Updated: 2021/Apr/14

BIO_should_retry(3)                 OpenSSL                BIO_should_retry(3)

       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

       libcrypto, -lcrypto

        #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);

       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

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

       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

       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.

       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.

       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.


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

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

       Licensed under the OpenSSL license (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

1.1.1i                            2018-09-23               BIO_should_retry(3)