Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > comp.lang.c > #390259

Re: Buffer contents well-defined after fgets() reaches EOF ?

From Ben Bacarisse <ben@bsb.me.uk>
Newsgroups comp.lang.c
Subject Re: Buffer contents well-defined after fgets() reaches EOF ?
Date 2025-02-10 01:32 +0000
Organization A noiseless patient Spider
Message-ID <8734gmpotb.fsf@bsb.me.uk> (permalink)
References <vo9g74$fu8u$1@dont-email.me>

Show all headers | View raw

Janis Papanagnou <janis_papanagnou+ng@hotmail.com> writes:

> To get the last line of a text file I'm using
>
>     char buf[BUFSIZ];
>     while (fgets (buf, BUFSIZ, fd) != NULL)
>         ;   // read to last line
>
> If the end of the file is reached my test shows that the previous
> contents of 'buf' are still existing (not cleared or overwritten).

Something that has not yet come up (as far as I can see) is that you
might need to handle an empty file.  In such a case, nothing gets
written and fgets returns NULL right away.  Processing buf in this
situation is then undefined.

One way to handle this is to put into buf something that can't get read
by fgets.  Two newlines is a good candidate:

  char buf[BUFSIZE] = "\n\n";

You can then test for that if need be, though of course it all depends
on what your application is doing.

-- 
Ben.

