BIO_should_retry (3)

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NAME

BIO_should_retry, BIO_should_read, BIO_should_write, BIO_should_io_special, BIO_retry_type, BIO_should_retry, BIO_get_retry_BIO, BIO_get_retry_reason - BIO retry functions

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);

DESCRIPTION

These functions determine why a 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

needs to read data.

BIO_should_write() is true if the cause of the condition is that a

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_retry_type() returns a mask of the cause of a retry condition consisting of the values

, , though current types will only set one of these.

BIO_get_retry_BIO() determines the precise reason for the special condition, it returns the

that caused this condition and if reason is not it contains the reason code. The meaning of the reason code and the action that should be taken depends on the type of that resulted in this condition.

BIO_get_retry_reason() returns the reason for a special condition if passed the relevant

for example as returned by BIO_get_retry_BIO().

NOTES

If BIO_should_retry() returns false then the precise ``error condition'' depends on the type that caused it and the return code of the operation. For example if a call to BIO_read() on a socket returns 0 and BIO_should_retry() is false then the cause will be that the connection closed. A similar condition on a file will mean that it has reached Some types may place additional information on the error queue. For more details see the individual type manual pages.

If the underlying I/O structure is in a blocking mode almost all current

types will not request a retry, because the underlying I/O calls will not. If the application knows that the type will never signal a retry then it need not call BIO_should_retry() after a failed call. This is typically done with file BIOs.

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(). An application can retry the failed call immediately or avoid this situation by setting on the underlying 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 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

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, though the performance may be poor if BIOs are present because long delays can occur during the initial handshake process.

It is possible for a

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.

BUGS

The OpenSSL 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 functions when the entire structure can be read or written.

SEE ALSO