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.\"	$OpenBSD: getenv.3,v 1.21 2014/07/11 09:24:03 tedu Exp $
.\"
.Dd $Mdocdate: July 11 2014 $
.Dt GETENV 3
.Os
.Sh NAME
.Nm getenv ,
.Nm putenv ,
.Nm setenv ,
.Nm unsetenv
.Nd environment variable functions
.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 .
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 .
The memory pointed to by
.Ar string
becomes part of the environment and must not be deallocated by the caller.
If the variable already exists, it will be overwritten.
A common source of bugs is to pass a
.Ar string
argument that is a locally scoped string buffer.
This will result in corruption of the environment after leaving
the scope in which the variable is defined.
For this reason, the
.Fn setenv
function is preferred over
.Fn putenv .
.Pp
The
.Fn unsetenv
function deletes all instances of the variable name pointed to by
.Fa name
from the list.
.Sh RETURN VALUES
.Rv -std putenv setenv unsetenv
.Pp
The
.Fn getenv
function returns a pointer to the requested value, or
.Dv NULL
if it could not be found.
If
.Fn getenv
is successful, the string returned should be considered read-only.
.Sh ERRORS
.Bl -tag -width Er
.It Bq Er EINVAL
The
.Fn setenv
or
.Fn unsetenv
function was passed an empty
.Ar name
or a NULL pointer, or was passed a
.Ar name
containing an
.Sq =
character.
.Pp
The
.Fn putenv
function was passed a
.Ar string
that did not contain an
.Sq =
character.
.It Bq Er ENOMEM
The
.Fn setenv
or
.Fn putenv
function failed because it was unable to allocate memory for the environment.
.El
.Sh SEE ALSO
.Xr csh 1 ,
.Xr sh 1 ,
.Xr execve 2 ,
.Xr issetugid 2 ,
.Xr environ 7
.Sh STANDARDS
The
.Fn getenv
function conforms to
.St -ansiC .
The
.Fn putenv ,
.Fn setenv ,
and
.Fn unsetenv
functions conform to
.St -p1003.1-2008 .
.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 .
.Sh CAVEATS
Library code must be careful about using
.Fn getenv
to read untrusted environment variables in setuid programs.
The
.Fn issetugid
function is provided for this purpose.