1BIO_s_bio(3)                        OpenSSL                       BIO_s_bio(3)
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NAME

6       BIO_s_bio, BIO_make_bio_pair, BIO_destroy_bio_pair, BIO_shutdown_wr,
7       BIO_set_write_buf_size, BIO_get_write_buf_size, BIO_new_bio_pair,
8       BIO_get_write_guarantee, BIO_ctrl_get_write_guarantee,
9       BIO_get_read_request, BIO_ctrl_get_read_request,
10       BIO_ctrl_reset_read_request - BIO pair BIO
11

SYNOPSIS

13        #include <openssl/bio.h>
14
15        BIO_METHOD *BIO_s_bio(void);
16
17        #define BIO_make_bio_pair(b1,b2)   (int)BIO_ctrl(b1,BIO_C_MAKE_BIO_PAIR,0,b2)
18        #define BIO_destroy_bio_pair(b)    (int)BIO_ctrl(b,BIO_C_DESTROY_BIO_PAIR,0,NULL)
19
20        #define BIO_shutdown_wr(b) (int)BIO_ctrl(b, BIO_C_SHUTDOWN_WR, 0, NULL)
21
22        #define BIO_set_write_buf_size(b,size) (int)BIO_ctrl(b,BIO_C_SET_WRITE_BUF_SIZE,size,NULL)
23        #define BIO_get_write_buf_size(b,size) (size_t)BIO_ctrl(b,BIO_C_GET_WRITE_BUF_SIZE,size,NULL)
24
25        int BIO_new_bio_pair(BIO **bio1, size_t writebuf1, BIO **bio2, size_t writebuf2);
26
27        #define BIO_get_write_guarantee(b) (int)BIO_ctrl(b,BIO_C_GET_WRITE_GUARANTEE,0,NULL)
28        size_t BIO_ctrl_get_write_guarantee(BIO *b);
29
30        #define BIO_get_read_request(b)    (int)BIO_ctrl(b,BIO_C_GET_READ_REQUEST,0,NULL)
31        size_t BIO_ctrl_get_read_request(BIO *b);
32
33        int BIO_ctrl_reset_read_request(BIO *b);
34

DESCRIPTION

36       BIO_s_bio() returns the method for a BIO pair. A BIO pair is a pair of
37       source/sink BIOs where data written to either half of the pair is
38       buffered and can be read from the other half. Both halves must usually
39       by handled by the same application thread since no locking is done on
40       the internal data structures.
41
42       Since BIO chains typically end in a source/sink BIO it is possible to
43       make this one half of a BIO pair and have all the data processed by the
44       chain under application control.
45
46       One typical use of BIO pairs is to place TLS/SSL I/O under application
47       control, this can be used when the application wishes to use a non
48       standard transport for TLS/SSL or the normal socket routines are
49       inappropriate.
50
51       Calls to BIO_read() will read data from the buffer or request a retry
52       if no data is available.
53
54       Calls to BIO_write() will place data in the buffer or request a retry
55       if the buffer is full.
56
57       The standard calls BIO_ctrl_pending() and BIO_ctrl_wpending() can be
58       used to determine the amount of pending data in the read or write
59       buffer.
60
61       BIO_reset() clears any data in the write buffer.
62
63       BIO_make_bio_pair() joins two separate BIOs into a connected pair.
64
65       BIO_destroy_pair() destroys the association between two connected BIOs.
66       Freeing up any half of the pair will automatically destroy the
67       association.
68
69       BIO_shutdown_wr() is used to close down a BIO b. After this call no
70       further writes on BIO b are allowed (they will return an error). Reads
71       on the other half of the pair will return any pending data or EOF when
72       all pending data has been read.
73
74       BIO_set_write_buf_size() sets the write buffer size of BIO b to size.
75       If the size is not initialized a default value is used. This is
76       currently 17K, sufficient for a maximum size TLS record.
77
78       BIO_get_write_buf_size() returns the size of the write buffer.
79
80       BIO_new_bio_pair() combines the calls to BIO_new(), BIO_make_bio_pair()
81       and BIO_set_write_buf_size() to create a connected pair of BIOs bio1,
82       bio2 with write buffer sizes writebuf1 and writebuf2. If either size is
83       zero then the default size is used.  BIO_new_bio_pair() does not check
84       whether bio1 or bio2 do point to some other BIO, the values are
85       overwritten, BIO_free() is not called.
86
87       BIO_get_write_guarantee() and BIO_ctrl_get_write_guarantee() return the
88       maximum length of data that can be currently written to the BIO. Writes
89       larger than this value will return a value from BIO_write() less than
90       the amount requested or if the buffer is full request a retry.
91       BIO_ctrl_get_write_guarantee() is a function whereas
92       BIO_get_write_guarantee() is a macro.
93
94       BIO_get_read_request() and BIO_ctrl_get_read_request() return the
95       amount of data requested, or the buffer size if it is less, if the last
96       read attempt at the other half of the BIO pair failed due to an empty
97       buffer.  This can be used to determine how much data should be written
98       to the BIO so the next read will succeed: this is most useful in
99       TLS/SSL applications where the amount of data read is usually
100       meaningful rather than just a buffer size. After a successful read this
101       call will return zero.  It also will return zero once new data has been
102       written satisfying the read request or part of it.  Note that
103       BIO_get_read_request() never returns an amount larger than that
104       returned by BIO_get_write_guarantee().
105
106       BIO_ctrl_reset_read_request() can also be used to reset the value
107       returned by BIO_get_read_request() to zero.
108

