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Groups > alt.os.development > #8354

Re: Smaller C

From "James Harris" <james.harris.1@gmail.com>
Newsgroups alt.os.development
Subject Re: Smaller C
Date 2015-07-13 11:10 +0100
Organization Aioe.org NNTP Server
Message-ID <mo02q5$i3g$1@speranza.aioe.org> (permalink)
References (14 earlier) <863a5219-999a-4028-b567-a8e1e282af5d@googlegroups.com> <79c0cfce-38ec-4bcc-bd5f-a25808ffb8ea@googlegroups.com> <39da71f8-7e21-4fcb-84da-263d5c81059f@googlegroups.com> <op.x1nizmxyyfako5@localhost> <mnuak9$f21$1@dont-email.me>

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"Alexei A. Frounze" <alexfrunews@gmail.com> wrote in message
news:d84b0130-4872-4bf4-b92e-2a5dabc1c279@googlegroups.com...
> On Sunday, July 12, 2015 at 11:12:49 AM UTC-7, James Harris wrote:
>> "Rod Pemberton" <...@fasdfrewar.cdm> wrote in message
>> news:op.x1nizmxyyfako5@localhost...
>>
>> ...
>>
>> > Did you know that stdin, stdout, stderr aren't static in Linux?
>> > ...
>> >
>> > FILE *out=stdout ; not valid with GLIBC, move assignment to main()
>> > etc
>>
>> Apparently stdin, stdout and stderr are defined to be macros so it
>> should not be possible to copy them.

That should say copy *to* them.

> There's no guarantee that the three expand to constant expressions and
> are suitable static object initializers.
> However, them being macros does not by itself preclude them from bing
> constant, just like I showed a BSD implementation.

That would be OK in some environments but not others. The text in the
man page I have for, say, stdin (i.e. type "man stdin" to see it) says

"Since the symbols stdin, stdout, and stderr are specified to be macros,
assigning to them is nonportable."

I would have thought that if they were defined that way then gcc would
issue a warning but from testing even with -Wall -Wextra no warning is
issued. There may, however, be another flag that will turn on such a
warning.

As stdin, stdout and stderr are pointers that it should be possible to
change I don't see how they can be constants, at least in a conforming
implementation. There are corresponding file descriptors, however, which
are integers and constants, i.e. STDIN_FILENO, STDOUT_FILENO,
STDERR_FILENO.

>> A man page says that the call
>>
>>   freopen
>>
>> (i.e. "f reopen" rather then "fre open") is provided for such
>> machinations.
>
> Such being which?

Reassigning to the file pointer.

> Reopening a stream (or rather, reassigning it to
> point to another file) has its own issues (e.g. it first closes
> one file, then opens another and you can't do much about closing
> errors or retrieving std* once it's been closed).

Agreed. I think we are *supposed* to use freopen if we want portable
code but you could probably fclose the old one first if you want to
catch errors.

> Using a FILE pointer variable, which you can change, has fewer
> associated problems.

Well, according to the man page that action is not portable.

I have been looking at freopen() and it is a bit of a weird one. The
signature is

  FILE *freopen(const char *path, const char *mode, FILE *stream);

The path and the mode are as in the fopen call but note that freopen has
*two* file pointers.

After some digging I think that the parameter FILE* is the stream that
should be reopened (repointed, if you like). The return FILE* is only
specified to be a FILE pointer

"Upon  successful  completion  fopen(),  fdopen()  and  freopen() return
a FILE pointer"

But it doesn't say what file it will point at. I can only guess that the
freopen call changes the parameter FILE pointer in some way and also
returns it the new value. The return is possibly there just to indicate
success (or failure if NULL).

A valid call, then, might be

  if (freopen("prog.c", "r", stdin)) ....

I still don't understand how that can change stdin unless freopen is a
macro. But it's not important. It is something I may never use.

