_putenv
, _wputenv
Creates, modifies, or removes environment variables. More secure versions of these functions are available; see _putenv_s
, _wputenv_s
.
Important
This API cannot be used in applications that execute in the Windows Runtime. For more information, see CRT functions not supported in Universal Windows Platform apps.
Syntax
int _putenv(
const char *envstring
);
int _wputenv(
const wchar_t *envstring
);
Parameters
envstring
Environment-string definition.
Return value
The functions return 0 if successful, or -1 if there's an error.
Remarks
The _putenv
function adds new environment variables or modifies the values of existing environment variables. Environment variables define the environment in which a process executes (for example, the default search path for libraries to be linked with a program). _wputenv
is a wide-character version of _putenv
; the envstring
argument to _wputenv
is a wide-character string.
By default, this function's global state is scoped to the application. To change this behavior, see Global state in the CRT.
Generic-text routine mappings
Tchar.h routine |
_UNICODE and _MBCS not defined |
_MBCS defined |
_UNICODE defined |
---|---|---|---|
_tputenv |
_putenv |
_putenv |
_wputenv |
The envstring
argument must be a pointer to a string of the form varname=value_string
, where varname
is the name of the environment variable to be added or modified and value_string
is the variable's value. If varname
is already part of the environment, its value is replaced by value_string
; otherwise, the new varname
variable and its value_string
value are added to the environment. You can remove a variable from the environment by specifying an empty value_string
, or in other words, by specifying only varname
=.
_putenv
and _wputenv
affect only the environment that is local to the current process; you can't use them to modify the command-level environment. That is, these functions operate only on data structures accessible to the run-time library. They don't operate on the environment segment created for a process by the operating system. When the current process terminates, the environment reverts to the level of the calling process (in most cases, the operating-system level). However, the modified environment can be passed to any new processes created by _spawn
, _exec
, or system
, and these new processes get any new items added by _putenv
and _wputenv
.
Don't change an environment entry directly: instead, use _putenv
or _wputenv
to change it. In particular, direct freeing elements of the _environ[]
global array might lead to invalid memory being addressed.
_getenv
and _putenv
use the global variable _environ
to access the environment table; _wgetenv
and _wputenv
use _wenviron
. _putenv
and _wputenv
might change the value of _environ
and _wenviron
, thus invalidating the _envp
argument to main
and the _wenvp
argument to wmain
. Therefore, it's safer to use _environ
or _wenviron
to access the environment information. For more information about the relation of _putenv
and _wputenv
to global variables, see _environ
, _wenviron
.
Note
The _putenv
and _getenv
families of functions are not thread-safe. _getenv
could return a string pointer while _putenv
is modifying the string, causing random failures. Make sure that calls to these functions are synchronized.
Requirements
Routine | Required header |
---|---|
_putenv |
<stdlib.h> |
_wputenv |
<stdlib.h> or <wchar.h> |
For more compatibility information, see Compatibility.
Example
For a sample of how to use _putenv
, see getenv
, _wgetenv
.
See also
Process and environment control
getenv
, _wgetenv
_searchenv
, _wsearchenv
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