NAME

       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_set_retry_reason - BIO
       retry functions

SYNOPSIS

        #include <openssl/bio.h>

        int BIO_should_read(BIO *b);
        int BIO_should_write(BIO *b);
        int BIO_should_io_special(iBIO *b);
        int BIO_retry_type(BIO *b);
        int BIO_should_retry(BIO *b);

        BIO *BIO_get_retry_BIO(BIO *bio, int *reason);
        int BIO_get_retry_reason(BIO *bio);
        void BIO_set_retry_reason(BIO *bio, int reason);

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_ex() or BIO_write_ex() 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 the BIO has insufficient
       data to return. Check for readability and/or retry the last operation.

       BIO_should_write() is true if the cause of the condition is that the BIO has pending data
       to write. Check for writability and/or retry the last operation.

       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
       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 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().

       BIO_set_retry_reason() sets the retry reason for a special condition for a given BIO. This
       would usually only be called by BIO implementations.

NOTES

       BIO_should_read(), BIO_should_write(), BIO_should_io_special(), BIO_retry_type(), and
       BIO_should_retry(), are implemented as macros.

       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_ex() 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. 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.

RETURN VALUES

       BIO_should_read(), BIO_should_write(), BIO_should_io_special(), and BIO_should_retry()
       return either 1 or 0 based on the actual conditions of the BIO.

       BIO_retry_type() returns a flag combination presenting the cause of a retry condition or
       false if there is no retry condition.

       BIO_get_retry_BIO() returns a valid BIO structure.

       BIO_get_retry_reason() returns the reason for a special condition.

SEE ALSO

       bio(7)

HISTORY

       The BIO_get_retry_reason() and BIO_set_retry_reason() functions were added in OpenSSL
       1.1.0.

COPYRIGHT

       Copyright 2000-2018 The OpenSSL Project Authors. All Rights Reserved.

       Licensed under the Apache License 2.0 (the "License").  You may not use this file except
       in compliance with the License.  You can obtain a copy in the file LICENSE in the source
       distribution or at <https://www.openssl.org/source/license.html>.