James

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Thread

Re: Smaller C "Alexei A. Frounze" <alexfrunews@gmail.com> - 2015-07-11 20:39 -0700
  Re: Smaller C "Rod Pemberton" <boo@fasdfrewar.cdm> - 2015-07-12 04:24 -0400
    Re: Smaller C "Alexei A. Frounze" <alexfrunews@gmail.com> - 2015-07-12 03:17 -0700
      Re: Smaller C "Rod Pemberton" <boo@fasdfrewar.cdm> - 2015-07-13 03:28 -0400
    Re: Smaller C "James Harris" <james.harris.1@gmail.com> - 2015-07-12 19:12 +0100
      Re: Smaller C "Alexei A. Frounze" <alexfrunews@gmail.com> - 2015-07-12 17:09 -0700
      Re: Smaller C "James Harris" <james.harris.1@gmail.com> - 2015-07-13 11:10 +0100
        Re: Smaller C "Rod Pemberton" <boo@fasdfrewar.cdm> - 2015-07-13 23:13 -0400
          Re: Smaller C "James Harris" <james.harris.1@gmail.com> - 2015-07-14 09:08 +0100
            Re: Smaller C "Rod Pemberton" <boo@fasdfrewar.cdm> - 2015-07-15 02:23 -0400
  Re: Smaller C "Alexei A. Frounze" <alexfrunews@gmail.com> - 2015-08-15 02:12 -0700
    Re: Smaller C "Alexei A. Frounze" <alexfrunews@gmail.com> - 2015-09-06 15:54 -0700
      Re: Smaller C "Benjamin David Lunt" <zfysz@fysnet.net> - 2015-09-07 12:19 -0700
        Re: Smaller C "Alexei A. Frounze" <alexfrunews@gmail.com> - 2015-09-07 13:30 -0700
          Re: Smaller C "Benjamin David Lunt" <zfysz@fysnet.net> - 2015-09-09 19:38 -0700
            Re: Smaller C "Benjamin David Lunt" <zfysz@fysnet.net> - 2015-09-09 19:49 -0700
            Re: Smaller C "Benjamin David Lunt" <zfysz@fysnet.net> - 2015-09-09 20:58 -0700
            Re: Smaller C "Alexei A. Frounze" <alexfrunews@gmail.com> - 2015-09-10 01:45 -0700
              Re: Smaller C "Benjamin David Lunt" <zfysz@fysnet.net> - 2015-09-10 10:45 -0700
                Re: Smaller C "Alexei A. Frounze" <alexfrunews@gmail.com> - 2015-09-10 23:20 -0700
                Re: Smaller C "wolfgang kern" <nowhere@never.at> - 2015-09-11 09:26 +0200
                Re: Smaller C "wolfgang kern" <nowhere@never.at> - 2015-09-11 09:50 +0200
                Re: Smaller C "Alexei A. Frounze" <alexfrunews@gmail.com> - 2015-09-11 00:58 -0700
                Re: Smaller C "Rod Pemberton" <boo@fasdfrewar.cdm> - 2015-09-11 17:38 -0400
                Re: Smaller C "James Harris" <james.harris.1@gmail.com> - 2015-09-11 23:39 +0100
                Re: Smaller C "Rod Pemberton" <boo@fasdfrewar.cdm> - 2015-09-11 19:39 -0400
                Re: Smaller C "James Harris" <james.harris.1@gmail.com> - 2015-09-12 11:22 +0100
                Re: Smaller C "Alexei A. Frounze" <alexfrunews@gmail.com> - 2015-09-12 02:21 -0700
                Re: Smaller C "wolfgang kern" <nowhere@never.at> - 2015-09-12 21:53 +0200
                Re: Smaller C "Rod Pemberton" <boo@fasdfrewar.cdm> - 2015-09-13 03:37 -0400
                Re: Smaller C "James Harris" <james.harris.1@gmail.com> - 2015-09-13 09:49 +0100
                Re: Smaller C "wolfgang kern" <nowhere@never.at> - 2015-09-13 12:58 +0200
                Re: Smaller C "Rod Pemberton" <boo@fasdfrewar.cdm> - 2015-09-13 07:32 -0400
                Re: Smaller C "James Harris" <james.harris.1@gmail.com> - 2015-09-13 19:05 +0100
                Re: Smaller C "James Harris" <james.harris.1@gmail.com> - 2015-09-14 13:19 +0100
                Re: Smaller C "Rod Pemberton" <boo@fasdfrewar.cdm> - 2015-09-15 00:01 -0400
                Re: Smaller C "James Harris" <james.harris.1@gmail.com> - 2015-09-18 14:48 +0100
                Re: Smaller C "Rod Pemberton" <boo@fasdfrewar.cdm> - 2015-09-26 16:23 -0400
                Re: Smaller C James Harris <james.harris.1@gmail.com> - 2015-09-27 00:23 +0100
                Re: Smaller C "Alexei A. Frounze" <alexfrunews@gmail.com> - 2015-09-26 16:37 -0700
                Re: Smaller C "Rod Pemberton" <boo@fasdfrewar.cdm> - 2015-09-27 10:17 -0400
                Re: Smaller C "Rod Pemberton" <boo@fasdfrewar.cdm> - 2015-09-27 10:24 -0400
                Re: Smaller C "Rod Pemberton" <boo@fasdfrewar.cdm> - 2015-09-27 10:46 -0400
                Re: Smaller C James Harris <james.harris.1@gmail.com> - 2016-01-16 16:14 +0000
