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.
putenv — change or add a value to an environment
int putenv(char *string);
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.
Upon successful completion, putenv() shall return 0; otherwise, it shall
return a non-zero value and set errno to indicate the error.
The putenv() function may fail if:
- Insufficient memory was available.
The following sections are informative.
The following example changes the value of the HOME environment variable
to the value /usr/home.
The putenv() function manipulates the environment pointed to by
environ, and can be used in conjunction with getenv().
static char *var = "HOME=/usr/home";
ret = putenv(var);
See exec() for restrictions on changing the environment in
This routine may use malloc() to enlarge the
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().
Refer to the RATIONALE section in setenv().
exec, free(), getenv(),
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 http://www.opengroup.org/unix/online.html .
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