NetNews Offline 2/NetNews Offline Volume 2.iso/news/comp/lang/c-part2/12896

Internet Message Format | 1996-08-05 | 2.3 KB

Path: mail2news.demon.co.uk!genesis.demon.co.uk
From: Lawrence Kirby <[e-mail]>
Newsgroups: comp.lang.c,comp.os.ms-windows.programmer.win32
Subject: Re: How can I check whether I file exists in a multi-user environment?
Date: Wed, 03 Apr 96 12:40:22 GMT
Organization: none
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References: <4jh4tl$[e-mail]> <[e-mail]> <4jjn1d$[e-mail]> <[e-mail]>
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In article <[e-mail]> [e-mail] "Doug Miller" writes:

>[e-mail] (Peter Seebach) wrote:

>>>#include <io.h>
>>>int file_exists (char *filename) {
>>>      return (access(filename, 0) == 0);
>>>} /* returns -1 if file exists, 0 if it does not */
>>
>>The reason this is a poor solution is that it doesn't test for existance,
>>it tests for accessibility.
>
>You should check your facts more carefully.  You evidently are unfamiliar with
> the behavior of
>access( ).  It is defined thusly:
>
>#include <io.h>
>int access (const char *filename, int amode);
>
>permissible values for amode are:
>        06      check for read and write permission
>        04      check for read permission
>        02      check for write permission
>        01      execute
>>>>>    00      check for existence of file     <<<<<

This is simply a poor description. access() is a function that is portable
across many platforms being derived from the Unix/POSIX definition (but isn't
standard C). It can only check for the existence of a file where that file is
accessible, it does not bypass filesystem permissions. Even under DOS/Windows
this is an issue for network drives. access() if first and foremost a test
for accessibility, only if that condition is met can it test for
existence/readability/writability. Worse than that in its correct POSIX
guise it may perform the test using different criteria than those
applied to fopen(), open() etc. which is why in general you should avoid
it and use something like stat() or fopen() directly.

--
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Lawrence Kirby | [e-mail]
Wilts, England | [e-mail]
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