另一种判断文件存在的方法－－_access和

本文介绍如何使用 _access 和 _waccess 函数来确定文件是否存在及读写权限。这两个函数允许开发者验证文件路径是否存在，并检查其是否具有指定的访问权限。

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_access, _waccess

Determine file-access permission.

int _access( const char *path, int mode );

int _waccess( const wchar_t *path, int mode );

Routine	Required Header	Optional Headers	Compatibility
_access	<io.h>	<errno.h>	Win 95, Win NT
_waccess	<wchar.h> or <io.h>	<errno.h>	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 0 if the file has the given mode. The function returns –1 if the named file does not exist or is not accessible in the given mode; in this case, errno is set as follows:

EACCES

Access denied: file’s permission setting does not allow specified access.

ENOENT

Filename or path not found.

Parameters

path

File or directory path

mode

Permission setting

Remarks

When used with files, the _access function determines whether the specified file exists and can be accessed as specified by the value of mode. When used with directories, _access determines only whether the specified directory exists; in Windows NT, all directories have read and write access.

mode Value	Checks File For
00	Existence only
02	Write permission
04	Read permission
06	Read and write permission

_waccess is a wide-character version of _access; the path argument to _waccess is a wide-character string. _waccess and _access behave identically otherwise.

Generic-Text Routine Mappings

TCHAR.H Routine	_UNICODE & _MBCS Not Defined	_MBCS Defined	_UNICODE Defined
_taccess	_access	_access	_waccess

Example

         /* ACCESS.C: This example uses _access to check the
        
         * file named "ACCESS.C" to see if it exists and if
        
         * writing is allowed.
        
         */
        
         #include  <io.h>
        
         #include  <stdio.h>
        
         #include  <stdlib.h>
        
         void 
         main( 
         void 
          )
        
         {
        
         /* Check for existence */
        
         if
         ( (_access( 
         "ACCESS.C"
         , 0 )) != -1 )
        
         {
        
         printf
         ( 
         "File ACCESS.C exists\n" 
          );
        
         /* Check for write permission */
        
         if
         ( (_access( 
         "ACCESS.C"
         , 2 )) != -1 )
        
         printf
         ( 
         "File ACCESS.C has write permission\n" 
          );
        
         if
         ( (_access( 
         "ACCESS.C"
         , 4 )) != -1 )
        
         printf
         ( 
         "File ACCESS.C has read permission\n" 
          );
        
         if
         ( (_access( 
         "ACCESS.C"
         , 6 )) != -1 )
        
         printf
         ( 
         "File ACCESS.C has read and write permission\n" 
         );
        
         }
        
         else
        
         {
        
         printf
         (
         "File ACCESS.C does not exists\n"
         );
        
         }
        
         }