SchOS/cmd/schdbg/main.c

// SPDX-License-Identifier: GPL-2.0-or-later
//
// schdbg -- ask the kernel a question.
//
// docs/architecture/debugging.md, milestone D2. The kernel half is /dev/dbg: write a
// line, read the answer. This is the half that makes that pleasant, and it is
// deliberately almost nothing -- because the protocol is lines of text, and a
// protocol whose client has to be clever is a protocol you cannot use when the
// client will not start.
//
//     schdbg              the views this kernel publishes
//     schdbg dma          one of them
//     schdbg all          all of them
//     schdbg -i           a prompt, for when you are asking several
//
// Everything here works equally well as:
//
//     echo dma > /dev/dbg && cat /dev/dbg
//
// which is the point, and is what you fall back to on a system where this
// program is one of the things that is broken.

#include "types.h"
#include "user.h"
#include "fcntl.h"

static void
puts_(const char *s)
{
    int n = 0;
    while (s[n]) n++;
    write(1, (void *)s, n);
}

// One question, one answer.
//
// The descriptor is opened fresh for each, because the answer is read from
// offset zero and the kernel replaces it on every write. Reusing one open
// descriptor across two questions would read the second answer from wherever
// the first one left off.
static int
ask(const char *cmd)
{
    char buf[512];
    int  fd, n;

    fd = open("/dev/dbg", O_RDWR);
    if (fd < 0) {
        puts_("schdbg: cannot open /dev/dbg\n");
        return -1;
    }

    if (cmd) {
        int len = 0;
        while (cmd[len]) len++;
        if (write(fd, (void *)cmd, len) != len) {
            puts_("schdbg: the kernel would not take that command\n");
            close(fd);
            return -1;
        }
        // Back to the start. Writing moved this descriptor's offset forward by
        // the length of the command, and the answer begins at zero -- so
        // without this the reply arrives with its first few characters missing,
        // which is subtle enough to look like a formatting bug in the view.
        lseek(fd, 0, 0);            /* SEEK_SET */
    }

    while ((n = read(fd, buf, sizeof(buf))) > 0)
        write(1, buf, n);

    close(fd);
    return 0;
}

// A line from the terminal, without the newline. Returns 0 at end of input.
static int
readline(char *buf, int cap)
{
    int n = 0;
    char c;

    for (;;) {
        int r = read(0, &c, 1);
        if (r <= 0)
            return n > 0;
        if (c == '\n') {
            buf[n] = 0;
            return 1;
        }
        if (n < cap - 1)
            buf[n++] = c;
    }
}

int
main(int argc, char *argv[])
{
    if (argc > 1 && argv[1][0] == '-' && argv[1][1] == 'i') {
        char line[128];

        puts_("schdbg: ask the kernel something. 'help' lists the views, "
              "'quit' leaves.\n");
        for (;;) {
            puts_("dbg> ");
            if (!readline(line, sizeof(line)))
                break;
            if (line[0] == 0)
                continue;
            if (line[0] == 'q' && line[1] == 'u' && line[2] == 'i' &&
                line[3] == 't' && line[4] == 0)
                break;
            ask(line);
        }
        return 0;
    }

    // No argument means "what can I ask", which is the question someone has
    // the first time and the one an empty response would not answer.
    if (argc < 2)
        return ask("help") < 0 ? 1 : 0;

    // Everything after the program name, rejoined with single spaces. The
    // shell split it and the kernel wants a line; `schdbg vma 4` has to arrive
    // as "vma 4" or the argument is silently dropped and the view quietly
    // ignores the narrowing it was asked for.
    {
        char line[128];
        int  n = 0;

        for (int i = 1; i < argc; i++) {
            if (i > 1 && n < (int)sizeof(line) - 1)
                line[n++] = ' ';
            for (int j = 0; argv[i][j] && n < (int)sizeof(line) - 1; j++)
                line[n++] = argv[i][j];
        }
        line[n] = 0;
        return ask(line) < 0 ? 1 : 0;
    }
}