NOTES

110       Both halves of a BIO pair should be freed. That is even if one half is
111       implicit freed due to a BIO_free_all() or SSL_free() call the other
112       half needs to be freed.
113
114       When used in bidirectional applications (such as TLS/SSL) care should
115       be taken to flush any data in the write buffer. This can be done by
116       calling BIO_pending() on the other half of the pair and, if any data is
117       pending, reading it and sending it to the underlying transport. This
118       must be done before any normal processing (such as calling select() )
119       due to a request and BIO_should_read() being true.
120
121       To see why this is important consider a case where a request is sent
122       using BIO_write() and a response read with BIO_read(), this can occur
123       during an TLS/SSL handshake for example. BIO_write() will succeed and
124       place data in the write buffer. BIO_read() will initially fail and
125       BIO_should_read() will be true. If the application then waits for data
126       to be available on the underlying transport before flushing the write
127       buffer it will never succeed because the request was never sent!
128

RETURN VALUES

130       BIO_new_bio_pair() returns 1 on success, with the new BIOs available in
131       bio1 and bio2, or 0 on failure, with NULL pointers stored into the
132       locations for bio1 and bio2. Check the error stack for more
133       information.
134
135       [XXXXX: More return values need to be added here]
136

EXAMPLE

138       The BIO pair can be used to have full control over the network access
139       of an application. The application can call select() on the socket as
140       required without having to go through the SSL-interface.
141
142        BIO *internal_bio, *network_bio;
143        ...
144        BIO_new_bio_pair(internal_bio, 0, network_bio, 0);
145        SSL_set_bio(ssl, internal_bio, internal_bio);
146        SSL_operations();
147        ...
148
149        application |   TLS-engine
150           |        |
151           +----------> SSL_operations()
152                    |     /\    ||
153                    |     ||    \/
154                    |   BIO-pair (internal_bio)
155           +----------< BIO-pair (network_bio)
156           |        |
157         socket     |
158
159         ...
160         SSL_free(ssl);                /* implicitly frees internal_bio */
161         BIO_free(network_bio);
162         ...
163
164       As the BIO pair will only buffer the data and never directly access the
165       connection, it behaves non-blocking and will return as soon as the
166       write buffer is full or the read buffer is drained. Then the
167       application has to flush the write buffer and/or fill the read buffer.
168
169       Use the BIO_ctrl_pending(), to find out whether data is buffered in the
170       BIO and must be transfered to the network. Use
171       BIO_ctrl_get_read_request() to find out, how many bytes must be written
172       into the buffer before the SSL_operation() can successfully be
173       continued.
174

WARNING

176       As the data is buffered, SSL_operation() may return with a
177       ERROR_SSL_WANT_READ condition, but there is still data in the write
178       buffer. An application must not rely on the error value of
179       SSL_operation() but must assure that the write buffer is always flushed
180       first. Otherwise a deadlock may occur as the peer might be waiting for
181       the data before being able to continue.
182

SEE ALSO

184       SSL_set_bio(3), ssl(3), bio(3), BIO_should_retry(3), BIO_read(3)
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1881.0.1e                            2013-02-11                      BIO_s_bio(3)
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