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1 .\" $OpenBSD: BIO_should_retry.3,v 1.5 2016/12/06 14:45:08 schwarze Exp $
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51 .Dd $Mdocdate: December 6 2016 $
52 .Dt BIO_SHOULD_RETRY 3
53 .Os
54 .Sh NAME
55 .Nm BIO_should_retry ,
56 .Nm BIO_should_read ,
57 .Nm BIO_should_write ,
58 .Nm BIO_should_io_special ,
59 .Nm BIO_retry_type ,
60 .Nm BIO_should_retry ,
61 .Nm BIO_get_retry_BIO ,
62 .Nm BIO_get_retry_reason
63 .Nd BIO retry functions
64 .Sh SYNOPSIS
65 .In openssl/bio.h
66 .Ft int
67 .Fo BIO_should_read
68 .Fa "BIO *b"
69 .Fc
70 .Ft int
71 .Fo BIO_should_write
72 .Fa "BIO *b"
73 .Fc
74 .Ft int
75 .Fo BIO_should_io_special
76 .Fa "BIO *b"
77 .Fc
78 .Ft int
79 .Fo BIO_retry_type
80 .Fa "BIO *b"
81 .Fc
82 .Ft int
83 .Fo BIO_should_retry
84 .Fa "BIO *b"
85 .Fc
86 .Fd #define BIO_FLAGS_READ 0x01
87 .Fd #define BIO_FLAGS_WRITE 0x02
88 .Fd #define BIO_FLAGS_IO_SPECIAL 0x04
89 .Fd #define BIO_FLAGS_RWS \e
90 .Fd \& (BIO_FLAGS_READ|BIO_FLAGS_WRITE|BIO_FLAGS_IO_SPECIAL)
91 .Fd #define BIO_FLAGS_SHOULD_RETRY 0x08
92 .Ft BIO *
93 .Fo BIO_get_retry_BIO
94 .Fa "BIO *bio"
95 .Fa "int *reason"
96 .Fc
97 .Ft int
98 .Fo BIO_get_retry_reason
99 .Fa "BIO *bio"
100 .Fc
101 .Sh DESCRIPTION
102 These functions determine why a BIO is not able to read or write data.
103 They will typically be called after a failed
104 .Xr BIO_read 3
105 or
106 .Xr BIO_write 3
107 call.
108 .Pp
109 .Fn BIO_should_retry
110 is true if the call that produced this condition
111 should be retried at a later time.
112 .Pp
113 If
114 .Fn BIO_should_retry
115 is false, the cause is an error condition.
116 .Pp
117 .Fn BIO_should_read
118 is true if the cause of the condition is that a BIO needs to read data.
119 .Pp
120 .Fn BIO_should_write
121 is true if the cause of the condition is that a BIO needs to write data.
122 .Pp
123 .Fn BIO_should_io_special
124 is true if some "special" condition
125 (i.e. a reason other than reading or writing) is the cause of the condition.
126 .Pp
127 .Fn BIO_retry_type
128 returns a mask of the cause of a retry condition consisting of the values
129 .Dv BIO_FLAGS_READ ,
130 .Dv BIO_FLAGS_WRITE ,
131 .Dv BIO_FLAGS_IO_SPECIAL
132 though current BIO types will only set one of these.
133 .Pp
134 .Fn BIO_get_retry_BIO
135 determines the precise reason for the special condition.
136 It returns the BIO that caused this condition and if
137 .Fa reason
138 is not
139 .Dv NULL
140 it contains the reason code.
141 The meaning of the reason code and the action that should be taken
142 depends on the type of BIO that resulted in this condition.
143 .Pp
144 .Fn BIO_get_retry_reason
145 returns the reason for a special condition
146 if passed the relevant BIO, for example as returned by
147 .Fn BIO_get_retry_BIO .
148 .Pp
149 .Fn BIO_should_retry ,
150 .Fn BIO_should_read ,
151 .Fn BIO_should_write ,
152 .Fn BIO_should_io_special ,
153 and
154 .Fn BIO_retry_type
155 are implemented as macros.
156 .Pp
157 If
158 .Fn BIO_should_retry
159 returns false, then the precise "error condition" depends on
160 the BIO type that caused it and the return code of the BIO operation.
161 For example if a call to
162 .Xr BIO_read 3
163 on a socket BIO returns 0 and
164 .Fn BIO_should_retry
165 is false, then the cause will be that the connection closed.
166 A similar condition on a file BIO will mean that it has reached EOF.
167 Some BIO types may place additional information on the error queue.
168 For more details see the individual BIO type manual pages.
169 .Pp
170 If the underlying I/O structure is in a blocking mode,
171 almost all current BIO types will not request a retry,
172 because the underlying I/O calls will not.
173 If the application knows that the BIO type will never
174 signal a retry then it need not call
175 .Fn BIO_should_retry
176 after a failed BIO I/O call.
177 This is typically done with file BIOs.
178 .Pp
179 SSL BIOs are the only current exception to this rule:
180 they can request a retry even if the underlying I/O structure
181 is blocking, if a handshake occurs during a call to
182 .Xr BIO_read 3 .
183 An application can retry the failed call immediately
184 or avoid this situation by setting
185 .Dv SSL_MODE_AUTO_RETRY
186 on the underlying SSL structure.
187 .Pp
188 While an application may retry a failed non-blocking call immediately,
189 this is likely to be very inefficient because the call will fail
190 repeatedly until data can be processed or is available.
191 An application will normally wait until the necessary condition
192 is satisfied.
193 How this is done depends on the underlying I/O structure.
194 .Pp
195 For example if the cause is ultimately a socket and
196 .Fn BIO_should_read
197 is true then a call to
198 .Xr select 2
199 may be made to wait until data is available
200 and then retry the BIO operation.
201 By combining the retry conditions of several non-blocking BIOs in a single
202 .Xr select 2
203 call it is possible to service several BIOs in a single thread,
204 though the performance may be poor if SSL BIOs are present because
205 long delays can occur during the initial handshake process.
206 .Pp
207 It is possible for a BIO to block indefinitely if the underlying I/O
208 structure cannot process or return any data.
209 This depends on the behaviour of the platforms I/O functions.
210 This is often not desirable: one solution is to use non-blocking I/O
211 and use a timeout on the
212 .Xr select 2
213 (or equivalent) call.
214 .Sh SEE ALSO
215 .Xr BIO_new 3 ,
216 .Xr BIO_read 3
217 .Sh BUGS
218 The OpenSSL ASN.1 functions cannot gracefully deal with non-blocking I/O:
219 they cannot retry after a partial read or write.
220 This is usually worked around by only passing the relevant data to ASN.1
221 functions when the entire structure can be read or written.