BIO_should_retry(3) OpenSSL BIO_should_retry(3)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
SYNOPSIS
#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);
DESCRIPTION
These functions determine why a BIO is not able to read or
write data. They will typically be called after a failed
BIO_read() or BIO_write() call.
BIO_should_retry() is true if the call that produced this
condition should then be retried at a later time.
If BIO_should_retry() is false then the cause is an error
condition.
BIO_should_read() is true if the cause of the condition is
that a BIO needs to read data.
BIO_should_write() is true if the cause of the condition
is that a BIO needs to read data.
BIO_should_io_special() is true if some "special"
condition, that is a reason other than reading or writing
is the cause of the condition.
BIO_get_retry_reason() returns a mask of the cause of a
retry condition consisting of the values BIO_FLAGS_READ,
BIO_FLAGS_WRITE, BIO_FLAGS_IO_SPECIAL though current BIO
types will only set one of these.
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 by BIO_get_retry_BIO().
NOTES
If 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() 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 BIO_should_retry() 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(). An application can retry the failed call
immediately or avoid this situation by setting
SSL_MODE_AUTO_RETRY 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 socket and
BIO_should_read() is true then a call to select() 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() 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() (or
equivalent) call.
BUGS
The OpenSSL ASN1 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 ASN1 functions when the
entire structure can be read or written.
SEE ALSO
TBA
16/Sep/2000 0.9.6j BIO_should_retry(3)
