Re: [dwm] input status text bug?

From: Ritesh Kumar <>
Date: Wed, 31 Oct 2007 11:00:46 -0400

On 10/31/07, Anselm R. Garbe <> wrote:
> Hi Ritesh,


> Well let's have a look at the relevant bits in the code:
> (a) switch(r = read(STDIN_FILENO, stext, sizeof stext - 1)) {
> [...]
> default:
> (b) for(stext[r] = '\0', p = stext + strlen(stext) - 1; p >=
> stext && *p == '\n'; *p-- = '\0');
> (c) for(; p >= stext && *p != '\n'; --p);
> (d) if(p > stext)
> (e) strncpy(stext, p + 1, sizeof stext);
> [...]
> (a) Reads the next bunch of stdin, beginning at stext[0] and
> ending at stext[r].
> (b) 0-terminates stext, and trims rightmost '\n' from right to left only.
> (c) If p still does not point to stext, it now seeks for a '\n'
> character leftwards.
> (d) If during (c) such a '\n' character was found, it is the
> begin of the last line which has been read.
> (e) Hence, we copy the last line read only to stext, beginning
> at the '\n' character + 1 to stext. It is 0-terminated already.
> So in my eyes, assumed the complete output of the input script
> is read, the code is correct and does what it is supposed
> to do: it only displays the last line read from stdin (this might
> be a bunch of data which is not '\n'-terminated yet during this read
> round of course).

The problem is that select will notify dwm the moment *anything* is
available on stdin... and that could be partial data.
There is one semantic difference between the code snippets. While the
original one retains the last line of given multiline input, the suggested
snippet shows the first line. However, its hard to think of a situation
where somebody would pour in multiline text (all in a small amount of time)
for the status as dwm would not be able to show all of it anyways regardless
of the semantics.

If you like to debug, you could add an
> fprintf(stderr, "'stext == %s'\n", stext);
> in there between (c) and (d). Then we can make sure that your
> guess is correct.
> If the input is not read during one read call as you believe,
> then at least there must be some flashing, because dwm will
> actually render the <some text> at the status bar, but maybe
> your system is really fast and you don't notice this.

I will do this and post the results.
You are right about my system... its really fast :). Actually, I did see a
flash but it was merely a flicker and happened only sometimes.

Received on Wed Oct 31 2007 - 16:00:48 UTC

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