|BIO_SHOULD_RETRY(3)||Library Functions Manual||BIO_SHOULD_RETRY(3)|
BIO_get_retry_reason — BIO
#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
BIO_should_retry() returns 1 if the call
that produced this condition should be retried at a later time, or 0 if an
BIO_should_read() returns 1 if the cause
of the retry condition is that a BIO needs to read data, or 0 otherwise.
BIO_should_write() returns 1 if the cause
of the retry condition is that a BIO needs to write data, or 0
BIO_should_io_special() returns 1 if some
special condition (i.e. a reason other than reading or writing) is the cause
of the retry condition, or 0 otherwise.
BIO_retry_type() returns the bitwise OR of
one or more of the flags
BIO_FLAGS_IO_SPECIAL representing the cause of the
current retry condition, or 0 if there is no retry condition. Current BIO
types only set one of the flags at a time.
BIO_get_retry_BIO() 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.
BIO_get_retry_reason() returns the reason
for a special condition if passed the relevant BIO, for example as returned
BIO_retry_type() are implemented as macros.
BIO_should_retry() 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
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
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
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
SSL_MODE_AUTO_RETRY on the underlying SSL
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 socket and
BIO_should_read() 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
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.
BIO_should_retry() first appeared in SSLeay 0.6.0.
BIO_get_retry_reason() first appeared in SSLeay
0.8.0. All these functions have been available since
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.
|December 19, 2018||OpenBSD-current|