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.
The putenv() function shall use the string argument to set
environment variable values. The string argument should point to a
string of the form "name=value". The putenv() function shall make
the value of the environment variable name equal to value by altering
an existing variable or creating a new one. In either case, the
string pointed to by string shall become part of the environment, so
altering the string shall change the environment.
The putenv() function need not be thread-safe.
Changing the Value of an Environment Variable
The following example changes the value of the HOME environment
variable to the value /usr/home.
#include <stdlib.h>...static char *var = "HOME=/usr/home";int ret;
ret = putenv(var);
The putenv() function manipulates the environment pointed to by
environ, and can be used in conjunction with getenv().
See exec() for restrictions on changing the environment in multi-
This routine may use malloc() to enlarge the environment.
A potential error is to call putenv() with an automatic variable as
the argument, then return from the calling function while string is
still part of the environment.
Although the space used by string is no longer used once a new string
which defines name is passed to putenv(), if any thread in the
application has used getenv() to retrieve a pointer to this variable,
it should not be freed by calling free(). 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 putenv() for the same variable.
The setenv() function is preferred over this function. One reason is
that putenv() is optional and therefore less portable. Another is
that using putenv() can slow down environment searches, as explained
in the RATIONALE section for getenv(3p).
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The Open
Group Base Specifications Issue 7, Copyright (C) 2013 by the
Institute of Electrical and Electronics Engineers, Inc and The Open
Group. (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1
applied.) 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 http://www.unix.org/online.html .
Any typographical or formatting errors that appear in this page are
most likely to have been introduced during the conversion of the
source files to man page format. To report such errors, see
IEEE/The Open Group 2013 PUTENV(3P)