Back to comp.lang.c | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread

Thread

Buffer contents well-defined after fgets() reaches EOF ? Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2025-02-09 06:59 +0100
  Re: Buffer contents well-defined after fgets() reaches EOF ? Kaz Kylheku <643-408-1753@kylheku.com> - 2025-02-09 06:23 +0000
  Re: Buffer contents well-defined after fgets() reaches EOF ? Andrey Tarasevich <noone@noone.net> - 2025-02-08 22:23 -0800
    Re: Buffer contents well-defined after fgets() reaches EOF ? Andrey Tarasevich <noone@noone.net> - 2025-02-08 23:12 -0800
      Re: Buffer contents well-defined after fgets() reaches EOF ? Lawrence D'Oliveiro <ldo@nz.invalid> - 2025-02-09 23:52 +0000
        Re: Buffer contents well-defined after fgets() reaches EOF ? Andrey Tarasevich <noone@noone.net> - 2025-02-09 17:06 -0800
          Re: Buffer contents well-defined after fgets() reaches EOF ? Andrey Tarasevich <noone@noone.net> - 2025-02-09 17:22 -0800
            Re: Buffer contents well-defined after fgets() reaches EOF ? Michael S <already5chosen@yahoo.com> - 2025-02-10 12:49 +0200
              Re: Buffer contents well-defined after fgets() reaches EOF ? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2025-02-13 07:14 -0800
                Re: Buffer contents well-defined after fgets() reaches EOF ? Michael S <already5chosen@yahoo.com> - 2025-02-14 16:51 +0200
                Re: Buffer contents well-defined after fgets() reaches EOF ? scott@slp53.sl.home (Scott Lurndal) - 2025-02-14 15:10 +0000
                Re: Buffer contents well-defined after fgets() reaches EOF ? Michael S <already5chosen@yahoo.com> - 2025-02-14 17:23 +0200
                Re: Buffer contents well-defined after fgets() reaches EOF ? scott@slp53.sl.home (Scott Lurndal) - 2025-02-14 16:46 +0000
                Re: Buffer contents well-defined after fgets() reaches EOF ? Kaz Kylheku <643-408-1753@kylheku.com> - 2025-02-14 17:28 +0000
                Re: Buffer contents well-defined after fgets() reaches EOF ? Kaz Kylheku <643-408-1753@kylheku.com> - 2025-02-14 17:22 +0000
                Re: Buffer contents well-defined after fgets() reaches EOF ? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2025-02-14 11:03 -0800
                Re: Buffer contents well-defined after fgets() reaches EOF ? Kaz Kylheku <643-408-1753@kylheku.com> - 2025-02-14 19:34 +0000
                Re: Buffer contents well-defined after fgets() reaches EOF ? Michael S <already5chosen@yahoo.com> - 2025-02-15 20:06 +0200
                Re: Buffer contents well-defined after fgets() reaches EOF ? scott@slp53.sl.home (Scott Lurndal) - 2025-02-14 20:01 +0000
                Re: Buffer contents well-defined after fgets() reaches EOF ? Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2025-02-14 20:51 +0100
                Re: Buffer contents well-defined after fgets() reaches EOF ? Michael S <already5chosen@yahoo.com> - 2025-02-15 19:02 +0200
                Re: Buffer contents well-defined after fgets() reaches EOF ? Michael S <already5chosen@yahoo.com> - 2025-02-15 19:29 +0200
                Re: Buffer contents well-defined after fgets() reaches EOF ? Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2025-02-16 04:29 +0100
                Re: Buffer contents well-defined after fgets() reaches EOF ? James Kuyper <jameskuyper@alumni.caltech.edu> - 2025-02-16 01:04 -0500
                Re: Buffer contents well-defined after fgets() reaches EOF ? Kaz Kylheku <643-408-1753@kylheku.com> - 2025-02-16 07:37 +0000
                Re: Buffer contents well-defined after fgets() reaches EOF ? Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2025-02-16 18:59 +0100
                Re: Buffer contents well-defined after fgets() reaches EOF ? Michael S <already5chosen@yahoo.com> - 2025-02-16 10:48 +0200
                Re: Buffer contents well-defined after fgets() reaches EOF ? Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2025-02-16 19:14 +0100
                Re: Buffer contents well-defined after fgets() reaches EOF ? Michael S <already5chosen@yahoo.com> - 2025-02-17 11:54 +0200
                Re: Buffer contents well-defined after fgets() reaches EOF ? Kaz Kylheku <643-408-1753@kylheku.com> - 2025-02-16 07:32 +0000
                Re: Buffer contents well-defined after fgets() reaches EOF ? Michael S <already5chosen@yahoo.com> - 2025-02-16 11:05 +0200
                Re: Buffer contents well-defined after fgets() reaches EOF ? Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2025-02-16 19:25 +0100
                Re: Buffer contents well-defined after fgets() reaches EOF ? Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2025-02-16 19:21 +0100
                Re: Buffer contents well-defined after fgets() reaches EOF ? scott@slp53.sl.home (Scott Lurndal) - 2025-02-16 20:26 +0000
                Re: Buffer contents well-defined after fgets() reaches EOF ? Michael S <already5chosen@yahoo.com> - 2025-02-15 19:41 +0200
                Re: Buffer contents well-defined after fgets() reaches EOF ? Michael S <already5chosen@yahoo.com> - 2025-02-15 20:29 +0200
                Re: Buffer contents well-defined after fgets() reaches EOF ? Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2025-02-16 04:33 +0100
                Re: Buffer contents well-defined after fgets() reaches EOF ? Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2025-02-14 20:23 +0100
                Re: Buffer contents well-defined after fgets() reaches EOF ? James Kuyper <jameskuyper@alumni.caltech.edu> - 2025-02-14 14:38 -0500
                Re: Buffer contents well-defined after fgets() reaches EOF ? Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2025-02-14 21:02 +0100
                Re: Buffer contents well-defined after fgets() reaches EOF ? Michael S <already5chosen@yahoo.com> - 2025-02-15 19:53 +0200
                Re: Buffer contents well-defined after fgets() reaches EOF ? Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2025-02-16 04:48 +0100
                Re: Buffer contents well-defined after fgets() reaches EOF ? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2025-02-15 08:37 -0800
                Re: Buffer contents well-defined after fgets() reaches EOF ? Michael S <already5chosen@yahoo.com> - 2025-02-15 20:08 +0200
