question about inet_ntoa(3)

[Anthony Roberts]

It returns a char*... but where is the buffer? free() prints a debug
message saying it wasn't allocated when I try to free it. That implies
that there's a buffer somewhere managed by someone else. I'm okay with
that, but it would be a bit weird.

The man pages on OpenBSD 3.4 and MacOS 10.3 don't say anything about
it... and OpenBSD would have a paragraph on the issue if it was a
problem. But I'm still confused, so I'd be a happy man if anyone could
confirm that the buffer isn't my problem.

TIA for any help,
Anthony

[Keith Thompson]

Re: question about inet_ntoa(3)

Anthony Roberts <acrobert-at-ucalgary-dot-ca@ucalgary.ca> writes:[color=blue]
> It returns a char*... but where is the buffer? free() prints a debug
> message saying it wasn't allocated when I try to free it. That implies
> that there's a buffer somewhere managed by someone else. I'm okay with
> that, but it would be a bit weird.
>
> The man pages on OpenBSD 3.4 and MacOS 10.3 don't say anything about
> it... and OpenBSD would have a paragraph on the issue if it was a
> problem. But I'm still confused, so I'd be a happy man if anyone could
> confirm that the buffer isn't my problem.[/color]

The inet_ntoa() function is not defined by the C standard. You'll
probably get better information in comp.unix.progr ammer.

In general, it's fairly common for functions that return strings to
return a pointer to a static buffer; the buffer is overwritten by
subsequent calls. (This can cause difficulties for multi-threaded
programs, but the C standard doesn't concern itself with
multi-threading.) I don't know whether inet_ntoa() does this.

--
Keith Thompson (The_Other_Keit h) kst-u@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <*> <http://users.sdsc.edu/~kst>
Schroedinger does Shakespeare: "To be *and* not to be"

[Anthony Roberts]

Re: question about inet_ntoa(3)

Keith Thompson wrote:[color=blue]
>
> The inet_ntoa() function is not defined by the C standard. You'll
> probably get better information in comp.unix.progr ammer.[/color]

I will try that.
[color=blue]
> In general, it's fairly common for functions that return strings to
> return a pointer to a static buffer; the buffer is overwritten by
> subsequent calls.[/color]

ewwww
[color=blue]
>(This can cause difficulties for multi-threaded
> programs, but the C standard doesn't concern itself with
> multi-threading.) I don't know whether inet_ntoa() does this.[/color]

If that were the case, you'd need to have a mutex to protect it.


Thank you for the fast and helpful response. :)

[Keith Thompson]

Re: question about inet_ntoa(3)

Anthony Roberts <acrobert-at-ucalgary-dot-ca@ucalgary.ca> writes:[color=blue]
> Keith Thompson wrote:[color=green]
> > The inet_ntoa() function is not defined by the C standard. You'll
> > probably get better information in comp.unix.progr ammer.[/color]
>
> I will try that.
>[color=green]
> > In general, it's fairly common for functions that return strings to
> > return a pointer to a static buffer; the buffer is overwritten by
> > subsequent calls.[/color]
>
> ewwww
>[color=green]
> >(This can cause difficulties for multi-threaded
> > programs, but the C standard doesn't concern itself with
> > multi-threading.) I don't know whether inet_ntoa() does this.[/color]
>
> If that were the case, you'd need to have a mutex to protect it.[/color]

It's also common to provide reentrant versions of such functions in
addition to the older non-reentrant versions. The reentrant version
typically has a name ending in "_r", and takes additional arguments
specifying the base address and size of a user-supplied buffer. I
don't know whether there is (or needs to be) such a function for
inet_ntoa(); again comp.unix.progr ammer is the place to ask about it
if your documentation fails you <OT>man inet_ntoa</OT>.

--
Keith Thompson (The_Other_Keit h) kst-u@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <*> <http://users.sdsc.edu/~kst>
Schroedinger does Shakespeare: "To be *and* not to be"