_dupenv_s, _wdupenv_s

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06/05/2017

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Gets a value from the current environment.

Important

This API cannot be used in applications that execute in the Windows Runtime. For more information, see CRT functions not supported with /ZW.

Syntax

errno_t _dupenv_s(  
   char **buffer,  
   size_t *numberOfElements,  
   const char *varname  
);  
errno_t _wdupenv_s(  
   wchar_t **buffer,  
   size_t *numberOfElements,  
   const wchar_t *varname  
);

Parameters

buffer
Buffer to store the variable's value.

numberOfElements
Size of buffer.

varname
Environment variable name.

Return Value

Zero on success, an error code on failure.

These functions validate their parameters; if buffer or varname is NULL, the invalid parameter handler is invoked as described in Parameter Validation. If execution is allowed to continue, the functions set errno to EINVAL and return EINVAL.

If these functions cannot allocate enough memory, they set buffer to NULL and numberOfElements to 0, and return ENOMEM.

Remarks

The _dupenv_s function searches the list of environment variables for varname. If the variable is found, _dupenv_s allocates a buffer and copies the variable's value into the buffer. The buffer's address and length are returned in buffer and numberOfElements. By allocating the buffer itself, _dupenv_s provides a more convenient alternative to getenv_s, _wgetenv_s.

Note

It is the calling program's responsibility to free the memory by calling free.

If the variable is not found, then buffer is set to NULL, numberOfElements is set to 0, and the return value is 0 because this situation is not considered to be an error condition.

If you are not interested in the size of the buffer you can pass NULL for numberOfElements.

_dupenv_s is not case sensitive in the Windows operating system. _dupenv_s uses the copy of the environment pointed to by the global variable _environ to access the environment. See the Remarks in getenv_s, _wgetenv_s for a discussion of _environ.

The value in buffer is a copy of the environment variable's value; modifying it has no effect on the environment. Use the _putenv_s, _wputenv_s function to modify the value of an environment variable.

_wdupenv_s is a wide-character version of _dupenv_s; the arguments of _wdupenv_s are wide-character strings. The _wenviron global variable is a wide-character version of _environ. See the Remarks in getenv_s, _wgetenv_s for more on _wenviron.

Generic-Text Routine Mappings

TCHAR.H routine	_UNICODE & _MBCS not defined	_MBCS defined	_UNICODE defined
`_tdupenv_s`	`_dupenv_s`	`_dupenv_s`	`_wdupenv_s`

Requirements

Routine	Required header
`_dupenv_s`	<stdlib.h>
`_wdupenv_s`	<stdlib.h> or <wchar.h>

For additional compatibility information, see Compatibility.

Example

// crt_dupenv_s.c  
#include  <stdlib.h>  
  
int main( void )  
{  
   char *pValue;  
   size_t len;  
   errno_t err = _dupenv_s( &pValue, &len, "pathext" );  
   if ( err ) return -1;  
   printf( "pathext = %s\n", pValue );  
   free( pValue );  
   err = _dupenv_s( &pValue, &len, "nonexistentvariable" );  
   if ( err ) return -1;  
   printf( "nonexistentvariable = %s\n", pValue );  
   free( pValue ); // It's OK to call free with NULL  
}

Sample Output

pathext = .COM;.EXE;.BAT;.CMD;.VBS;.VBE;.JS;.JSE;.WSF;.WSH;.pl  
nonexistentvariable = (null)

.NET Framework Equivalent

System::Environment::GetEnvironmentVariable

共用方式為