.\"	$OpenBSD: BIO_push.3,v 1.5 2016/12/06 12:54:19 schwarze Exp $
.\"	OpenSSL doc/man3/BIO_push.pod 76ed5a42 Jun 29 13:38:55 2014 +0100
.\"	OpenSSL doc/man7/bio.pod a9c85cea Nov 11 09:33:55 2016 +0100
.\"
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.Dd $Mdocdate: December 6 2016 $
.Dt BIO_PUSH 3
.Os
.Sh NAME
.Nm BIO_push ,
.Nm BIO_pop
.Nd add and remove BIOs from a chain
.Sh SYNOPSIS
.In openssl/bio.h
.Ft BIO *
.Fo BIO_push
.Fa "BIO *b"
.Fa "BIO *append"
.Fc
.Ft BIO *
.Fo BIO_pop
.Fa "BIO *b"
.Fc
.Sh DESCRIPTION
BIOs can be joined together to form chains.
A chain normally consist of one or more filter BIOs
and one source/sink BIO at the end.
Data read from or written to the first BIO traverses the chain
to the end.
A single BIO can be regarded as a chain with one component.
.Pp
The
.Fn BIO_push
function appends the BIO
.Fa append
to
.Fa b
and returns
.Fa b .
.Pp
.Fn BIO_pop
removes the BIO
.Fa b
from a chain and returns the next BIO in the chain, or
.Dv 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.
.Pp
The names of these functions are perhaps a little misleading.
.Fn BIO_push
joins two BIO chains whereas
.Fn BIO_pop
deletes a single BIO from a chain;
the deleted BIO does not need to be at the end of a chain.
.Pp
The process of calling
.Fn BIO_push
and
.Fn BIO_pop
on a BIO may have additional consequences: a
.Xr BIO_ctrl 3
call is made to the affected BIOs.
Any effects will be noted in the descriptions of individual BIOs.
.Sh RETURN VALUES
.Fn BIO_push
returns the beginning of the chain,
.Fa b .
.Pp
.Fn BIO_pop
returns the next BIO in the chain, or
.Dv NULL
if there is no next BIO.
.Sh EXAMPLES
For these examples suppose
.Sy md1
and
.Sy md2
are digest BIOs,
.Sy b64
is a Base64 BIO and
.Sy f
is a file BIO.
.Pp
If the call
.Pp
.Dl BIO_push(b64, f);
.Pp
is made then the new chain will be
.Sy b64-f .
After making the calls
.Bd -literal -offset indent
BIO_push(md2, b64);
BIO_push(md1, md2);
.Ed
.Pp
the new chain is
.Sy md1-md2-b64-f .
Data written to
.Sy md1
will be digested
by
.Sy md1
and
.Sy md2 ,
Base64-encoded and written to
.Sy f .
.Pp
It should be noted that reading causes data to pass
in the reverse direction.
That is, data is read from
.Sy f ,
Base64-decoded and digested by
.Sy md1
and
.Sy md2 .
If this call is made:
.Pp
.Dl BIO_pop(md2);
.Pp
The call will return
.Sy b64
and the new chain will be
.Sy md1-b64-f ;
data can be written to
.Sy md1
as before.
.Sh SEE ALSO
.Xr BIO_find_type 3 ,
.Xr BIO_new 3 ,
.Xr BIO_read 3