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_putenv_s, _wputenv_s

Creates, modifies, or removes environment variables. These functions are versions of _putenv, _wputenv that have security enhancements, as described in Security features in the CRT.


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.


errno_t _putenv_s(
   const char *varname,
   const char *value_string
errno_t _wputenv_s(
   const wchar_t *varname,
   const wchar_t *value_string


The environment variable name.

The value to set the environment variable to.

Return value

Returns 0 if successful, or an error code.

Error conditions

varname value_string Return value

If one of the error conditions occurs, these functions invoke an invalid parameter handler, as described in Parameter validation. If execution is allowed to continue, these functions return EINVAL and set errno to EINVAL.


The _putenv_s 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_s is a wide-character version of _putenv_s; the envstring argument to _wputenv_s 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_s _putenv_s _putenv_s _wputenv_s

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 are added to the environment. You can remove a variable from the environment by specifying an empty string (that is, "") for value_string.

_putenv_s and _wputenv_s affect only the environment that is local to the current process; you can't use them to modify the command-level environment. These functions operate only on data structures that are accessible to the run-time library and not on the environment "segment" that the operating system creates for a process. When the current process terminates, the environment reverts to the level of the calling process, which in most cases is the operating-system level. However, the modified environment can be passed to any new processes that are created by _spawn, _exec, or system, and these new processes get any new items that are added by _putenv_s and _wputenv_s.

Don't change an environment entry directly; instead, use _putenv_s or _wputenv_s to change it. In particular, directly freeing elements of the _environ[] global array might cause invalid memory to be addressed.

getenv and _putenv_s use the global variable _environ to access the environment table; _wgetenv and _wputenv_s use _wenviron. _putenv_s and _wputenv_s may change the value of _environ and _wenviron, and thereby invalidate 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 relationship of _putenv_s and _wputenv_s to global variables, see _environ, _wenviron.


The _putenv_s and _getenv_s families of functions are not thread-safe. _getenv_s could return a string pointer while _putenv_s is modifying the string, and thereby cause random failures. Make sure that calls to these functions are synchronized.


Routine Required header
_putenv_s <stdlib.h>
_wputenv_s <stdlib.h> or <wchar.h>

For more compatibility information, see Compatibility.


For a sample that shows how to use _putenv_s, see getenv_s, _wgetenv_s.

See also

Process and environment control
getenv, _wgetenv
_searchenv, _wsearchenv