getenv, _wgetenv
Get a value from the current environment.
char *getenv( const char *varname );
wchar_t *_wgetenv( const wchar_t *varname );
| Routine | Required Header | Compatibility |
| getenv | <stdlib.h> | ANSI, Win 95, Win NT |
| _wgetenv | <stdlib.h> or <wchar.h> | Win 95, Win NT |
For additional compatibility information, see Compatibility in the Introduction.
Libraries
| LIBC.LIB | Single thread static library, retail version |
| LIBCMT.LIB | Multithread static library, retail version |
| MSVCRT.LIB | Import library for MSVCRT.DLL, retail version |
Return Value
Each of these functions returns a pointer to the environment table entry containing varname. It is not safe to modify the value of the environment variable using the returned pointer. Use the _putenv function to modify the value of an environment variable. The return value is NULL if varname is not found in the environment table.
Parameter
varname
Environment variable name
Remarks
The getenv function searches the list of environment variables for varname. getenv is not case sensitive in Windows NT and Windows 95. getenv and _putenv use the copy of the environment pointed to by the global variable _environ to access the environment. getenv operates only on the data structures accessible to the run-time library and not on the environment “segment” created for the process by the operating system. Therefore, programs that use the envp argument to main or wmain may retrieve invalid information.
_wgetenv is a wide-character version of getenv; the argument and return value of _wgetenv are wide-character strings. The _wenviron global variable is a wide-character version of _environ.
In an MBCS program (for example, in an SBCS ASCII program), _wenviron is initially NULL because the environment is composed of multibyte-character strings. Then, on the first call to _wputenv, or on the first call to _wgetenv if an (MBCS) environment already exists, a corresponding wide-character string environment is created and is then pointed to by _wenviron.
Similarly in a Unicode (_wmain) program, _environ is initially NULL because the environment is composed of wide-character strings. Then, on the first call to _putenv, or on the first call to getenv if a (Unicode) environment already exists, a corresponding MBCS environment is created and is then pointed to by _environ.
When two copies of the environment (MBCS and Unicode) exist simultaneously in a program, the run-time system must maintain both copies, resulting in slower execution time. For example, whenever you call _putenv, a call to _wputenv is also executed automatically, so that the two environment strings correspond.
Caution In rare instances, when the run-time system is maintaining both a Unicode version and a multibyte version of the environment, these two environment versions may not correspond exactly. This is because, although any unique multibyte-character string maps to a unique Unicode string, the mapping from a unique Unicode string to a multibyte-character string is not necessarily unique. For more information, see _environ, _wenviron.
Generic-Text Routine Mappings
| TCHAR.H Routine | _UNICODE & _MBCS Not Defined | _MBCS Defined | _UNICODE Defined |
| _tgetenv | getenv | getenv | _wgetenv |
To check or change the value of the TZ environment variable, use getenv, _putenv and _tzset as necessary. For more information about TZ, see _tzset and see _daylight, timezone, and _tzname.
Example
/* GETENV.C: This program uses getenv to retrieve
* the LIB environment variable and then uses
* _putenv to change it to a new value.
*/
#include <stdlib.h>
#include <stdio.h>
void main( void )
{
char *libvar;
/* Get the value of the LIB environment variable. */
libvar = getenv( "LIB" );
if( libvar != NULL )
printf( "Original LIB variable is: %s/n", libvar );
/* Attempt to change path. Note that this only affects the environment
* variable of the current process. The command processor's environment
* is not changed.
*/
_putenv( "LIB=c://mylib;c://yourlib" );
/* Get new value. */
libvar = getenv( "LIB" );
if( libvar != NULL )
printf( "New LIB variable is: %s/n", libvar );
}
Output
Original LIB variable is: C:/progra~1/devstu~1/vc/lib
New LIB variable is: c:/mylib;c:/yourlib