=pod

=head1 NAME

BIO_ctrl, BIO_callback_ctrl, BIO_ptr_ctrl, BIO_int_ctrl, BIO_reset,
BIO_seek, BIO_tell, BIO_flush, BIO_eof, BIO_set_close, BIO_get_close,
BIO_pending, BIO_wpending, BIO_ctrl_pending, BIO_ctrl_wpending,
BIO_get_info_callback, BIO_set_info_callback, BIO_info_cb
- BIO control operations

=head1 SYNOPSIS

 #include <openssl/bio.h>

 typedef int BIO_info_cb(BIO *b, int state, int res);

 long BIO_ctrl(BIO *bp, int cmd, long larg, void *parg);
 long BIO_callback_ctrl(BIO *b, int cmd, BIO_info_cb *cb);
 char *BIO_ptr_ctrl(BIO *bp, int cmd, long larg);
 long BIO_int_ctrl(BIO *bp, int cmd, long larg, int iarg);

 int BIO_reset(BIO *b);
 int BIO_seek(BIO *b, int ofs);
 int BIO_tell(BIO *b);
 int BIO_flush(BIO *b);
 int BIO_eof(BIO *b);
 int BIO_set_close(BIO *b, long flag);
 int BIO_get_close(BIO *b);
 int BIO_pending(BIO *b);
 int BIO_wpending(BIO *b);
 size_t BIO_ctrl_pending(BIO *b);
 size_t BIO_ctrl_wpending(BIO *b);

 int BIO_get_info_callback(BIO *b, BIO_info_cb **cbp);
 int BIO_set_info_callback(BIO *b, BIO_info_cb *cb);

=head1 DESCRIPTION

BIO_ctrl(), BIO_callback_ctrl(), BIO_ptr_ctrl() and BIO_int_ctrl()
are BIO "control" operations taking arguments of various types.
These functions are not normally called directly, various macros
are used instead. The standard macros are described below, macros
specific to a particular type of BIO are described in the specific
BIOs manual page as well as any special features of the standard
calls.

BIO_reset() typically resets a BIO to some initial state, in the case
of file related BIOs for example it rewinds the file pointer to the
start of the file.

BIO_seek() resets a file related BIO's (that is file descriptor and
FILE BIOs) file position pointer to B<ofs> bytes from start of file.

BIO_tell() returns the current file position of a file related BIO.

BIO_flush() normally writes out any internally buffered data, in some
cases it is used to signal EOF and that no more data will be written.

BIO_eof() returns 1 if the BIO has read EOF, the precise meaning of
"EOF" varies according to the BIO type.

BIO_set_close() sets the BIO B<b> close flag to B<flag>. B<flag> can
take the value BIO_CLOSE or BIO_NOCLOSE. Typically BIO_CLOSE is used
in a source/sink BIO to indicate that the underlying I/O stream should
be closed when the BIO is freed.

BIO_get_close() returns the BIOs close flag.

BIO_pending(), BIO_ctrl_pending(), BIO_wpending() and BIO_ctrl_wpending()
return the number of pending characters in the BIOs read and write buffers.
Not all BIOs support these calls. BIO_ctrl_pending() and BIO_ctrl_wpending()
return a size_t type and are functions, BIO_pending() and BIO_wpending() are
macros which call BIO_ctrl().

=head1 RETURN VALUES

BIO_reset() normally returns 1 for success and 0 or -1 for failure. File
BIOs are an exception, they return 0 for success and -1 for failure.

BIO_seek() and BIO_tell() both return the current file position on success
and -1 for failure, except file BIOs which for BIO_seek() always return 0
for success and -1 for failure.

BIO_flush() returns 1 for success and 0 or -1 for failure.

BIO_eof() returns 1 if EOF has been reached 0 otherwise.

BIO_set_close() always returns 1.

BIO_get_close() returns the close flag value: BIO_CLOSE or BIO_NOCLOSE.

BIO_pending(), BIO_ctrl_pending(), BIO_wpending() and BIO_ctrl_wpending()
return the amount of pending data.

=head1 NOTES

BIO_flush(), because it can write data may return 0 or -1 indicating
that the call should be retried later in a similar manner to BIO_write_ex().
The BIO_should_retry() call should be used and appropriate action taken
is the call fails.

The return values of BIO_pending() and BIO_wpending() may not reliably
determine the amount of pending data in all cases. For example in the
case of a file BIO some data may be available in the FILE structures
internal buffers but it is not possible to determine this in a
portably way. For other types of BIO they may not be supported.

Filter BIOs if they do not internally handle a particular BIO_ctrl()
operation usually pass the operation to the next BIO in the chain.
This often means there is no need to locate the required BIO for
a particular operation, it can be called on a chain and it will
be automatically passed to the relevant BIO. However, this can cause
unexpected results: for example no current filter BIOs implement
BIO_seek(), but this may still succeed if the chain ends in a FILE
or file descriptor BIO.

Source/sink BIOs return an 0 if they do not recognize the BIO_ctrl()
operation.

=head1 BUGS

Some of the return values are ambiguous and care should be taken. In
particular a return value of 0 can be returned if an operation is not
supported, if an error occurred, if EOF has not been reached and in
the case of BIO_seek() on a file BIO for a successful operation.

=head1 COPYRIGHT

Copyright 2000-2020 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
L<https://www.openssl.org/source/license.html>.

=cut
