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 func-
tions
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
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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 con-
dition 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 typ-
ically 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 perfor-
mance 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 under-
lying I/O structure cannot process or return any data. This
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BIO_SHOULD_RETRY(3) OpenSSL BIO_SHOULD_RETRY(3)
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
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