=pod =head1 NAME BIO_should_retry, BIO_should_read, BIO_should_write - BIO retry functions =head1 SYNOPSIS #include #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, B, B 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 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