BIO_SHOULD_RETRY(3) BSD Library Functions Manual BIO_SHOULD_RETRY(3)NAME
BIO_should_retry, BIO_should_read, BIO_should_write,
BIO_should_io_special, BIO_retry_type, 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(3) or BIO_write(3)
call.
BIO_should_retry() is true if the call that produced this condition
should be retried at a later time.
If BIO_should_retry() is false, 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 write 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_retry_type() returns a mask of the cause of a retry condition con‐
sisting 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 condi‐
tion. 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(3) on a socket BIO returns
0 and BIO_should_retry() is false, then the cause will be that the con‐
nection 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(3). 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 repeat‐
edly 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 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 behav‐
iour 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.
BUGS
The OpenSSL ASN1 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 ASN1 functions when the
entire structure can be read or written.
BSD April 23, 2024 BSD
