NetNews Usenet Archive 1992 #19/NN_1992_19.iso/spool/comp/sys/hp/10035

Internet Message Format | 1992-09-03 | 4.5 KB

From: [e-mail] (John Newman)
Date: Thu, 3 Sep 1992 23:17:43 GMT
Subject: Re: attaching xdb to a running process...
Message-ID: <[e-mail]>
Organization: Hewlett-Packard, Fort Collins, CO, USA
Path: sparky!uunet!usc!sdd.hp.com!hp-cv!hp-pcd!hpfcso!jmn
Newsgroups: comp.sys.hp
References: <[e-mail]>
Lines: 182


> I suspect that I just dont know enough about xdb to see what it needs to
> do to get a process started, but the only manual Ive seen is a really good
> users manual,

Thanks.

> and this is a little beyond that.
>
> If anyone knows more about xdb and what signals it uses to start/stop a
> process once it has gotten it with ptrace, please let me know.

Xdb uses ptrace, not a signal, to (re)start process.  The only signal
xdb uses itself is SIGTRAP.

---

I played around with your example program, and got it to work by doing
this:

    1. put in an short signal handler for SIGUSR1 (any innocuous signal
       will do here).

    2. Set up the handler in exec_debugger(), before the fork.

I needed the handler in order to get back out of the pause().
But, here's the key:

    3. Once the debugger is attached, I used the following commands:

        >b
            This sets a breakpoint at the return addr from
            the call to pause().  This gives control back to
            the debugger (user) when the pause() returns.

        >p $signal = 16
        >C
            This passes the SIGUSR1 to the process and completes
            the pause().

       If you use 's' instead of 'C' here, you end up at the 1st stmt
       in the signal handler.

Check out the discussion of $signal and Continue (capital 'C') in TFM.

You can avoid manually doing this every time with a playback file.
Put the above 3 commands into a file, and list it with -p in your
call to exec:

    execlp( ... , "-p", "playback", proc, NULL);

NOTE:  Using the 'C' with a breakpoint works great on PA-RISC if archive
libs are used (compiled with -Wl,-a,archive on HP-UX 8.0x).  If a shared libc
(libc.sl) is used, then you'll need to give an explicit location to the
'b' command.  This is a limitation having to do with shared-libs,
stack unwinding, and attaching while in the kernel.  I'll leave the
grody details out of this discussion.

Using the 's' to step into the signal-handler works okay regardless.

BTW, Chan's suggestion to use "while(i) ;" will work find too. Just
put something like this into the playback:  p i=0; s

         ___
        /  /        John Newman
HEWLETT/hp/PACKARD  Colorado Language Lab
      /__/          [e-mail]

--------- --------- --------- --------- --------- --------- --------- -----
This response does not represent the official position of, or statement by,
the Hewlett-Packard Company.  The above data is provided for informational
purposes only.  It is supplied without warranty of any kind.

#---------------------------------- cut here ------------------
# This is a shell archive.  Remove anything before this line,
# then unpack it by saving it in a file and typing "sh file".
#
# This archive contains:
#    prog.c        playback
#

LANG=""; export LANG
PATH=/bin:/usr/bin:$PATH; export PATH

echo x - prog.c
cat >prog.c <<'@EOF'

/*
 * Compile with: cc -g -Aa -Wl,-a,archive
 */
#define _POSIX_SOURCE
#include <sys/stdsyms.h>
#include <sys/signal.h>
#include <stdio.h>

main(int argc, char *argv[])
{

    int pid, pid1, pid2, i;
    int exitcode;

    printf("in main program\n");

    pid = fork();

    if (pid != 0) {        /* parent */
        waitpid(pid, &exitcode, 0);
        exit(0);
    }

    /*
     * here is the child
     */

    printf(" in the child\n");

    pid1 = fork();

    if (pid1 != 0) {    /* parent */
        pid2 = getpid();
        printf(" in the first child (debugger parent) %d\n", pid2);

        exec_debugger(argv[0], pid2);

        pause();

        printf("starting the loop\n");
        for (i=1; i < 100; i++) {
            printf("I is %d\n",i);
        }
        exit(1);
    }
}

void catch (int sig, unsigned long code, struct sigcontext *scp)
{
    if (sig == SIGUSR1)
        (void) signal (SIGUSR1,SIG_IGN);

    return ;
}

exec_debugger(char *proc, int pid)
{
    char pidstr[20];
    char *display;
    extern char *getenv(char *);

    if ((display = getenv("DISPLAY")) == (char *)NULL || !*display) {
        display = "local:0";
    }

    (void) signal(SIGUSR1,(void (*) ())catch);

    sprintf(pidstr,"%d",pid);
    printf("starting the debugger %s\n", pidstr);

    if (fork() == 0) {
        execlp("/usr/bin/X11/hpterm", "/usr/bin/X11/hpterm", "-display",
            display, "-e", "/usr/bin/xdb", "-P", pidstr,
#ifdef USE_PLAYBK
            "-p", "playback",
#endif
            proc, NULL);
        exit(2);
    }
}

@EOF

chmod 666 prog.c

echo x - playback
cat >playback <<'@EOF'
b
p $signal = 16
C
@EOF

chmod 666 playback

exit 0