BIO_should_retry(3)

     BIO_should_retry(3)    0.9.6h (2000-09-16)    BIO_should_retry(3)

     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 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 BIO_FLAGS_READ,
          BIO_FLAGS_WRITE, BIO_FLAGS_IO_SPECIAL though current BIO
          types will only set one of these.

          BIO_get_retry_BIO() determines the precise reason for the
          special condition, it returns the BIO that caused this

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          condition and if 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 passed the relevant BIO, for example as
          returned by BIO_get_retry_BIO().

     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 almost
          all current BIO types will not request a retry, 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.

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

          It is possible for a BIO to block indefinitely if the
          underlying I/O structure cannot process or return any data.

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          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 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.

     SEE ALSO
          TBA

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