_e___g_u_a_r_a_n_t_e_e_(_). _B_I_O___c_t_r_l___r_e_s_e_t___r_e_a_d___r_e_q_u_e_s_t_(_) can also be used to reset the value returned by _B_I_O___g_e_t___r_e_a_d___r_e_q_u_e_s_t_(_) to zero. NNOOTTEESS Both halves of a BIO pair should be freed. That is even if one half is implicit freed due to a _B_I_O___f_r_e_e___a_l_l_(_) or _S_S_L___f_r_e_e_(_) call the other half needs to be freed. When used in bidirectional applications (such as TLS/SSL) care should be taken to flush any data in the write buffer. This can be done by calling _B_I_O___p_e_n_d_i_n_g_(_) 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 _s_e_l_e_c_t_(_) ) due to a request and _B_I_O___s_h_o_u_l_d___r_e_a_d_(_) being true. To see why this is important consider a case where a request is sent using _B_I_O___w_r_i_t_e_(_) and a response read with _B_I_O___r_e_a_d_(_), this can occur during an TLS/SSL handshake for example. _B_I_O___w_r_i_t_e_(_) will succeed and place data in the write buffer. _B_I_O___r_e_a_d_(_) will initially fail and _B_I_O___s_h_o_u_l_d___r_e_a_d_(_) 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! RREETTUURRNN VVAALLUUEESS _B_I_O___n_e_w___b_i_o___p_a_i_r_(_) returns 1 on success, with the new BIOs available in bbiioo11 and bbiioo22, or 0 on failure, with NULL pointers stored into the locations for bbiioo11 and bbiioo22. Check the error stack for more informa- tion. [XXXXX: More return values need to be added here] EEXXAAMMPPLLEE The BIO pair can be used to have full control over the network access of an application. The application can call _s_e_l_e_c_t_(_) on the socket as required without having to go through the SSL-interface. 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(); ... application | TLS-engine | | +----------> SSL_operations() | /\ || | || \/ | BIO-pair (internal_bio) +----------< BIO-pair (network_bio) | | socket | ... SSL_free(ssl); /* implicitly frees internal_bio */ BIO_free(network_bio); ... As the BIO 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 applica- tion has to flush the write buffer and/or fill the read buffer. Use the _B_I_O___c_t_r_l___p_e_n_d_i_n_g_(_), to find out whether data is buffered in the BIO and must be transfered to the network. Use _B_I_O___c_t_r_l___g_e_t___r_e_a_d___r_e_q_u_e_s_t_(_) to find out, how many bytes must be written into the buffer before the _S_S_L___o_p_e_r_a_t_i_o_n_(_) can successfully be contin- ued. WWAARRNNIINNGG As the data is buffered, _S_S_L___o_p_e_r_a_t_i_o_n_(_) may return with a ERROR_SSL_WANT_READ condition, but there is still data in the write buffer. An application must not rely on the error value of _S_S_L___o_p_e_r_a_- _t_i_o_n_(_) 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. SSEEEE AALLSSOO _S_S_L___s_e_t___b_i_o(3), _s_s_l(3), _b_i_o(3), _B_I_O___s_h_o_u_l_d___r_e_t_r_y(3), _B_I_O___r_e_a_d(3) 1.0.1u 2016-09-22 BIO_s_bio(3)