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.\"     @(#)getenv.3	8.2 (Berkeley) 12/11/93
.\" $FreeBSD$
.\"
.Dd June 20, 2007
.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 *
.Fn getenv "const char *name"
.Ft int
.Fn setenv "const char *name" "const char *value" "int overwrite"
.Ft int
.Fn putenv "char *string"
.Ft int
.Fn unsetenv "const char *name"
.Sh DESCRIPTION
These functions set, unset and fetch environment variables from the
host
.Em environment list .
.Pp
The
.Fn getenv
function obtains the current value of the environment variable,
.Fa name .
The application should not modify the string pointed
to by the
.Fn getenv
function.
.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 getenv ,
.Fn setenv
or
.Fn unsetenv
failed because the
.Fa name
is a
.Dv NULL
pointer, points to an empty string, or points to a string containing an
.Dq Li \&=
character.
.Pp
The function
.Fn putenv
failed because
.Fa string
is a
.Dv NULL
pointer or
.Fa string
is without an
.Dq Li \&=
character.
.It Bq Er ENOMEM
The function
.Fn setenv ,
.Fn unsetenv
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 .
The
.Fn setenv ,
.Fn putenv
and
.Fn unsetenv
functions conforms to
.St -p1003.1-2001 .
.Sh HISTORY
The functions
.Fn setenv
and
.Fn unsetenv
appeared in
.At v7 .
The
.Fn putenv
function appeared in
.Bx 4.3 Reno .
.Sh BUGS
Successive calls to
.Fn setenv
that assign a larger-sized
.Fa value
than any previous value to the same
.Fa name
will result in a memory leak.
The
.Fx
semantics for this function
(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.