.\" Copyright 1993 David Metcalfe (david@prism.demon.co.uk)
.\"
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.\" 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)
.TH PUTENV 3  "April 8, 1993" "GNU" "Linux Programmer's Manual"
.SH NAME
putenv \- change or add an environment variable
.SH SYNOPSIS
.nf
.B #include <stdlib.h>
.sp
.BI "int putenv(const char *" string );
.fi
.SH DESCRIPTION
The \fBputenv()\fP 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 \fBputenv()\fP function returns zero on success, or \-1 if an error
occurs.
.SH ERRORS
.TP
.B ENOMEM
Insufficient space to allocate new environment.
.SH NOTES
The \fBputenv()\fP function is not required to be reentrant, and the
one in libc4, libc5 and glibc2.0 is not, but the glibc2.1 version is.
.LP
Description for libc4, libc5, glibc:
If the argument \fIstring\fP is of the form \fIname\fP,
and does not contain an `=' character, then the variable \fIname\fP
is removed from the environment.
If \fBputenv()\fP has to allocate a new array \fIenviron\fP,
and the previous array was also allocated by \fBputenv()\fP,
then it will be freed. In no case will the old storage associated
to the environment variable itself be freed.
.LP
The libc4 and libc5 and glibc 2.1.2 versions conform to SUSv2:
the pointer \fIstring\fP given to \fBputenv()\fP is used.
In particular, this string becomes part of the environment;
changing it later will change the environment.
(Thus, it is an error is to call \fBputenv()\fP with an automatic variable
as the argument, then return from the calling function while \fIstring\fP
is still part of the environment.)
However, glibc 2.0-2.1.1 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. This has been fixed in glibc2.1.2.
.LP
The BSD4.4 version, like glibc 2.0, uses a copy.
.LP
SUSv2 removes the `const' from the prototype, and so does glibc 2.1.3.
.SH "CONFORMING TO"
SVID 3, POSIX, BSD 4.3
.SH "SEE ALSO"
.BR getenv (3),
.BR setenv (3),
.BR unsetenv (3),
.BR environ (5)