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.\"     @(#)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 \&= .
The behavior is undefined when an equal sign appears at any other location in
.Fa name .
.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 .