BIO retry functions
#define BIO_FLAGS_READ 0x01
#define BIO_FLAGS_WRITE 0x02
#define BIO_FLAGS_IO_SPECIAL 0x04
#define BIO_FLAGS_RWS \
#define BIO_FLAGS_SHOULD_RETRY 0x08
These functions determine why a BIO is not able to read or write data. They will typically be called after a failed BIO_read(3) or BIO_write(3) call.
is true if the call that produced this condition should be retried at a
is false, the cause is an error condition.
is true if the cause of the condition is that a BIO needs to read data.
is true if the cause of the condition is that a BIO needs to write data.
is true if some "special" condition (i.e. a reason other than
reading or writing) is the cause of the condition.
returns a mask of the cause of a retry condition consisting of the values
BIO_FLAGS_IO_SPECIAL though current BIO types will
only set one of these.
determines the precise reason for the special condition. It returns the BIO
that caused this condition and if reason is not
NULL it contains the reason code. The meaning of the
reason code and the action that should be taken depends on the type of BIO
that resulted in this condition.
returns the reason for a special condition if passed the relevant BIO, for
example as returned by
BIO_retry_type() are implemented as macros.
returns false, then the precise "error condition" depends on the
BIO type that caused it and the return code of the BIO operation. For
example if a call to
BIO_read(3) on a socket BIO returns 0 and
BIO_should_retry() is false, then the cause will be
that the connection closed. A similar condition on a file BIO will mean that
it has reached EOF. Some BIO types may place additional information on the
error queue. For more details see the individual BIO type manual pages.
If the underlying I/O structure is in a
blocking mode, almost all current BIO types will not request a retry,
because the underlying I/O calls will not. If the application knows that the
BIO type will never signal a retry then it need not call
after a failed BIO I/O call. This is typically done with file BIOs.
SSL 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
BIO_read(3). An application can retry the failed call immediately or
avoid this situation by setting
on the underlying SSL structure.
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.
For example if the cause is ultimately a
is true then a call to
select(2) may be made to wait until data is available and then retry
the BIO operation. By combining the retry conditions of several non-blocking
BIOs in a single
select(2) call it is possible to service several BIOs in a single
thread, though the performance may be poor if SSL BIOs are present because
long delays can occur during the initial handshake process.
It is possible for a BIO 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 select(2) (or equivalent) call.
The OpenSSL ASN.1 functions cannot gracefully deal with non-blocking I/O: they cannot retry after a partial read or write. This is usually worked around by only passing the relevant data to ASN.1 functions when the entire structure can be read or written.