_putenv_s, _wputenv_s

Creates, modifies, or removes environment variables. These are versions of _putenv, _wputenv but 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 with /ZW.

errno_t _putenv_s(
   const char *name,
   const char *value 
errno_t _wputenv_s(
   const wchar_t *name,
   const wchar_t *value


  • name
    The environment variable name.

  • value
    The value to set the environment variable to.

Return Value

Returns 0 if successful, or an error code.

Error Conditions



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.

Generic-Text Routine Mappings

TCHAR.H routine

_UNICODE & _MBCS not defined

_MBCS defined

_UNICODE defined





name is the name of the environment variable to be added or modified and value is the variable's value. If name is already part of the environment, its value is replaced by value; otherwise, the new name variable and its value are added to the environment. You can remove a variable from the environment by specifying an empty string (that is, "") for value.

_putenv_s and _wputenv_s affect only the environment that is local to the current process; you cannot 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.

Do not 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 is 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.



Required header




<stdlib.h> or <wchar.h>

For additional compatibility information, see Compatibility.


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

.NET Framework Equivalent

Not applicable. To call the standard C function, use PInvoke. For more information, see Platform Invoke Examples.

See Also


Process and Environment Control

getenv, _wgetenv

_searchenv, _wsearchenv