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
#define BIO_should_read(a) ((a)->flags &
BIO_should_write(a) ((a)->flags & BIO_FLAGS_WRITE)
#define BIO_should_io_special(a) ((a)->flags &
BIO_retry_type(a) ((a)->flags & BIO_FLAGS_RWS)
#define BIO_should_retry(a) ((a)->flags &
#define BIO_FLAGS_READ 0x01 #define
BIO_FLAGS_WRITE 0x02 #define BIO_FLAGS_IO_SPECIAL 0x04
#define BIO_FLAGS_SHOULD_RETRY 0x08
BIO * BIO_get_retry_BIO(
int *reason ); int BIO_get_retry_reason(
BIO *bio );
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.
The BIO_should_retry() function is true if the call that
produced this condition should then be retried at a later
If BIO_should_retry() is false then the cause is an error
The BIO_should_read() function is true if the cause of the
condition is that a BIO needs to read data.
The BIO_should_write() function is true if the cause of
the condition is that a BIO needs to read data.
The BIO_should_io_special() function is true if some special
condition, other than reading or writing, is the
cause of the condition.
The BIO_get_retry_reason() function returns a mask of the
cause of a retry condition consisting of the values
BIO_FLAGS_READ, BIO_FLAGS_WRITE, and BIO_FLAGS_IO_SPECIAL.
Current BIO types will only set one of these.
The BIO_get_retry_BIO() function 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.
The BIO_get_retry_reason() function returns the reason for
a special condition if passed the relevant BIO, for example
as returned by BIO_get_retry_BIO().
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 can retry a failed nonblocking 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
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 behavior of the platforms I/O
functions. This is often not desirable. One solution is to
use nonblocking I/O and use a timeout on the select() (or
The OpenSSL ASN1 functions cannot gracefully deal with
nonblocking 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.
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