.\" Copyright (c) 1988, 1991, 1993
.\"	The Regents of the University of California.  All rights reserved.
.\"
.\" This code is derived from software contributed to Berkeley by
.\" the American National Standards Committee X3, on Information
.\" Processing Systems.
.\"
.\" 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 acknowledgement:
.\"	This product includes software developed by the University of
.\"	California, Berkeley and its contributors.
.\" 4. Neither the name of the University nor the names of its contributors
.\"    may be used to endorse or promote products derived from this software
.\"    without specific prior written permission.
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
.\" ANY EXPRESS 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 REGENTS OR 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.
.\"
.\"     @(#)getenv.3	8.2 (Berkeley) 12/11/93
.\" $FreeBSD: src/lib/libc/stdlib/getenv.3,v 1.16 2004/07/07 19:57:13 ru Exp $
.\"
.Dd December 11, 1993
.Dt GETENV 3
.Os
.Sh NAME
.Nm getenv ,
.Nm putenv ,
.Nm setenv ,
.Nm unsetenv
.Nd environment variable functions
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In stdlib.h
.Ft char *
.Fo getenv
.Fa "const char *name"
.Fc
.Ft int
.Fo putenv
.Fa "char *string"
.Fc
.Ft int
.Fo setenv
.Fa "const char *name"
.Fa "const char *value"
.Fa "int overwrite"
.Fc
.Ft int
.Fo unsetenv
.Fa "const char *name"
.Fc
.Sh DESCRIPTION
These functions set, unset and fetch environment variables from the
host
.Em environment list .
For compatibility with differing environment conventions,
the given arguments
.Fa name
and
.Fa value
may be appended and prepended,
respectively,
with an equal sign
.Dq Li \&= .
.Pp
The
.Fn getenv
function obtains the current value of the environment variable,
.Fa name .
.Pp
The
.Fn setenv
function inserts or resets the environment variable
.Fa name
in the current environment list.
If the variable
.Fa name
does not exist in the list,
it is inserted with the given
.Fa value .
If the variable does exist, the argument
.Fa overwrite
is tested; if
.Fa overwrite
is
zero, the
variable is not reset, otherwise it is reset
to the given
.Fa value .
.Pp
The
.Fn putenv
function takes an argument of the form ``name=value'' and is
equivalent to:
.Bd -literal -offset indent
setenv(name, value, 1);
.Ed
.Pp
The string pointed to by
.Fa string
becomes part of the environment.
A program should not alter or free the string,
and should not use stack or other transient string variables
as arguments to
.Fn putenv .
The
.Fn setenv
function is strongly preferred to
.Fn putenv .
.Pp
The
.Fn unsetenv
function
deletes all instances of the variable name pointed to by
.Fa name
from the list.
Note that only the variable name (e.g., "NAME") should be given;
"NAME=value" will not work.
.Sh RETURN VALUES
The
.Fn getenv
function returns the value of the environment variable as a
.Dv NUL Ns
-terminated string.
If the variable
.Fa name
is not in the current environment,
.Dv NULL
is returned.
.Pp
.Rv -std setenv putenv unsetenv
.Sh ERRORS
.Bl -tag -width Er
.It Bq Er EINVAL
The function
.Fn unsetenv
failed because
.Fa name
was not found in the environment list.
.It Bq Er ENOMEM
The function
.Fn setenv
or
.Fn putenv
failed because it was unable to allocate memory for the environment.
.El
.Sh LEGACY SYNOPSIS
.Fd #include <stdlib.h>
.Pp
.Ft void
.br
.Fo unsetenv
.Fa "const char *name"
.Fc ;
.Pp
.Fn unsetenv
doesn't return a value.
.Sh COMPATIBILITY
.Fn putenv
no longer copies its input buffer.
This often appears in crash logs as a crash in
.Fn getenv .
Avoid passing local buffers or freeing the memory
that is passed to
.Fn putenv .
Use
.Fn setenv ,  
which still makes an internal copy of its buffers.
.Pp
.Fn unsetenv
no longer parses the variable name;
e.g., unsetenv ("FOO=BAR") no longer works.
Use unsetenv("FOO").
.Fn unsetenv
also now returns a status value and will set
.Va errno
to EINVAL if
.Fa name
is not a defined environment variable.
.Sh SEE ALSO
.Xr csh 1 ,
.Xr sh 1 ,
.Xr execve 2 ,
.Xr compat 5 ,
.Xr environ 7
.Sh STANDARDS
The
.Fn getenv
function conforms to
.St -isoC .
.Sh BUGS
Successive calls to
.Fn setenv
or
.Fn putenv
assigning a differently sized
.Fa value
to the same
.Fa name
will result in a memory leak.
The
.Fx
semantics for these functions
(namely, that the contents of
.Fa value
are copied and that old values remain accessible indefinitely) make this
bug unavoidable.
Future versions may eliminate one or both of these
semantic guarantees in order to fix the bug.
.Sh HISTORY
The functions
.Fn setenv
and
.Fn unsetenv
appeared in
.At v7 .
The
.Fn putenv
function appeared in
.Bx 4.3 Reno .