Re: [Bug-readline] [PATCH] Gnuplot on macOS does not suspend by Control-z

[Rin Okuyama]

Thank you for your kind reply!

> Does GNUplot install a handler for SIGTSTP or ignore it? The behavior
> you describe, in the absence of any other changes, sounds like something
> sets the DSUSP character to ^Z.

No, it does not install any signal handlers.

> It must be something specific to GNUplot, since the simple readline test
> programs handle ^Z as expected when run on MacOS.

I examined differences between the sample programs and gnuplot, then
I found that gnuplot specifies its own wrapper for getc(3) to
rl_getc_function:

----
int getc_wrapper(FILE *fp) {
        fd_set fds;
        int fd = fileno(fp);
        int ierr;
        if (external_fd >= 0) {
                do {
                        FD_ZERO(&fds);
                        FD_SET(fd, &fds);
                        FD_SET(external_fd, &fds);
                        ierr = select(external_fd + 1, &fds, 0, 0, NULL);
                        ...
                } while (!FD_ISSET(fd, &fds));
        }
        return getc(fp);
}
----
(This is a simplified version. Please refer to getc_wrapper() in
src/plot.c and X11_waitforinput() in term/x11.trm for more details.)

The reasons why gnuplot does not suspend by Control-z, and does by
any key following Control-z are,

(1) when external_fd >= 0, select(2) cannot catch SIGTSTP, and
(2) when external_fd < 0, getc(3) cannot catch SIGTSTP

Therefore I modified the code as below:

----
int getc_wrapper(FILE *fp) {
        fd_set fds;
        int fd = fileno(fp);
        int ierr;
        if (external_fd >= 0) {
                do {
                        FD_ZERO(&fds);
                        FD_SET(fd, &fds);
                        FD_SET(external_fd, &fds);
#ifdef HAVE_LIBREADLINE
                        extern int _rl_caught_signal;
                        void _rl_signal_handler(int);
                        if (_rl_caught_signal)
                                _rl_signal_handler(_rl_caught_signal);
#endif
                        ierr = select(external_fd + 1, &fds, 0, 0, NULL);
                        ...
                } while (!FD_ISSET(fd, &fds));
        }
#ifdef HAVE_LIBREADLINE
        int rl_getc(FILE*);
        return rl_getc(fp);
#else
        return getc(fp);
#endif
}
----

Then, it successfully suspends regardless of external_fd.

However, as you can see, this is a kind of dirty hack. I wonder whether
there are some portable ways to do like this? I guess that rl_getc() in
the second ifdef block can be replaced by something like this:

----
int (*readline_getc)(FILE*) = NULL;
...
readline_getc = rl_getc_function;
rl_getc_function = getc_wrapper;
...
int gnuplot_getc(FILE *fp) {
        if (readline_getc)
                return readline_getc(fp);
        else
                return getc(fp);
}
----

But, how about the first ifdef block?

Best regards,
Rin Okuyama

On 2017/11/22 4:36, Chet Ramey wrote:
> On 11/21/17 7:53 AM, Rin Okuyama wrote:
> > Hi,
> >
> > Gnuplot on macOS does not suspend by Control-z in its command line.
> > Please find the details below and attached patch.
>
> Does GNUplot install a handler for SIGTSTP or ignore it? The behavior
> you describe, in the absence of any other changes, sounds like something
> sets the DSUSP character to ^Z.
>
> > At the moment, readline does not disable VSUSP, and expect SIGTSTP to
> > be appropriately sent by the tty driver. However, unfortunately, it
> > does not work on macOS.
>
> It must be something specific to GNUplot, since the simple readline test
> programs handle ^Z as expected when run on MacOS.