--- /dev/null
+=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 is likely to
+fail repeatedly until data can be processed. 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 the 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
projects / openssl.git / blobdiff