                Re: Smaller C Rod Pemberton <NoHaveNotOne@bcczxcfre.cmm> - 2016-01-23 13:20 -0500
                Re: Smaller C James Harris <james.harris.1@gmail.com> - 2016-01-27 13:52 +0000
                Re: Smaller C "Rod Pemberton" <boo@fasdfrewar.cdm> - 2015-09-27 10:28 -0400
                Re: Smaller C James Harris <james.harris.1@gmail.com> - 2016-01-16 16:31 +0000
                Re: Smaller C Rod Pemberton <NoHaveNotOne@bcczxcfre.cmm> - 2016-01-23 13:20 -0500
                Re: Smaller C "James Harris" <james.harris.1@gmail.com> - 2015-09-18 16:21 +0100
                Re: Smaller C "Alexei A. Frounze" <alexfrunews@gmail.com> - 2015-09-20 17:25 -0700
                Re: Smaller C "wolfgang kern" <nowhere@never.at> - 2015-09-13 12:43 +0200
                Re: Smaller C "James Harris" <james.harris.1@gmail.com> - 2015-09-13 09:21 +0100
                Re: Smaller C "wolfgang kern" <nowhere@never.at> - 2015-09-13 13:02 +0200
                Re: Smaller C "James Harris" <james.harris.1@gmail.com> - 2015-09-13 19:09 +0100
                Re: Smaller C "Benjamin David Lunt" <zfysz@fysnet.net> - 2015-09-11 13:32 -0700
                Re: Smaller C "Rod Pemberton" <boo@fasdfrewar.cdm> - 2015-09-11 18:51 -0400
                Re: Smaller C "Benjamin David Lunt" <zfysz@fysnet.net> - 2015-09-11 18:16 -0700
                Re: Smaller C "Alexei A. Frounze" <alexfrunews@gmail.com> - 2015-09-12 02:36 -0700
                Re: Smaller C "James Harris" <james.harris.1@gmail.com> - 2015-09-12 11:56 +0100
                Re: Smaller C "Rod Pemberton" <boo@fasdfrewar.cdm> - 2015-09-13 03:45 -0400
                Re: Smaller C "James Harris" <james.harris.1@gmail.com> - 2015-09-13 10:28 +0100
                Re: Smaller C "Alexei A. Frounze" <alexfrunews@gmail.com> - 2015-09-13 02:53 -0700
                Re: Smaller C "Rod Pemberton" <boo@fasdfrewar.cdm> - 2015-09-13 11:17 -0400
                Re: Smaller C "Alexei A. Frounze" <alexfrunews@gmail.com> - 2015-09-13 16:54 -0700
                Re: Smaller C "Rod Pemberton" <boo@fasdfrewar.cdm> - 2015-09-13 21:39 -0400
                Re: Smaller C "Alexei A. Frounze" <alexfrunews@gmail.com> - 2015-09-13 20:31 -0700
                Re: Smaller C "Rod Pemberton" <boo@fasdfrewar.cdm> - 2015-09-13 09:03 -0400
                Re: Smaller C "James Harris" <james.harris.1@gmail.com> - 2015-09-13 19:48 +0100
                Re: Smaller C "Rod Pemberton" <boo@fasdfrewar.cdm> - 2015-09-13 21:33 -0400
                Re: Smaller C "James Harris" <james.harris.1@gmail.com> - 2015-09-14 23:17 +0100
                Re: Smaller C "Rod Pemberton" <boo@fasdfrewar.cdm> - 2015-09-20 17:37 -0400
                Re: Smaller C "James Harris" <james.harris.1@gmail.com> - 2015-09-20 23:46 +0100
  Re: Smaller C "Benjamin David Lunt" <zfysz@fysnet.net> - 2015-09-11 21:37 -0700
    Re: Smaller C "Benjamin David Lunt" <zfysz@fysnet.net> - 2015-09-11 22:29 -0700
      Re: Smaller C "Alexei A. Frounze" <alexfrunews@gmail.com> - 2015-09-12 03:25 -0700
        Re: Smaller C "Benjamin David Lunt" <zfysz@fysnet.net> - 2015-09-12 11:18 -0700
          Re: Smaller C "Benjamin David Lunt" <zfysz@fysnet.net> - 2015-09-12 11:39 -0700
          Re: Smaller C "Rod Pemberton" <boo@fasdfrewar.cdm> - 2015-09-13 03:47 -0400
            Re: Smaller C "Rod Pemberton" <boo@fasdfrewar.cdm> - 2015-09-13 11:31 -0400
          Re: Smaller C "Alexei A. Frounze" <alexfrunews@gmail.com> - 2015-09-13 02:00 -0700
            Re: Smaller C "Rod Pemberton" <boo@fasdfrewar.cdm> - 2015-09-13 11:31 -0400
            Re: Smaller C "Benjamin David Lunt" <zfysz@fysnet.net> - 2015-09-13 09:15 -0700
    Re: Smaller C "Alexei A. Frounze" <alexfrunews@gmail.com> - 2015-09-12 02:45 -0700
      Re: Smaller C "Benjamin David Lunt" <zfysz@fysnet.net> - 2015-09-12 10:55 -0700
  Re: Smaller C "Benjamin David Lunt" <zfysz@fysnet.net> - 2015-09-15 19:23 -0700
    Re: Smaller C "Alexei A. Frounze" <alexfrunews@gmail.com> - 2015-09-16 03:03 -0700
      Re: Smaller C "Benjamin David Lunt" <zfysz@fysnet.net> - 2015-09-16 10:17 -0700

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