BIO_should_retry, BIO_should_read, BIO_should_write, BIO_should_io_spe‐
cial, BIO_retry_type, BIO_get_retry_BIO, BIO_get_retry_reason - BIO
#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 * 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()
The BIO_should_retry() function 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.
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
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 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 cur‐
rent 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 hand‐
shake 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 con‐
ditions 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 underlying I/O
structure cannot process or return any data. This depends on the behav‐
ior 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 equivalent) call.
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.