'\" t
.\" Copyright 1993 David Metcalfe (david@prism.demon.co.uk)
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.\" References consulted:
.\"     Linux libc source code
.\"     Lewine's _POSIX Programmer's Guide_ (O'Reilly & Associates, 1991)
.\"     386BSD man pages
.\"	Single UNIX Specification, Version 2
.\" Modified Thu Apr  8 15:00:12 1993, David Metcalfe
.\" Modified Sat Jul 24 18:44:45 1993, Rik Faith (faith@cs.unc.edu)
.\" Modified Fri Feb 14 21:47:50 1997 by Andries Brouwer (aeb@cwi.nl)
.\" Modified Mon Oct 11 11:11:11 1999 by Andries Brouwer (aeb@cwi.nl)
.\" Modified Wed Nov 10 00:02:26 1999 by Andries Brouwer (aeb@cwi.nl)
.\" Modified Sun May 20 22:17:20 2001 by Andries Brouwer (aeb@cwi.nl)
.TH putenv 3 2023-10-31 "Linux man-pages 6.7"
.SH NAME
putenv \- change or add an environment variable
.SH LIBRARY
Standard C library
.RI ( libc ", " \-lc )
.SH SYNOPSIS
.nf
.B #include <stdlib.h>
.P
.BI "int putenv(char *" string );
.\" Not: const char *
.fi
.P
.RS -4
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.RE
.P
.BR putenv ():
.nf
    _XOPEN_SOURCE
        || /* glibc >= 2.19: */ _DEFAULT_SOURCE
        || /* glibc <= 2.19: */ _SVID_SOURCE
.fi
.SH DESCRIPTION
The
.BR putenv ()
function adds or changes the value of environment
variables.
The argument \fIstring\fP is of the form \fIname\fP=\fIvalue\fP.
If \fIname\fP does not already exist in the environment, then
\fIstring\fP is added to the environment.
If \fIname\fP does exist,
then the value of \fIname\fP in the environment is changed to
\fIvalue\fP.
The string pointed to by \fIstring\fP becomes part of the environment,
so altering the string changes the environment.
.SH RETURN VALUE
The
.BR putenv ()
function returns zero on success.
On failure, it returns a nonzero value, and
.I errno
is set to indicate the error.
.SH ERRORS
.TP
.B ENOMEM
Insufficient space to allocate new environment.
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.TS
allbox;
lbx lb lb
l l l.
Interface	Attribute	Value
T{
.na
.nh
.BR putenv ()
T}	Thread safety	MT-Unsafe const:env
.TE
.SH STANDARDS
POSIX.1-2008.
.SH HISTORY
POSIX.1-2001, SVr2, 4.3BSD-Reno.
.P
The
.BR putenv ()
function is not required to be reentrant, and the
one in glibc 2.0 is not, but the glibc 2.1 version is.
.\" .P
.\" Description for libc4, libc5, glibc:
.\" If the argument \fIstring\fP is of the form \fIname\fP,
.\" and does not contain an \[aq]=\[aq] character, then the variable \fIname\fP
.\" is removed from the environment.
.\" If
.\" .BR putenv ()
.\" has to allocate a new array \fIenviron\fP,
.\" and the previous array was also allocated by
.\" .BR putenv (),
.\" then it will be freed.
.\" In no case will the old storage associated
.\" to the environment variable itself be freed.
.P
Since glibc 2.1.2, the glibc implementation conforms to SUSv2:
the pointer \fIstring\fP given to
.BR putenv ()
is used.
In particular, this string becomes part of the environment;
changing it later will change the environment.
(Thus, it is an error to call
.BR putenv ()
with an automatic variable
as the argument, then return from the calling function while \fIstring\fP
is still part of the environment.)
However, from glibc 2.0 to glibc 2.1.1, it differs:
a copy of the string is used.
On the one hand this causes a memory leak, and on the other hand
it violates SUSv2.
.P
The 4.3BSD-Reno version, like glibc 2.0, uses a copy;
this is fixed in all modern BSDs.
.P
SUSv2 removes the \fIconst\fP from the prototype, and so does glibc 2.1.3.
.P
The GNU C library implementation provides a nonstandard extension.
If
.I string
does not include an equal sign:
.P
.in +4n
.EX
putenv("NAME");
.EE
.in
.P
then the named variable is removed from the caller's environment.
.SH SEE ALSO
.BR clearenv (3),
.BR getenv (3),
.BR setenv (3),
.BR unsetenv (3),
.BR environ (7)