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114 lines
4.6 KiB
114 lines
4.6 KiB
=pod
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=head1 NAME
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BIO_should_retry, BIO_should_read, BIO_should_write,
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BIO_should_io_special, BIO_retry_type, BIO_should_retry,
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BIO_get_retry_BIO, BIO_get_retry_reason - BIO retry functions
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=head1 SYNOPSIS
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#include <openssl/bio.h>
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#define BIO_should_read(a) ((a)->flags & BIO_FLAGS_READ)
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#define BIO_should_write(a) ((a)->flags & BIO_FLAGS_WRITE)
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#define BIO_should_io_special(a) ((a)->flags & BIO_FLAGS_IO_SPECIAL)
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#define BIO_retry_type(a) ((a)->flags & BIO_FLAGS_RWS)
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#define BIO_should_retry(a) ((a)->flags & BIO_FLAGS_SHOULD_RETRY)
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#define BIO_FLAGS_READ 0x01
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#define BIO_FLAGS_WRITE 0x02
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#define BIO_FLAGS_IO_SPECIAL 0x04
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#define BIO_FLAGS_RWS (BIO_FLAGS_READ|BIO_FLAGS_WRITE|BIO_FLAGS_IO_SPECIAL)
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#define BIO_FLAGS_SHOULD_RETRY 0x08
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BIO * BIO_get_retry_BIO(BIO *bio, int *reason);
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int BIO_get_retry_reason(BIO *bio);
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=head1 DESCRIPTION
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These functions determine why a BIO is not able to read or write data.
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They will typically be called after a failed BIO_read() or BIO_write()
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call.
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BIO_should_retry() is true if the call that produced this condition
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should then be retried at a later time.
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If BIO_should_retry() is false then the cause is an error condition.
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BIO_should_read() is true if the cause of the condition is that a BIO
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needs to read data.
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BIO_should_write() is true if the cause of the condition is that a BIO
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needs to read data.
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BIO_should_io_special() is true if some "special" condition, that is a
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reason other than reading or writing is the cause of the condition.
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BIO_retry_type() returns a mask of the cause of a retry condition
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consisting of the values B<BIO_FLAGS_READ>, B<BIO_FLAGS_WRITE>,
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B<BIO_FLAGS_IO_SPECIAL> though current BIO types will only set one of
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these.
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BIO_get_retry_BIO() determines the precise reason for the special
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condition, it returns the BIO that caused this condition and if
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B<reason> is not NULL it contains the reason code. The meaning of
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the reason code and the action that should be taken depends on
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the type of BIO that resulted in this condition.
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BIO_get_retry_reason() returns the reason for a special condition if
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passed the relevant BIO, for example as returned by BIO_get_retry_BIO().
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=head1 NOTES
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If BIO_should_retry() returns false then the precise "error condition"
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depends on the BIO type that caused it and the return code of the BIO
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operation. For example if a call to BIO_read() on a socket BIO returns
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0 and BIO_should_retry() is false then the cause will be that the
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connection closed. A similar condition on a file BIO will mean that it
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has reached EOF. Some BIO types may place additional information on
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the error queue. For more details see the individual BIO type manual
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pages.
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If the underlying I/O structure is in a blocking mode almost all current
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BIO types will not request a retry, because the underlying I/O
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calls will not. If the application knows that the BIO type will never
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signal a retry then it need not call BIO_should_retry() after a failed
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BIO I/O call. This is typically done with file BIOs.
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SSL BIOs are the only current exception to this rule: they can request a
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retry even if the underlying I/O structure is blocking, if a handshake
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occurs during a call to BIO_read(). An application can retry the failed
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call immediately or avoid this situation by setting SSL_MODE_AUTO_RETRY
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on the underlying SSL structure.
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While an application may retry a failed non blocking call immediately
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this is likely to be very inefficient because the call will fail
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repeatedly until data can be processed or is available. An application
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will normally wait until the necessary condition is satisfied. How
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this is done depends on the underlying I/O structure.
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For example if the cause is ultimately a socket and BIO_should_read()
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is true then a call to select() may be made to wait until data is
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available and then retry the BIO operation. By combining the retry
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conditions of several non blocking BIOs in a single select() call
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it is possible to service several BIOs in a single thread, though
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the performance may be poor if SSL BIOs are present because long delays
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can occur during the initial handshake process.
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It is possible for a BIO to block indefinitely if the underlying I/O
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structure cannot process or return any data. This depends on the behaviour of
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the platforms I/O functions. This is often not desirable: one solution
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is to use non blocking I/O and use a timeout on the select() (or
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equivalent) call.
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=head1 BUGS
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The OpenSSL ASN1 functions cannot gracefully deal with non blocking I/O:
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that is they cannot retry after a partial read or write. This is usually
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worked around by only passing the relevant data to ASN1 functions when
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the entire structure can be read or written.
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=head1 SEE ALSO
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TBA
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