Debugging with GDB: Sample Session

Next: Invocation, Previous: Summary, Up: Top [Contents][Index]

## 1 A Sample GDB Session

You can use this manual at your leisure to read all about GDB. However, a handful of commands are enough to get started using the debugger. This chapter illustrates those commands.

One of the preliminary versions of GNU `m4` (a generic macro processor) exhibits the following bug: sometimes, when we change its quote strings from the default, the commands used to capture one macro definition within another stop working. In the following short `m4` session, we define a macro `foo` which expands to `0000`; we then use the `m4` built-in `defn` to define `bar` as the same thing. However, when we change the open quote string to `<QUOTE>` and the close quote string to `<UNQUOTE>`, the same procedure fails to define a new synonym `baz`:

    $ cd gnu/m4
    $ ./m4
    define(foo,0000)

    foo
    0000
    define(bar,defn(‘foo’))

    bar
    0000
    changequote(<QUOTE>,<UNQUOTE>)

    define(baz,defn(<QUOTE>foo<UNQUOTE>))
    baz
    Ctrl-d
    m4: End of input: 0: fatal error: EOF in string

Let us use GDB to try to see what is going on.

    $ gdb m4
    GDB is free software and you are welcome to distribute copies
     of it under certain conditions; type "show copying" to see
     the conditions.
    There is absolutely no warranty for GDB; type "show warranty"
     for details.

    GDB 8.0.1.20171114-git, Copyright 1999 Free Software Foundation, Inc...
    (gdb)

GDB reads only enough symbol data to know where to find the rest when needed; as a result, the first prompt comes up very quickly. We now tell GDB to use a narrower display width than usual, so that examples fit in this manual.

    (gdb) set width 70

We need to see how the `m4` built-in `changequote` works. Having looked at the source, we know the relevant subroutine is `m4_changequote`, so we set a breakpoint there with the GDB `break` command.

    (gdb) break m4_changequote
    Breakpoint 1 at 0x62f4: file builtin.c, line 879.

Using the `run` command, we start `m4` running under GDB control; as long as control does not reach the `m4_changequote` subroutine, the program runs as usual:

    (gdb) run
    Starting program: /work/Editorial/gdb/gnu/m4/m4
    define(foo,0000)

    foo
    0000

To trigger the breakpoint, we call `changequote`. GDB suspends execution of `m4`, displaying information about the context where it stops.

    changequote(<QUOTE>,<UNQUOTE>)

    Breakpoint 1, m4_changequote (argc=3, argv=0x33c70)
        at builtin.c:879
    879         if (bad_argc(TOKEN_DATA_TEXT(argv[0]),argc,1,3))

Now we use the command `n` (`next`) to advance execution to the next line of the current function.

    (gdb) n
    882         set_quotes((argc >= 2) ? TOKEN_DATA_TEXT(argv[1])\
     : nil,

`set_quotes` looks like a promising subroutine. We can go into it by using the command `s` (`step`) instead of `next`. `step` goes to the next line to be executed in any subroutine, so it steps into `set_quotes`.

    (gdb) s
    set_quotes (lq=0x34c78 "<QUOTE>", rq=0x34c88 "<UNQUOTE>")
        at input.c:530
    530         if (lquote != def_lquote)

The display that shows the subroutine where `m4` is now suspended (and its arguments) is called a stack frame display. It shows a summary of the stack. We can use the `backtrace` command (which can also be spelled `bt`), to see where we are in the stack as a whole: the `backtrace` command displays a stack frame for each active subroutine.

    (gdb) bt
    #0  set_quotes (lq=0x34c78 "<QUOTE>", rq=0x34c88 "<UNQUOTE>")
        at input.c:530
    #1  0x6344 in m4_changequote (argc=3, argv=0x33c70)
        at builtin.c:882
    #2  0x8174 in expand_macro (sym=0x33320) at macro.c:242
    #3  0x7a88 in expand_token (obs=0x0, t=209696, td=0xf7fffa30)
        at macro.c:71
    #4  0x79dc in expand_input () at macro.c:40
    #5  0x2930 in main (argc=0, argv=0xf7fffb20) at m4.c:195

We step through a few more lines to see what happens. The first two times, we can use ‘`s`’; the next two times we use `n` to avoid falling into the `xstrdup` subroutine.

    (gdb) s
    0x3b5c  532         if (rquote != def_rquote)
    (gdb) s
    0x3b80  535         lquote = (lq == nil || *lq == '\0') ?  \
    def_lquote : xstrdup(lq);
    (gdb) n
    536         rquote = (rq == nil || *rq == '\0') ? def_rquote\
     : xstrdup(rq);
    (gdb) n
    538         len_lquote = strlen(rquote);

The last line displayed looks a little odd; we can examine the variables `lquote` and `rquote` to see if they are in fact the new left and right quotes we specified. We use the command `p` (`print`) to see their values.

    (gdb) p lquote
    $1 = 0x35d40 "<QUOTE>"
    (gdb) p rquote
    $2 = 0x35d50 "<UNQUOTE>"

`lquote` and `rquote` are indeed the new left and right quotes. To look at some context, we can display ten lines of source surrounding the current line with the `l` (`list`) command.

    (gdb) l
    533             xfree(rquote);
    534
    535         lquote = (lq == nil || *lq == '\0') ? def_lquote\
     : xstrdup (lq);
    536         rquote = (rq == nil || *rq == '\0') ? def_rquote\
     : xstrdup (rq);
    537
    538         len_lquote = strlen(rquote);
    539         len_rquote = strlen(lquote);
    540     }
    541
    542     void

Let us step past the two lines that set `len_lquote` and `len_rquote`, and then examine the values of those variables.

    (gdb) n
    539         len_rquote = strlen(lquote);
    (gdb) n
    540     }
    (gdb) p len_lquote
    $3 = 9
    (gdb) p len_rquote
    $4 = 7

That certainly looks wrong, assuming `len_lquote` and `len_rquote` are meant to be the lengths of `lquote` and `rquote` respectively. We can set them to better values using the `p` command, since it can print the value of any expression—and that expression can include subroutine calls and assignments.

    (gdb) p len_lquote=strlen(lquote)
    $5 = 7
    (gdb) p len_rquote=strlen(rquote)
    $6 = 9

Is that enough to fix the problem of using the new quotes with the `m4` built-in `defn`? We can allow `m4` to continue executing with the `c` (`continue`) command, and then try the example that caused trouble initially:

    (gdb) c
    Continuing.

    define(baz,defn(<QUOTE>foo<UNQUOTE>))

    baz
    0000

Success! The new quotes now work just as well as the default ones. The problem seems to have been just the two typos defining the wrong lengths. We allow `m4` exit by giving it an EOF as input:

    Ctrl-d
    Program exited normally.

The message ‘`Program exited normally.`’ is from GDB; it indicates `m4` has finished executing. We can end our GDB session with the GDB `quit` command.

    (gdb) quit

Next: Invocation, Previous: Summary, Up: Top [Contents][Index]