BIO retry functions
#define BIO_FLAGS_READ 0x01
#define BIO_FLAGS_WRITE 0x02
#define BIO_FLAGS_IO_SPECIAL 0x04
#define BIO_FLAGS_RWS \
#define BIO_FLAGS_SHOULD_RETRY 0x08
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.
returns 1 if the call that produced this condition should be retried at a
later time, or 0 if an error occurred.
returns 1 if the cause of the retry condition is that a BIO needs to read
data, or 0 otherwise.
returns 1 if the cause of the retry condition is that a BIO needs to write
data, or 0 otherwise.
returns 1 if some special condition (i.e. a reason other than reading or
writing) is the cause of the retry condition, or 0 otherwise.
returns the bitwise OR of one or more of the flags
BIO_FLAGS_IO_SPECIAL representing the cause of the
current retry condition, or 0 if there is no retry condition. Current BIO
types only set one of the flags at a time.
determines the precise reason for the special condition. It walks the BIO
chain starting at bio and returns the BIO that caused
this condition. If there is no special condition, bio
itself is returned. If reason is not a
NULL pointer, *reason is set
to one of the following reason codes:
- There is no special condition.
- accept(2) would have blocked. This can occur for BIOs created from BIO_s_accept(3) or BIO_f_ssl(3).
- connect(2) would have blocked. This can occur for BIOs created from BIO_s_connect(3) or BIO_f_ssl(3).
- An application callback set by SSL_CTX_set_client_cert_cb(3) has asked to be called again. This can occur for BIOs created from BIO_f_ssl(3).
returns one of the above reason codes for a special condition that occurred
in bio. It does not walk the chain and returns 0 if no
special condition occurred in bio itself.
sets the retry reason for a special condition for the given
bio. It is intended to be called by functions
implementing a BIO type rather than by functions merely using BIOs.
BIO_retry_type() are implemented as macros.
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 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
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
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
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
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(2) (or equivalent) call.
BIO_should_retry() first appeared in SSLeay 0.6.0.
BIO_get_retry_reason() first appeared in SSLeay
0.8.0. All these functions have been available since
BIO_set_retry_reason() first appeared in
OpenSSL 1.1.0 and has been available since OpenBSD
The OpenSSL ASN.1 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 ASN.1 functions when the entire structure can be read or written.