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.\" ========================================================================
.\"
.IX Title "BIO_PUSH 3"
.TH BIO_PUSH 3 "2023-09-11" "1.1.1w" "OpenSSL"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
.if n .ad l
.nh
.SH "NAME"
BIO_push, BIO_pop, BIO_set_next \- add and remove BIOs from a chain
.SH "SYNOPSIS"
.IX Header "SYNOPSIS"
.Vb 1
\& #include <openssl/bio.h>
\&
\& BIO *BIO_push(BIO *b, BIO *next);
\& BIO *BIO_pop(BIO *b);
\& void BIO_set_next(BIO *b, BIO *next);
.Ve
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
\&\fIBIO_push()\fR pushes \fIb\fR on \fInext\fR.
If \fIb\fR is \s-1NULL\s0 the function does nothing and returns \fInext\fR.
Otherwise it prepends \fIb\fR, which may be a single \s-1BIO\s0 or a chain of BIOs,
to \fInext\fR (unless \fInext\fR is \s-1NULL\s0).
It then makes a control call on \fIb\fR and returns \fIb\fR.
.PP
\&\fIBIO_pop()\fR removes the \s-1BIO \s0\fIb\fR from any chain is is part of.
If \fIb\fR is \s-1NULL\s0 the function does nothing and returns \s-1NULL.\s0
Otherwise it makes a control call on \fIb\fR and
returns the next \s-1BIO\s0 in the chain, or \s-1NULL\s0 if there is no next \s-1BIO.\s0
The removed \s-1BIO\s0 becomes a single \s-1BIO\s0 with no association with
the original chain, it can thus be freed or be made part of a different chain.
.PP
\&\fIBIO_set_next()\fR replaces the existing next \s-1BIO\s0 in a chain with the \s-1BIO\s0 pointed to
by \fInext\fR. The new chain may include some of the same BIOs from the old chain
or it may be completely different.
.SH "NOTES"
.IX Header "NOTES"
The names of these functions are perhaps a little misleading. \fIBIO_push()\fR
joins two \s-1BIO\s0 chains whereas \fIBIO_pop()\fR deletes a single \s-1BIO\s0 from a chain,
the deleted \s-1BIO\s0 does not need to be at the end of a chain.
.PP
The process of calling \fIBIO_push()\fR and \fIBIO_pop()\fR on a \s-1BIO\s0 may have additional
consequences (a control call is made to the affected BIOs).
Any effects will be noted in the descriptions of individual BIOs.
.SH "RETURN VALUES"
.IX Header "RETURN VALUES"
\&\fIBIO_push()\fR returns the head of the chain,
which usually is \fIb\fR, or \fInext\fR if \fIb\fR is \s-1NULL.\s0
.PP
\&\fIBIO_pop()\fR returns the next \s-1BIO\s0 in the chain,
or \s-1NULL\s0 if there is no next \s-1BIO.\s0
.SH "EXAMPLES"
.IX Header "EXAMPLES"
For these examples suppose \fImd1\fR and \fImd2\fR are digest BIOs,
\&\fIb64\fR is a base64 \s-1BIO\s0 and \fIf\fR is a file \s-1BIO.\s0
.PP
If the call:
.PP
.Vb 1
\& BIO_push(b64, f);
.Ve
.PP
is made then the new chain will be \fIb64\-f\fR. After making the calls
.PP
.Vb 2
\& BIO_push(md2, b64);
\& BIO_push(md1, md2);
.Ve
.PP
the new chain is \fImd1\-md2\-b64\-f\fR. Data written to \fImd1\fR will be digested
by \fImd1\fR and \fImd2\fR, base64 encoded, and finally written to \fIf\fR.
.PP
It should be noted that reading causes data to pass in the reverse
direction, that is data is read from \fIf\fR, base64 decoded,
and digested by \fImd2\fR and then \fImd1\fR.
.PP
The call:
.PP
.Vb 1
\& BIO_pop(md2);
.Ve
.PP
will return \fIb64\fR and the new chain will be \fImd1\-b64\-f\fR.
Data can be written to and read from \fImd1\fR as before,
except that \fImd2\fR will no more be applied.
.SH "SEE ALSO"
.IX Header "SEE ALSO"
bio
.SH "HISTORY"
.IX Header "HISTORY"
The \fIBIO_set_next()\fR function was added in OpenSSL 1.1.0.
.SH "COPYRIGHT"
.IX Header "COPYRIGHT"
Copyright 2000\-2021 The OpenSSL Project Authors. All Rights Reserved.
.PP
Licensed under the OpenSSL license (the \*(L"License\*(R").  You may not use
this file except in compliance with the License.  You can obtain a copy
in the file \s-1LICENSE\s0 in the source distribution or at
<https://www.openssl.org/source/license.html>.

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