BIO_SET_FLAGS(3SSL)

NAME
BIO_set_flags, BIO_clear_flags, BIO_test_flags, BIO_get_flags, BIO_set_retry_read, BIO_set_retry_write, BIO_set_retry_special, BIO_clear_retry_flags, BIO_get_retry_flags - manipulate and interpret BIO flags

SYNOPSIS
 #include <openssl/bio.h>

 void BIO_set_flags(BIO *b, int flags);
 void BIO_clear_flags(BIO *b, int flags);
 int  BIO_test_flags(const BIO *b, int flags);
 int  BIO_get_flags(const BIO *b);

 void BIO_set_retry_read(BIO *b);
 void BIO_set_retry_write(BIO *b);
 void BIO_set_retry_special(BIO *b);
 void BIO_clear_retry_flags(BIO *b);
 int  BIO_get_retry_flags(BIO *b);

DESCRIPTION
A BIO has an internal set of bit flags that describe its state. These functions and macros are used primarily by BIO implementations and by code that builds BIO chains to manipulate those flags.

BIO_set_flags() sets the bits given in flags in the BIO b. Any bits already set in the BIO's flag word remain set.

BIO_clear_flags() clears the bits given in flags from the BIO b. Any other bits in the flag word are left unchanged.

BIO_test_flags() tests the bits given in flags in the BIO b and returns a nonzero value if any of them are currently set and zero otherwise.

BIO_get_flags() returns the current flag word from the BIO b. This is equivalent to testing for all bits and returning the result.

The following convenience macros are built on top of these primitives and are used to maintain the retry state of a BIO:

BIO_set_retry_read() marks the BIO b as being in a retryable state by setting the BIO_FLAGS_SHOULD_RETRY flag. In addition, it sets the BIO_FLAGS_READ flag to indicate that the retry condition is associated with a read operation.

BIO_set_retry_write() marks the BIO b as being in a retryable state by setting the BIO_FLAGS_SHOULD_RETRY flag. In addition, it sets the BIO_FLAGS_WRITE flag to indicate that the retry condition is associated with a write operation.

BIO_set_retry_special() marks the BIO b as being in a retryable state by setting the BIO_FLAGS_SHOULD_RETRY flag. In addition, it sets the BIO_FLAGS_IO_SPECIAL flag to indicate that the retry condition is associated with a read operation some "special" condition. The precise meaning of this condition depends on the BIO type.

BIO_clear_retry_flags() clears all retry-related bits from b, i.e. BIO_FLAGS_READ, BIO_FLAGS_WRITE, BIO_FLAGS_IO_SPECIAL, and BIO_FLAGS_SHOULD_RETRY.

BIO_get_retry_flags() returns retry-related bits that are currently set in b. The result is a subset of BIO_FLAGS_RWS|BIO_FLAGS_SHOULD_RETRY.

The retry bits are interpreted by the higher level macros BIO_should_read(), BIO_should_write(), BIO_should_io_special(), BIO_retry_type() and BIO_should_retry(), as documented in BIO_should_retry(3). Application code will typically use those macros rather than manipulate the underlying flags directly.

The following flag bits are currently defined for use with BIO_set_flags(), BIO_clear_flags() and BIO_test_flags():

BIO_FLAGS_READ
    The last I/O operation should be retried when the BIO becomes readable. This flag is normally set by the BIO implementation via BIO_set_retry_read() after a failed read operation.

BIO_FLAGS_WRITE
    The last I/O operation should be retried when the BIO becomes writable. This flag is normally set by the BIO implementation via BIO_set_retry_write() after a failed write operation.

BIO_FLAGS_IO_SPECIAL
    The last I/O operation should be retried when some "special" condition becomes true. The precise meaning of this condition depends on the BIO type and is usually obtained via BIO_get_retry_BIO() and BIO_get_retry_reason() as described in BIO_should_retry(3). This flag is normally set by the BIO implementation via BIO_set_retry_special().

BIO_FLAGS_RWS
    The bitwise OR of BIO_FLAGS_READ, BIO_FLAGS_WRITE and BIO_FLAGS_IO_SPECIAL. This mask is used when clearing or extracting the retry-direction bits.

BIO_FLAGS_SHOULD_RETRY
    Set if the last I/O operation on the BIO should be retried at a later time. If this bit is not set then the condition is treated as an error. This flag is normally set by the BIO implementation.

The following flag can be used only with the base64 encoding BIO. The result of using this flag with other BIOs is unpredictable.

BIO_FLAGS_BASE64_NO_NL
    When set on a base64 filter BIO this flag disables the generation of newline characters in the encoded output and causes newlines to be ignored in the input. See also BIO_f_base64(3). The flag has no effect on any other built-in BIO types.

The following flags can be used only with the memory buffer BIO and secure memory buffer BIO. The result of using these flags with other BIOs is unpredictable.

BIO_FLAGS_MEM_RDONLY
    When set on a memory BIO this flag indicates that the underlying buffer is read only. Attempts to write to such a BIO will fail. The flag has no effect on any other built-in BIO types.

BIO_FLAGS_NONCLEAR_RST
    On a memory BIO this flag modifies the behaviour of BIO_reset(). When it is set, resetting the BIO does not clear the underlying buffer but only resets the current read position. The flag has no effect on any other built-in BIO types.

BIO_FLAGS_IN_EOF
    This flag is for internal use only. It should not be used outside BIO implementations.

A range of additional flag values is reserved for internal use by OpenSSL to track kernel TLS (KTLS) state. This range and the corresponding flag macros are not part of the public API and must not be used by applications.

RETURN VALUES
BIO_get_flags() returns a bit mask of the flags currently set on the BIO.

BIO_test_flags() returns a bit mask consisting of those flags from the argument that are currently set in the BIO. Consequently, it returns a nonzero value if and only if at least one of the requested flags is set.

BIO_get_retry_flags() returns a bit mask consisting of those flags from BIO_FLAGS_READ, BIO_FLAGS_WRITE, BIO_FLAGS_IO_SPECIAL, and BIO_FLAGS_SHOULD_RETRY that are currently set in the BIO.

NOTES
Ordinary application code will rarely need to call BIO_set_flags(), BIO_clear_flags() or BIO_test_flags() directly. They are intended for BIO implementations and for code that forwards retry state from one BIO in a chain to another. After a failed I/O operation, applications should normally use BIO_should_retry() and related macros as described in BIO_should_retry(3) instead of inspecting the flags directly.

These functions and macros are not thread-safe. If a single BIO is accessed from multiple threads, the caller must provide appropriate external synchronisation.

SEE ALSO
BIO_should_retry(3), BIO_f_base64(3), bio(7)

HISTORY
The functions and macros described here have been available in OpenSSL since at least 1.1.0 (BIO_FLAGS_IN_EOF since 1.1.1).

COPYRIGHT
Copyright 2025-2026 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>.