xref: /openssl/doc/man3/BIO_should_retry.pod (revision 6e4618a0)
1=pod
2
3=head1 NAME
4
5BIO_should_read, BIO_should_write,
6BIO_should_io_special, BIO_retry_type, BIO_should_retry,
7BIO_get_retry_BIO, BIO_get_retry_reason, BIO_set_retry_reason - BIO retry
8functions
9
10=head1 SYNOPSIS
11
12 #include <openssl/bio.h>
13
14 int BIO_should_read(BIO *b);
15 int BIO_should_write(BIO *b);
16 int BIO_should_io_special(iBIO *b);
17 int BIO_retry_type(BIO *b);
18 int BIO_should_retry(BIO *b);
19
20 BIO *BIO_get_retry_BIO(BIO *bio, int *reason);
21 int BIO_get_retry_reason(BIO *bio);
22 void BIO_set_retry_reason(BIO *bio, int reason);
23
24=head1 DESCRIPTION
25
26These functions determine why a BIO is not able to read or write data.
27They will typically be called after a failed BIO_read_ex() or BIO_write_ex()
28call.
29
30BIO_should_retry() is true if the call that produced this condition
31should then be retried at a later time.
32
33If BIO_should_retry() is false then the cause is an error condition.
34
35BIO_should_read() is true if the cause of the condition is that the BIO
36has insufficient data to return. Check for readability and/or retry the
37last operation.
38
39BIO_should_write() is true if the cause of the condition is that the BIO
40has pending data to write. Check for writability and/or retry the
41last operation.
42
43BIO_should_io_special() is true if some "special" condition, that is a
44reason other than reading or writing is the cause of the condition.
45
46BIO_retry_type() returns a mask of the cause of a retry condition
47consisting of the values B<BIO_FLAGS_READ>, B<BIO_FLAGS_WRITE>,
48B<BIO_FLAGS_IO_SPECIAL> though current BIO types will only set one of
49these.
50
51BIO_get_retry_BIO() determines the precise reason for the special
52condition, it returns the BIO that caused this condition and if
53B<reason> is not NULL it contains the reason code. The meaning of
54the reason code and the action that should be taken depends on
55the type of BIO that resulted in this condition.
56
57BIO_get_retry_reason() returns the reason for a special condition if
58passed the relevant BIO, for example as returned by BIO_get_retry_BIO().
59
60BIO_set_retry_reason() sets the retry reason for a special condition for a given
61BIO. This would usually only be called by BIO implementations.
62
63=head1 NOTES
64
65BIO_should_read(), BIO_should_write(), BIO_should_io_special(),
66BIO_retry_type(), and BIO_should_retry(), are implemented as macros.
67
68If BIO_should_retry() returns false then the precise "error condition"
69depends on the BIO type that caused it and the return code of the BIO
70operation. For example if a call to BIO_read_ex() on a socket BIO returns
710 and BIO_should_retry() is false then the cause will be that the
72connection closed. A similar condition on a file BIO will mean that it
73has reached EOF. Some BIO types may place additional information on
74the error queue. For more details see the individual BIO type manual
75pages.
76
77If the underlying I/O structure is in a blocking mode almost all current
78BIO types will not request a retry, because the underlying I/O
79calls will not. If the application knows that the BIO type will never
80signal a retry then it need not call BIO_should_retry() after a failed
81BIO I/O call. This is typically done with file BIOs.
82
83SSL BIOs are the only current exception to this rule: they can request a
84retry even if the underlying I/O structure is blocking, if a handshake
85occurs during a call to BIO_read(). An application can retry the failed
86call immediately or avoid this situation by setting SSL_MODE_AUTO_RETRY
87on the underlying SSL structure.
88
89While an application may retry a failed non blocking call immediately
90this is likely to be very inefficient because the call will fail
91repeatedly until data can be processed or is available. An application
92will normally wait until the necessary condition is satisfied. How
93this is done depends on the underlying I/O structure.
94
95For example if the cause is ultimately a socket and BIO_should_read()
96is true then a call to select() may be made to wait until data is
97available and then retry the BIO operation. By combining the retry
98conditions of several non blocking BIOs in a single select() call
99it is possible to service several BIOs in a single thread, though
100the performance may be poor if SSL BIOs are present because long delays
101can occur during the initial handshake process.
102
103It is possible for a BIO to block indefinitely if the underlying I/O
104structure cannot process or return any data. This depends on the behaviour of
105the platforms I/O functions. This is often not desirable: one solution
106is to use non blocking I/O and use a timeout on the select() (or
107equivalent) call.
108
109=head1 BUGS
110
111The OpenSSL ASN1 functions cannot gracefully deal with non blocking I/O:
112that is they cannot retry after a partial read or write. This is usually
113worked around by only passing the relevant data to ASN1 functions when
114the entire structure can be read or written.
115
116=head1 RETURN VALUES
117
118BIO_should_read(), BIO_should_write(), BIO_should_io_special(), and
119BIO_should_retry() return either 1 or 0 based on the actual conditions
120of the B<BIO>.
121
122BIO_retry_type() returns a flag combination presenting the cause of a retry
123condition or false if there is no retry condition.
124
125BIO_get_retry_BIO() returns a valid B<BIO> structure.
126
127BIO_get_retry_reason() returns the reason for a special condition.
128
129=head1 SEE ALSO
130
131L<bio(7)>
132
133=head1 HISTORY
134
135The BIO_get_retry_reason() and BIO_set_retry_reason() functions were added in
136OpenSSL 1.1.0.
137
138=head1 COPYRIGHT
139
140Copyright 2000-2018 The OpenSSL Project Authors. All Rights Reserved.
141
142Licensed under the Apache License 2.0 (the "License").  You may not use
143this file except in compliance with the License.  You can obtain a copy
144in the file LICENSE in the source distribution or at
145L<https://www.openssl.org/source/license.html>.
146
147=cut
148