Date: Thu, 22 Jan 1998 00:25:11 +0100 (GMT)
From: "X DOT PONS AT UAB DOT ES" <ILGES AT cc DOT uab DOT es>
Subject: access() in DJGPP and TC/BC
To: djgpp AT delorie DOT com
Message-id: <01ISNOZW1WPE008ULX@cc.uab.es>
Organization: Universitat Autonoma de Barcelona
MIME-version: 1.0
Content-type: TEXT/PLAIN; CHARSET=ISO-8859-1
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I have found a problem when porting TC/BC code to DJGPP because of
the different meanings of the accesibility mode ("amode", the second
parameter) of the access() function.

* DJGPP defines these accessibility modes:
  R_OK (=0x02)  Request if the file is readable.  Since all files are
                readable under MS-DOS, this access mode always exists.
  W_OK (=0x04)  Request if the file is writable.
  X_OK (=0x08)  Request if the file is executable.
  F_OK (=0x01)  Request if the file exists.
  D_OK (=0x10)  Request if the file is really a directory.

* TC/BC define these accessibility modes:
    06  ¦ Check for read and write permission
    04  ¦ Check for read permission
    02  ¦ Check for write permission
    01  ¦ Execute (ignored)
    00  ¦ Check for existence of file

This is problematic because the meanings are very different although
the code compiles and runs perfectly. Note, for example, that a 4
(read permission) in TC/BC implies a W_OK (write permission) in DJGPP.

access() is not an ANSI-C function, so probably it is not an "error"
of the compilers, but it is easy to do a mistake, specially when porting
from TC/BC (where numbers are used, not #defines) to DJGPP (where the
compiler simply interpret those numbers in a different way).

I would like:

 - Warn those of you that use access() in both compilers (probably most or
   all of you know about it, but it could be useful for some people).
   Recommend to use words, not numbers.
 - Ask if anybody knows a reason for that difference.
 - Post to the group my proposal to avoid this. It is a small #include
   that can be included both in DJGPP and TC/BC codes and that allows
   using DJGPP notation under TC/BC. Nevertheless, be aware that it
   does not solve the confusion when access() is (was) already written
   using TC/BC numerical codes instead of the proposed words.


#ifdef __DJGPP__
#include <unistd.h>
#else
#include <io.h>
#endif

#if defined (__BORLANDC__) || defined (__TURBOC__)

#define F_OK    0
#define R_OK    4 /* Gives the same result as F_OK */
#define W_OK    2
#define X_OK    1 /* TC/BC returns 0 if the file exists; -1 if it does not.
                     DJGPP returns 0 if the file exists and it is executable
                           (EXE, COM);  -1 if it does not exist or if it does
                           not recognize it as executable (BAT and PIF files
                           also return -1). */
#define D_OK    0 /* TC/BC only determines whether the directory exists.
                     If you submit c:\dos\xcopy.xxx returns 0 although
                     xcopy.xxx does not exist (but c:\dos does).
                     DJGPP returns 0 only if it is a directory (c:\dos). */

#endif


Hope this helps,

Xavier Pons