Section: POSIX Programmer's Manual (3P)
This manual page is part of the POSIX Programmer's Manual.
The Linux implementation of this interface may differ (consult
the corresponding Linux manual page for details of Linux behavior),
or the interface may not be implemented on Linux.
--- change or add a value to an environment
int putenv(char *string);
function shall use the
argument to set environment variable values. The
argument should point to a string of the form "name
function shall make the value of the environment variable
by altering an existing variable or creating a new one. In either
case, the string pointed to by
shall become part of the environment, so altering the string shall
change the environment.
function need not be thread-safe.
Upon successful completion,
shall return 0; otherwise, it shall return a non-zero value and set
to indicate the error.
function may fail if:
Insufficient memory was available.
The following sections are informative.
Changing the Value of an Environment Variable
The following example changes the value of the
environment variable to the value
static char *var = "HOME=/usr/home";
ret = putenv(var);
function manipulates the environment pointed to by
and can be used in conjunction with
for restrictions on changing the environment in multi-threaded
This routine may use
to enlarge the environment.
A potential error is to call
with an automatic variable as the argument, then return from the
calling function while
is still part of the environment.
Although the space used by
is no longer used once a new string which defines
is passed to
if any thread in the application has used
to retrieve a pointer to this variable, it should not be freed by calling
If the changed environment variable is one known by the system (such as
the locale environment variables) the application should never free the
buffer used by earlier calls to
for the same variable.
function is preferred over this function. One reason is that
is optional and therefore less portable. Another is that using
can slow down environment searches, as explained in the RATIONALE
Refer to the RATIONALE section in
The Base Definitions volume of POSIX.1-2017,
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1-2017, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, 2018 Edition,
Copyright (C) 2018 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group.
In the event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online at
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