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_putenv, _wputenv

Create, modify, or remove environment variables.

int _putenv(
   const char *envstring 
int _wputenv(
   const wchar_t *envstring 


Environment-string definition.

Return Value

Return 0 if successful, or –1 in the case of an error.


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.

Generic-Text Routine Mappings

TCHAR.H routine _UNICODE & _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=string, where varname is the name of the environment variable to be added or modified and string is the variable's value. If varname is already part of the environment, its value is replaced by string; otherwise, the new varname variable and its string value are added to the environment. You can remove a variable from the environment by specifying an empty string — in other words, by specifying only varname=.

_putenv and _wputenv affect only the environment that is local to the current process; you cannot use them to modify the command-level environment. That is, these functions operate only on data structures accessible to the run-time library and not 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.

Do not change an environment entry directly: instead, use _putenv or _wputenv to change it. In particular, direct freeing elements of the _environ[] global array may 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 may change the value of _environ and _wenviron, thus invalidating 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 relation of _putenv and _wputenv to global variables, see _environ, _wenviron.


Routine Required header Compatibility
_putenv <stdlib.h> Win 98, Win Me, Win NT, Win 2000, Win XP
_wputenv <stdlib.h> or <wchar.h> Win NT, Win 2000, Win XP

For additional compatibility information, see Compatibility in the Introduction.


All versions of the C run-time libraries.


For a sample of how to use _putenv, see getenv.

See Also

Process and Environment Control Routines | getenv | _searchenv | Run-Time Routines and .NET Framework Equivalents