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They will typically be called after a failed \fIBIO_read()\fR or \fIBIO_write()\fR call. .PP \&\fIBIO_should_retry()\fR is true if the call that produced this condition should then be retried at a later time. .PP If \fIBIO_should_retry()\fR is false then the cause is an error condition. .PP \&\fIBIO_should_read()\fR is true if the cause of the condition is that a \s-1BIO\s0 needs to read data. .PP \&\fIBIO_should_write()\fR is true if the cause of the condition is that a \s-1BIO\s0 needs to read data. .PP \&\fIBIO_should_io_special()\fR is true if some \*(L"special\*(R" condition, that is a reason other than reading or writing is the cause of the condition. .PP \&\fIBIO_retry_type()\fR returns a mask of the cause of a retry condition consisting of the values \fB\s-1BIO_FLAGS_READ\s0\fR, \fB\s-1BIO_FLAGS_WRITE\s0\fR, \&\fB\s-1BIO_FLAGS_IO_SPECIAL\s0\fR though current \s-1BIO\s0 types will only set one of these. .PP \&\fIBIO_get_retry_BIO()\fR determines the precise reason for the special condition, it returns the \s-1BIO\s0 that caused this condition and if \&\fBreason\fR is not \s-1NULL\s0 it contains the reason code. The meaning of the reason code and the action that should be taken depends on the type of \s-1BIO\s0 that resulted in this condition. .PP \&\fIBIO_get_retry_reason()\fR returns the reason for a special condition if passed the relevant \s-1BIO\s0, for example as returned by \fIBIO_get_retry_BIO()\fR. .SH "NOTES" .IX Header "NOTES" If \fIBIO_should_retry()\fR returns false then the precise \*(L"error condition\*(R" depends on the \s-1BIO\s0 type that caused it and the return code of the \s-1BIO\s0 operation. For example if a call to \fIBIO_read()\fR on a socket \s-1BIO\s0 returns 0 and \fIBIO_should_retry()\fR is false then the cause will be that the connection closed. A similar condition on a file \s-1BIO\s0 will mean that it has reached \s-1EOF\s0. Some \s-1BIO\s0 types may place additional information on the error queue. For more details see the individual \s-1BIO\s0 type manual pages. .PP If the underlying I/O structure is in a blocking mode almost all current \&\s-1BIO\s0 types will not request a retry, because the underlying I/O calls will not. If the application knows that the \s-1BIO\s0 type will never signal a retry then it need not call \fIBIO_should_retry()\fR after a failed \&\s-1BIO\s0 I/O call. This is typically done with file BIOs. .PP \&\s-1SSL\s0 BIOs are the only current exception to this rule: they can request a retry even if the underlying I/O structure is blocking, if a handshake occurs during a call to \fIBIO_read()\fR. An application can retry the failed call immediately or avoid this situation by setting \s-1SSL_MODE_AUTO_RETRY\s0 on the underlying \s-1SSL\s0 structure. .PP While an application may retry a failed non blocking call immediately this is likely to be very inefficient because the call will fail repeatedly until data can be processed or is available. An application will normally wait until the necessary condition is satisfied. How this is done depends on the underlying I/O structure. .PP For example if the cause is ultimately a socket and \fIBIO_should_read()\fR is true then a call to \fIselect()\fR may be made to wait until data is available and then retry the \s-1BIO\s0 operation. By combining the retry conditions of several non blocking BIOs in a single \fIselect()\fR call it is possible to service several BIOs in a single thread, though the performance may be poor if \s-1SSL\s0 BIOs are present because long delays can occur during the initial handshake process. .PP It is possible for a \s-1BIO\s0 to block indefinitely if the underlying I/O structure cannot process or return any data. This depends on the behaviour of the platforms I/O functions. This is often not desirable: one solution is to use non blocking I/O and use a timeout on the \fIselect()\fR (or equivalent) call. .SH "BUGS" .IX Header "BUGS" The OpenSSL \s-1ASN1\s0 functions cannot gracefully deal with non blocking I/O: that is they cannot retry after a partial read or write. This is usually worked around by only passing the relevant data to \s-1ASN1\s0 functions when the entire structure can be read or written. .SH "SEE ALSO" .IX Header "SEE ALSO" \&\s-1TBA\s0