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=head1 NAME

BIO_push, BIO_pop - add and remove BIOs from a chain.

=head1 SYNOPSIS

 #include <openssl/bio.h>

 BIO *	BIO_push(BIO *b,BIO *append);
 BIO *	BIO_pop(BIO *b);

=head1 DESCRIPTION

The BIO_push() function appends the BIO B<append> to B<b>, it returns
B<b>.

BIO_pop() removes the BIO B<b> from a chain and returns the next BIO
in the chain, or NULL if there is no next BIO. The removed BIO then
becomes a single BIO with no association with the original chain,
it can thus be freed or attached to a different chain.

=head1 NOTES

The names of these functions are perhaps a little misleading. BIO_push()
joins two BIO chains whereas BIO_pop() deletes a single BIO from a chain,
the deleted BIO does not need to be at the end of a chain.

The process of calling BIO_push() and BIO_pop() on a BIO may have additional
consequences (a control call is made to the affected BIOs) any effects will
be noted in the descriptions of individual BIOs.

=head1 EXAMPLES

For these examples suppose B<md1> and B<md2> are digest BIOs, B<b64> is
a base64 BIO and B<f> is a file BIO.

If the call:

 BIO_push(b64, f);

is made then the new chain will be B<b64-chain>. After making the calls

 BIO_push(md2, b64);
 BIO_push(md1, md2);

the new chain is B<md1-md2-b64-f>. Data written to B<md1> will be digested
by B<md1> and B<md2>, B<base64> encoded and written to B<f>.

It should be noted that reading causes data to pass in the reverse
direction, that is data is read from B<f>, base64 B<decoded> and digested
by B<md1> and B<md2>. If the call:

 BIO_pop(md2);

The call will return B<b64> and the new chain will be B<md1-b64-f> data can
be written to B<md1> as before.

=head1 RETURN VALUES

BIO_push() returns the end of the chain, B<b>.

BIO_pop() returns the next BIO in the chain, or NULL if there is no next
BIO.

=head1 SEE ALSO

TBA