getenv_s, _wgetenv_s

Статья
10/16/2008

Get a value from the current environment. These are versions of getenv, _wgetenv with security enhancements as described in Security Enhancements in the CRT.

errno_t getenv_s( 
   size_t *pReturnValue,
   char* buffer,
   size_t numberOfElements,
   const char *varname 
);
errno_t _wgetenv_s( 
   size_t *pReturnValue,
   wchar_t *buffer,
   size_t numberOfElements,
   const wchar_t *varname 
);
template <size_t size>
errno_t getenv_s( 
   size_t *pReturnValue,
   char (&buffer)[size],
   const char *varname 
); // C++ only
template <size_t size>
errno_t _wgetenv_s( 
   size_t *pReturnValue,
   wchar_t (&buffer)[size],
   const wchar_t *varname 
); // C++ only

Параметры

pReturnValue
The buffer size needed, or 0 if the variable is not found.
buffer
Buffer to store the variable and its value.
numberOfElements
Size of buffer.
varname
Environment variable name.

Возвращаемое значение

Zero if successful, an error code on failure.

Error Conditions

pReturnValue	buffer	numberOfElements	varname	Return Value
NULL	any	any	any	EINVAL
any	NULL	>0	any	EINVAL
any	any	any	NULL	EINVAL

Any of the above error conditions will invoke an invalid parameter handler, as described in Parameter Validation. If execution is allowed to continue, the functions set errno to EINVAL and return EINVAL.

Also, if the buffer is too small, these functions return ERANGE. They do not invoke an invalid parameter handler. They will write out the required buffer size in pReturnValue, enabling programs to call the function again with a larger buffer.

Заметки

The getenv_s function searches the list of environment variables for varname. getenv_s is not case sensitive in the Windows operating system. getenv_s and _putenv_s use the copy of the environment pointed to by the global variable _environ to access the environment. getenv_s 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.

It is not safe to modify the value of the environment variable using the pointer returned in buffer. Use the _putenv_s function to modify the value of an environment variable. The value of buffer is set to an empty string if varname is not found in the environment table.

_wgetenv_s is a wide-character version of getenv_s; the argument and return value of _wgetenv_s 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_s 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_s 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.

Внимание!
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.

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.

Примечание.
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, causing random failures. Make sure that calls to these functions are synchronized.

In C++, using these functions is simplified by template overloads; the overloads can infer buffer length automatically, eliminating the need to specify a size argument. For more information, see Secure Template Overloads.

Generic-Text Routine Mappings

TCHAR.H routine	_UNICODE & _MBCS not defined	_MBCS defined	_UNICODE defined
_tgetenv_s	getenv_s	getenv_s	_wgetenv_s

To check or change the value of the TZ environment variable, use getenv_s, _putenv and _tzset as necessary. For more information about TZ, see _tzset and see _daylight, timezone, and _tzname.

Требования

Routine	Required header
getenv_s	<stdlib.h>
_wgetenv_s	<stdlib.h> or <wchar.h>

For additional compatibility information, see Compatibility in the Introduction.

Пример

// crt_getenv_s.c
// This program uses getenv_s 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;
   size_t requiredSize;

   getenv_s( &requiredSize, NULL, 0, "LIB");

   libvar = (char*) malloc(requiredSize * sizeof(char));
   if (!libvar)
   {
      printf("Failed to allocate memory!\n");
      exit(1);
   }

   // Get the value of the LIB environment variable.
   getenv_s( &requiredSize, libvar, requiredSize, "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_s( "LIB", "c:\\mylib;c:\\yourlib" );

   getenv_s( &requiredSize, NULL, 0, "LIB");

   libvar = (char*) malloc(requiredSize * sizeof(char));
   if (!libvar)
   {
      printf("Failed to allocate memory!\n");
      exit(1);
   }

   // Get the new value of the LIB environment variable. 
   getenv_s( &requiredSize, libvar, requiredSize, "LIB" );

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

Original LIB variable is: c:\vctools\lib;c:\vctools\atlmfc\lib;c:\vctools\PlatformSDK\lib;c:\vctools\Visual Studio SDKs\DIA Sdk\lib;c:\vctools\Visual Studio SDKs\BSC Sdk\lib
New LIB variable is: c:\mylib;c:\yourlib