Re: ptrace/select/signal errno weirdness

[Brian Wellington]

On 15 Aug 2002, Linus Torvalds wrote:

> In article <[e-mail]>,
> Brian Wellington  <[e-mail]> wrote:
> >When sending a SIGINT to a ptraced process (run under gdb), an interrupted 
> >select() call returns with errno==514.  linux/include/linux/errno.h says:
> >
> >/* Should never be seen by user programs */
> >#define ERESTARTSYS     512
> >#define ERESTARTNOINTR  513
> >#define ERESTARTNOHAND  514     /* restart if no handler.. */
> >#define ENOIOCTLCMD     515     /* No ioctl command */
> >
> >As gdb is a user program, and the printf is printing it, there's something
> >wrong.
> 
> No, there's nothing wrong.
> 
> The process _itself_ never sees these magic error numbers, because they
> are internal to the kernel, and the only time they are seen is by a
> tracer that sees them - at the same time as the kernel backed up the
> instruction pointer so that the traced process will not actually return
> from the system call, it will re-do the system call.

If that's the case, then how does
	fprintf(stderr, "select: %s\n", strerror(errno));
print
	select: Unknown error 514
?

That's the traced process printing the error, not the tracing process.

Brian