_chmod, _wchmod

Changes the file-permission settings.


int _chmod( const char *filename, int pmode );
int _wchmod( const wchar_t *filename, int pmode );


Name of the existing file.

Permission setting for the file.

Return value

These functions return 0 if the permission setting is successfully changed. A return value of -1 indicates failure. If the specified file couldn't be found, errno is set to ENOENT; if a parameter is invalid, errno is set to EINVAL.


The _chmod function changes the permission setting of the file specified by filename. The permission setting controls the read and write access to the file. The integer expression pmode contains one or both of the following manifest constants, defined in SYS\Stat.h.

pmode Meaning
_S_IREAD Only reading permitted.
_S_IWRITE Writing permitted. (In effect, permits reading and writing.)
_S_IREAD | _S_IWRITE Reading and writing permitted.

When both constants are given, they're joined with the bitwise or operator (|). If write permission isn't given, the file is read-only. Note that all files are always readable; it isn't possible to give write-only permission. Thus, the modes _S_IWRITE and _S_IREAD | _S_IWRITE are equivalent.

_wchmod is a wide-character version of _chmod; the filename argument to _wchmod is a wide-character string. _wchmod and _chmod behave identically otherwise.

This function validates its parameters. If pmode isn't a combination of one of the manifest constants or incorporates an alternate set of constants, the function simply ignores them. If filename is NULL, the invalid parameter handler is invoked, as described in Parameter validation. If execution is allowed to continue, errno is set to EINVAL and the function returns -1.

By default, this function's global state is scoped to the application. To change it, see Global state in the CRT.

Generic-text routine mappings

Tchar.h routine _UNICODE and _MBCS not defined _MBCS defined _UNICODE defined
_tchmod _chmod _chmod _wchmod


Routine Required header Optional header
_chmod <io.h> <sys/types.h>, <sys/stat.h>, <errno.h>
_wchmod <io.h> or <wchar.h> <sys/types.h>, <sys/stat.h>, <errno.h>

For more compatibility information, see Compatibility.


// crt_chmod.c
// This program uses _chmod to
// change the mode of a file to read-only.
// It then attempts to modify the file.

#include <sys/types.h>
#include <sys/stat.h>
#include <io.h>
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>

// Change the mode and report error or success
void set_mode_and_report(char * filename, int mask)
   // Check for failure
   if( _chmod( filename, mask ) == -1 )
      // Determine cause of failure and report.
      switch (errno)
         case EINVAL:
            fprintf( stderr, "Invalid parameter to chmod.\n");
         case ENOENT:
            fprintf( stderr, "File %s not found\n", filename );
            // Should never be reached
            fprintf( stderr, "Unexpected error in chmod.\n" );
      if (mask == _S_IREAD)
        printf( "Mode set to read-only\n" );
      else if (mask & _S_IWRITE)
        printf( "Mode set to read/write\n" );

int main( void )

   // Create or append to a file.
   system( "echo /* End of file */ >> crt_chmod.c_input" );

   // Set file mode to read-only:
   set_mode_and_report("crt_chmod.c_input ", _S_IREAD );

   system( "echo /* End of file */ >> crt_chmod.c_input " );

   // Change back to read/write:
   set_mode_and_report("crt_chmod.c_input ", _S_IWRITE );

   system( "echo /* End of file */ >> crt_chmod.c_input " );

A line of text.

      A line of text.Mode set to read-only
Access is denied.
Mode set to read/write

See also

File handling
_access, _waccess
_creat, _wcreat
_fstat, _fstat32, _fstat64, _fstati64, _fstat32i64, _fstat64i32
_open, _wopen
_stat, _wstat functions