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.TH PUTENV 3C "Jun 22, 2016"
.SH NAME
putenv \- change or add value to environment
.SH SYNOPSIS
.LP
.nf
#include <stdlib.h>

\fBint\fR \fBputenv\fR(\fBchar *\fR\fIstring\fR);
.fi

.SH DESCRIPTION
.LP
The \fBputenv()\fR function makes the value of the environment variable
\fIname\fR equal to \fIvalue\fR by altering an existing variable or creating a
new one. In either case, the string pointed to by \fIstring\fR becomes part of
the environment, so altering the string will change the environment.
.sp
.LP
The \fIstring\fR argument points to a string of the form
\fIname\fR\fB=\fR\fIvalue\fR. The space used by \fIstring\fR is no longer used
once a new string-defining \fIname\fR is passed to \fBputenv()\fR. If
there is no equals sign (\fB=\fR) present as a separator, then
\fIstring\fR is treated as the \fIname\fR of an environment variable to
remove from the environment, as though \fBunsetenv\fR(3C) had been
called.
.sp
.LP
The \fBputenv()\fR function uses \fBmalloc\fR(3C) to enlarge the environment.
.sp
.LP
After \fBputenv()\fR is called, environment variables are not in alphabetical
order.
.SH RETURN VALUES
.LP
Upon successful completion, \fBputenv()\fR returns 0. Otherwise, it returns a
non-zero value and sets \fBerrno\fR to indicate the error.
.SH ERRORS
.LP
The \fBputenv()\fR function may fail if:
.sp
.ne 2
.na
\fB\fBENOMEM\fR\fR
.ad
.RS 10n
Insufficient memory was available.
.RE

.SH USAGE
.LP
The \fBputenv()\fR function can be safely called from multithreaded programs.
Caution must be exercised when using this function and \fBgetenv\fR(3C) in
multithreaded programs. These functions examine and modify the environment
list, which is shared by all threads in a program.  The system prevents the
list from being accessed simultaneously by two different threads.  It does not,
however, prevent two threads from successively accessing the environment list
using \fBputenv()\fR or \fBgetenv()\fR.
.SH ATTRIBUTES
.LP
See \fBattributes\fR(7) for descriptions of the following attributes:
.sp

.sp
.TS
box;
c | c
l | l .
ATTRIBUTE TYPE	ATTRIBUTE VALUE
_
Interface Stability	Standard
_
MT-Level	Safe
.TE

.SH SEE ALSO
.LP
.BR exec (2),
.BR getenv (3C),
.BR malloc (3C),
.BR attributes (7),
.BR environ (7),
.BR standards (7)
.SH WARNINGS
.LP
The \fIstring\fR argument should not be an automatic variable.  It should be
declared static if it is declared within a function because it cannot be
automatically declared. A potential error is to call \fBputenv()\fR with a
pointer to an automatic variable as the argument and to then exit the calling
function while \fIstring\fR is still part of the environment.