getenv, _wgetenv

Articolo
07/24/2011

Get a value from the current environment. More secure versions of these functions are available; see getenv_s, _wgetenv_s.

char *getenv( 
   const char *varname 
);
wchar_t *_wgetenv( 
   const wchar_t *varname 
);

Parameters

varname
Environment variable name.

Return Value

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.

Remarks

The getenv function searches the list of environment variables for varname. getenv is not case sensitive in the Windows operating system. 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.

If varname is NULL, this function invokes an invalid parameter handler, as described in Parameter Validation. If execution is allowed to continue, this function sets errno to EINVAL and returns NULL.

_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.

Avviso

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.

Nota

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.

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 _daylight, timezone, and _tzname.

Requirements

Routine	Required header
getenv	<stdlib.h>
_wgetenv	<stdlib.h> or <wchar.h>

For additional compatibility information, see Compatibility in the Introduction.

Example

// crt_getenv.c
// compile with: /W3
// 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>

int main( void )
{
   char *libvar;

   // Get the value of the LIB environment variable.
   libvar = getenv( "LIB" ); // C4996
   // Note: getenv is deprecated; consider using getenv_s instead

   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" ); // C4996
   // Note: _putenv is deprecated; consider using putenv_s instead

   // Get new value.
   libvar = getenv( "LIB" ); // C4996

   if( libvar != NULL )
      printf( "New LIB variable is: %s\n", libvar );
}

Original LIB variable is: C:\progra~1\devstu~1\vc\lib
New LIB variable is: c:\mylib;c:\yourlib

.NET Framework Equivalent

System::Environment::GetEnvironmentVariable

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