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.\"	$OpenBSD: getenv.3,v 1.12 2007/05/31 19:19:31 jmc Exp $
.\"
.Dd $Mdocdate: May 31 2007 $
.Dt GETENV 3
.Os
.Sh NAME
.Nm getenv ,
.Nm putenv ,
.Nm setenv ,
.Nm unsetenv
.Nd environment variable functions
.Sh SYNOPSIS
.Fd #include <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 "const char *string"
.Ft void
.Fn unsetenv "const char *name"
.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 .
If the variable
.Fa name
is not in the current environment, a null pointer is returned.
.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
.Ar name Ns = Ns Ar value
and is equivalent to:
.Bd -literal -offset indent
setenv(name, value, 1);
.Ed
.Pp
The
.Fn unsetenv
function deletes all instances of the variable name pointed to by
.Fa name
from the list.
.Sh RETURN VALUES
The functions
.Fn setenv
and
.Fn putenv
return zero if successful; otherwise the global variable
.Va errno
is set to indicate the error and \-1 is returned.
.Pp
If
.Fn getenv
is successful, the string returned should be considered read-only.
.Sh ERRORS
.Bl -tag -width Er
.It Bq Er ENOMEM
The function
.Fn setenv
or
.Fn putenv
failed because they were unable to allocate memory for the environment.
.El
.Sh SEE ALSO
.Xr csh 1 ,
.Xr sh 1 ,
.Xr execve 2 ,
.Xr environ 7
.Sh STANDARDS
The
.Fn getenv
function conforms to
.St -ansiC .
.Sh HISTORY
The function
.Fn getenv
appeared in
.At v7
and
.Bx 3 .
The functions
.Fn setenv
and
.Fn unsetenv
appeared in
.Bx 4.3 Tahoe .
The
.Fn putenv
function appeared in
.Bx 4.3 Reno .