Creates, modifies, or removes environment variables. More secure versions of these functions are available; see _putenv_s, _wputenv_s.
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.
_UNICODE and _MBCS not defined
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 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 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.
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.
<stdlib.h> or <wchar.h>
For more compatibility information, see Compatibility in the Introduction.