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.\" $OpenBSD: BIO_read.3,v 1.5 2016/12/06 14:45:08 schwarze Exp $
.\" OpenSSL 99d63d46 Oct 26 13:56:48 2016 -0400
.\"
.\" This file was written by Dr. Stephen Henson <steve@openssl.org>.
.\" Copyright (c) 2000, 2016 The OpenSSL Project. All rights reserved.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\"
.\" 1. Redistributions of source code must retain the above copyright
.\" notice, this list of conditions and the following disclaimer.
.\"
.\" 2. Redistributions in binary form must reproduce the above copyright
.\" notice, this list of conditions and the following disclaimer in
.\" the documentation and/or other materials provided with the
.\" distribution.
.\"
.\" 3. All advertising materials mentioning features or use of this
.\" software must display the following acknowledgment:
.\" "This product includes software developed by the OpenSSL Project
.\" for use in the OpenSSL Toolkit. (http://www.openssl.org/)"
.\"
.\" 4. The names "OpenSSL Toolkit" and "OpenSSL Project" must not be used to
.\" endorse or promote products derived from this software without
.\" prior written permission. For written permission, please contact
.\" openssl-core@openssl.org.
.\"
.\" 5. Products derived from this software may not be called "OpenSSL"
.\" nor may "OpenSSL" appear in their names without prior written
.\" permission of the OpenSSL Project.
.\"
.\" 6. Redistributions of any form whatsoever must retain the following
.\" acknowledgment:
.\" "This product includes software developed by the OpenSSL Project
.\" for use in the OpenSSL Toolkit (http://www.openssl.org/)"
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE OpenSSL PROJECT ``AS IS'' AND ANY
.\" EXPRESSED OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
.\" PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE OpenSSL PROJECT OR
.\" ITS CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
.\" SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
.\" LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
.\" STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
.\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED
.\" OF THE POSSIBILITY OF SUCH DAMAGE.
.\"
.Dd $Mdocdate: December 6 2016 $
.Dt BIO_READ 3
.Os
.Sh NAME
.Nm BIO_read ,
.Nm BIO_gets ,
.Nm BIO_write ,
.Nm BIO_puts
.Nd BIO I/O functions
.Sh SYNOPSIS
.In openssl/bio.h
.Ft int
.Fo BIO_read
.Fa "BIO *b"
.Fa "void *buf"
.Fa "int len"
.Fc
.Ft int
.Fo BIO_gets
.Fa "BIO *b"
.Fa "char *buf"
.Fa "int size"
.Fc
.Ft int
.Fo BIO_write
.Fa "BIO *b"
.Fa "const void *buf"
.Fa "int len"
.Fc
.Ft int
.Fo BIO_puts
.Fa "BIO *b"
.Fa "const char *buf"
.Fc
.Sh DESCRIPTION
.Fn BIO_read
attempts to read
.Fa len
bytes from BIO
.Fa b
and places the data in
.Fa buf .
.Pp
.Fn BIO_gets
performs the BIOs "gets" operation and places the data in
.Fa buf .
Usually this operation will attempt to read a line of data
from the BIO of maximum length
.Fa len No - 1 .
There are exceptions to this however, for example
.Fn BIO_gets
on a digest BIO will calculate and return the digest
and other BIOs may not support
.Fn BIO_gets
at all.
The returned string is always NUL-terminated.
.Pp
.Fn BIO_write
attempts to write
.Fa len
bytes from
.Fa buf
to BIO
.Fa b .
.Pp
.Fn BIO_puts
attempts to write a null terminated string
.Fa buf
to BIO
.Fa b .
.Pp
One technique sometimes used with blocking sockets
is to use a system call (such as
.Xr select 2 ,
.Xr poll 2
or equivalent) to determine when data is available and then call
.Xr read 2
to read the data.
The equivalent with BIOs (that is call
.Xr select 2
on the underlying I/O structure and then call
.Fn BIO_read
to read the data) should
.Em not
be used because a single call to
.Fn BIO_read
can cause several reads (and writes in the case of SSL BIOs)
on the underlying I/O structure and may block as a result.
Instead
.Xr select 2
(or equivalent) should be combined with non-blocking I/O
so successive reads will request a retry instead of blocking.
.Pp
See
.Xr BIO_should_retry 3
for details of how to determine the cause of a retry and other I/O issues.
.Pp
If the
.Fn BIO_gets
function is not supported by a BIO then it is possible to
work around this by adding a buffering BIO
.Xr BIO_f_buffer 3
to the chain.
.Sh RETURN VALUES
All these functions return either the amount of data successfully
read or written (if the return value is positive) or that no data
was successfully read or written if the result is 0 or -1.
If the return value is -2, then the operation is not implemented
in the specific BIO type.
The trailing NUL is not included in the length returned by
.Fn BIO_gets .
.Pp
A 0 or -1 return is not necessarily an indication of an error.
In particular when the source/sink is non-blocking or of a certain type
it may merely be an indication that no data is currently available and that
the application should retry the operation later.
.Sh SEE ALSO
.Xr BIO_new 3 ,
.Xr BIO_should_retry 3
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