                Re: Buffer contents well-defined after fgets() reaches EOF ? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2025-02-18 20:17 -0800
                Re: Buffer contents well-defined after fgets() reaches EOF ? Lawrence D'Oliveiro <ldo@nz.invalid> - 2025-02-21 05:58 +0000
            Re: Buffer contents well-defined after fgets() reaches EOF ? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2025-02-15 08:12 -0800
          Re: Buffer contents well-defined after fgets() reaches EOF ? Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2025-02-10 02:44 +0100
          Re: Buffer contents well-defined after fgets() reaches EOF ? Lawrence D'Oliveiro <ldo@nz.invalid> - 2025-02-10 03:28 +0000
            Re: Buffer contents well-defined after fgets() reaches EOF ? Andrey Tarasevich <noone@noone.net> - 2025-02-09 20:11 -0800
              Re: Buffer contents well-defined after fgets() reaches EOF ? Lawrence D'Oliveiro <ldo@nz.invalid> - 2025-02-10 07:21 +0000
                Re: Buffer contents well-defined after fgets() reaches EOF ? scott@slp53.sl.home (Scott Lurndal) - 2025-02-10 16:39 +0000
                Re: Buffer contents well-defined after fgets() reaches EOF ? James Kuyper <jameskuyper@alumni.caltech.edu> - 2025-02-10 13:58 -0500
                Re: Buffer contents well-defined after fgets() reaches EOF ? Lawrence D'Oliveiro <ldo@nz.invalid> - 2025-02-11 01:03 +0000
                Re: Buffer contents well-defined after fgets() reaches EOF ? James Kuyper <jameskuyper@alumni.caltech.edu> - 2025-02-10 22:33 -0500
                Re: Buffer contents well-defined after fgets() reaches EOF ? Kaz Kylheku <643-408-1753@kylheku.com> - 2025-02-11 03:42 +0000
                Re: Buffer contents well-defined after fgets() reaches EOF ? Lawrence D'Oliveiro <ldo@nz.invalid> - 2025-02-11 04:54 +0000
                Re: Buffer contents well-defined after fgets() reaches EOF ? James Kuyper <jameskuyper@alumni.caltech.edu> - 2025-02-11 13:07 -0500
                Re: Buffer contents well-defined after fgets() reaches EOF ? Lawrence D'Oliveiro <ldo@nz.invalid> - 2025-02-11 21:47 +0000
                Re: Buffer contents well-defined after fgets() reaches EOF ? James Kuyper <jameskuyper@alumni.caltech.edu> - 2025-02-11 17:44 -0500
                Re: Buffer contents well-defined after fgets() reaches EOF ? Lawrence D'Oliveiro <ldo@nz.invalid> - 2025-02-12 06:16 +0000
                Re: Buffer contents well-defined after fgets() reaches EOF ? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2025-02-11 13:59 -0800
                Re: Buffer contents well-defined after fgets() reaches EOF ? James Kuyper <jameskuyper@alumni.caltech.edu> - 2025-02-11 17:58 -0500
                Re: Buffer contents well-defined after fgets() reaches EOF ? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2025-02-11 15:40 -0800
                Re: Buffer contents well-defined after fgets() reaches EOF ? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2025-02-17 21:11 -0800
                Re: Buffer contents well-defined after fgets() reaches EOF ? Andrey Tarasevich <noone@noone.net> - 2025-02-10 22:32 -0800
                Re: Buffer contents well-defined after fgets() reaches EOF ? Andrey Tarasevich <noone@noone.net> - 2025-02-10 22:38 -0800
                Re: Buffer contents well-defined after fgets() reaches EOF ? Kaz Kylheku <643-408-1753@kylheku.com> - 2025-02-11 12:04 +0000
                Re: Buffer contents well-defined after fgets() reaches EOF ? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2025-02-13 07:29 -0800
    Re: Buffer contents well-defined after fgets() reaches EOF ? Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2025-02-09 08:13 +0100
      Re: Buffer contents well-defined after fgets() reaches EOF ? Michael S <already5chosen@yahoo.com> - 2025-02-09 12:50 +0200
        Re: Buffer contents well-defined after fgets() reaches EOF ? Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2025-02-09 18:29 +0100
          Re: Buffer contents well-defined after fgets() reaches EOF ? Mark Bourne <nntp.mbourne@spamgourmet.com> - 2025-02-10 21:57 +0000
      Re: Buffer contents well-defined after fgets() reaches EOF ? Andrey Tarasevich <noone@noone.net> - 2025-02-09 07:27 -0800
        Re: Buffer contents well-defined after fgets() reaches EOF ? Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2025-02-09 18:34 +0100
          Re: Buffer contents well-defined after fgets() reaches EOF ? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2025-02-09 16:57 -0800
            Re: Buffer contents well-defined after fgets() reaches EOF ? Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2025-02-10 02:35 +0100
              Re: Buffer contents well-defined after fgets() reaches EOF ? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2025-02-09 20:37 -0800
                Re: Buffer contents well-defined after fgets() reaches EOF ? Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2025-02-10 07:08 +0100
                Re: Buffer contents well-defined after fgets() reaches EOF ? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2025-02-09 22:41 -0800
                Re: Buffer contents well-defined after fgets() reaches EOF ? Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2025-02-10 07:54 +0100
                Re: Buffer contents well-defined after fgets() reaches EOF ? Mark Bourne <nntp.mbourne@spamgourmet.com> - 2025-02-10 23:22 +0000
                Re: Buffer contents well-defined after fgets() reaches EOF ? Kaz Kylheku <643-408-1753@kylheku.com> - 2025-02-11 00:59 +0000
                Re: Buffer contents well-defined after fgets() reaches EOF ? Lawrence D'Oliveiro <ldo@nz.invalid> - 2025-02-17 02:50 +0000
      Re: Buffer contents well-defined after fgets() reaches EOF ? Mark Bourne <nntp.mbourne@spamgourmet.com> - 2025-02-10 22:57 +0000
        Re: Buffer contents well-defined after fgets() reaches EOF ? Mark Bourne <nntp.mbourne@spamgourmet.com> - 2025-02-10 23:24 +0000
  Re: Buffer contents well-defined after fgets() reaches EOF ? Ben Bacarisse <ben@bsb.me.uk> - 2025-02-10 01:32 +0000
    Re: Buffer contents well-defined after fgets() reaches EOF ? Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2025-02-10 02:40 +0100

csiph-web