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Filename : BIO_should_io_special.3
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.\" ========================================================================
.\"
.IX Title "BIO_should_retry 3"
.TH BIO_should_retry 3 "2016-09-22" "1.0.1u" "OpenSSL"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
.if n .ad l
.nh
.SH "NAME"
BIO_should_retry, BIO_should_read, BIO_should_write,
BIO_should_io_special, BIO_retry_type, BIO_should_retry,
BIO_get_retry_BIO, BIO_get_retry_reason \- BIO retry functions
.SH "SYNOPSIS"
.IX Header "SYNOPSIS"
.Vb 1
\& #include <openssl/bio.h>
\&
\& #define BIO_should_read(a)             ((a)\->flags & BIO_FLAGS_READ)
\& #define BIO_should_write(a)            ((a)\->flags & BIO_FLAGS_WRITE)
\& #define BIO_should_io_special(a)       ((a)\->flags & BIO_FLAGS_IO_SPECIAL)
\& #define BIO_retry_type(a)              ((a)\->flags & BIO_FLAGS_RWS)
\& #define BIO_should_retry(a)            ((a)\->flags & BIO_FLAGS_SHOULD_RETRY)
\&
\& #define BIO_FLAGS_READ         0x01
\& #define BIO_FLAGS_WRITE        0x02
\& #define BIO_FLAGS_IO_SPECIAL   0x04
\& #define BIO_FLAGS_RWS (BIO_FLAGS_READ|BIO_FLAGS_WRITE|BIO_FLAGS_IO_SPECIAL)
\& #define BIO_FLAGS_SHOULD_RETRY 0x08
\&
\& BIO *  BIO_get_retry_BIO(BIO *bio, int *reason);
\& int    BIO_get_retry_reason(BIO *bio);
.Ve
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
These functions determine why a \s-1BIO\s0 is not able to read or write data.
They will typically be called after a failed \fBBIO_read()\fR or \fBBIO_write()\fR
call.
.PP
\&\fBBIO_should_retry()\fR is true if the call that produced this condition
should then be retried at a later time.
.PP
If \fBBIO_should_retry()\fR is false then the cause is an error condition.
.PP
\&\fBBIO_should_read()\fR is true if the cause of the condition is that a \s-1BIO\s0
needs to read data.
.PP
\&\fBBIO_should_write()\fR is true if the cause of the condition is that a \s-1BIO\s0
needs to read data.
.PP
\&\fBBIO_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
\&\fBBIO_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
\&\fBBIO_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
\&\fBBIO_get_retry_reason()\fR returns the reason for a special condition if
passed the relevant \s-1BIO,\s0 for example as returned by \fBBIO_get_retry_BIO()\fR.
.SH "NOTES"
.IX Header "NOTES"
If \fBBIO_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 \fBBIO_read()\fR on a socket \s-1BIO\s0 returns
0 and \fBBIO_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 \fBBIO_should_retry()\fR after a failed
\&\s-1BIO I/O\s0 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 \fBBIO_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 \fBBIO_should_read()\fR
is true then a call to \fBselect()\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 \fBselect()\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 \fBselect()\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