openssl/doc/crypto/BIO_should_retry.pod

=pod

=head1 NAME

	BIO_should_retry, BIO_should_read, BIO_should_write - BIO retry functions

=head1 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);

=head1 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 B<BIO_FLAGS_READ>, B<BIO_FLAGS_WRITE>,
B<BIO_FLAGS_IO_SPECIAL> though current BIO types will only set one of
these (Q: is this correct?).

BIO_get_retry_BIO() determines the precise reason for the special
condition, it returns the BIO that caused this condition and if 
B<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
pass the relevant BIO, for example as returned by BIO_get_retry_BIO().

=head1 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 then most BIO
types will not signal a retry condition, 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.

The presence of an SSL BIO is an exception to this rule: it can
request a retry because the handshake process is underway (either
initially or due to a session renegotiation) even if the underlying
I/O structure (for example a socket) is in a blocking mode.

The action an application should take after a BIO has signalled that a
retry is required depends on the BIO that caused the retry.

If the underlying I/O structure is in a blocking mode then the BIO
call can be retried immediately. That is something like this can be
done:

 do {
    len = BIO_read(bio, buf, len);
 } while((len <= 0) && BIO_should_retry(bio));

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. 

The cause of the retry condition may not be the same as the call that
made it: for example if BIO_write() fails BIO_should_read() can be
true. One possible reason for this is that an SSL handshake is taking
place.

Even if data is read from the underlying I/O structure this does not
imply that the next BIO I/O call will succeed. For example if an
encryption BIO reads only a fraction of a block it will not be
able to pass any data to the application until a complete block has
been read.

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() (or
equivalent) call.

=head1 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.

=head1 SEE ALSO

TBA