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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
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NAME
putenv — change or add a value to an environment
SYNOPSIS
#include <stdlib.h> int putenv(char *string);
DESCRIPTION
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.
RETURN VALUE
Upon successful completion, putenv() shall return 0; otherwise, it shall return a non-zero value and set
errno to indicate the error.
ERRORS
The putenv() function may fail if:
ENOMEM Insufficient memory was available.
The following sections are informative.
EXAMPLES
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);
APPLICATION USAGE
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-threaded applications.
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().
RATIONALE
Refer to the RATIONALE section in setenv().
FUTURE DIRECTIONS
None.
SEE ALSO
exec, free(), getenv(), malloc(), setenv()
The Base Definitions volume of POSIX.1‐2017, <stdlib.h>
COPYRIGHT
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 http://www.opengroup.org/unix/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
https://www.kernel.org/doc/man-pages/reporting_bugs.html .