Chapter 3.A Sample gdb SessionYou can use this manual at your leisure to read all about gdb. However, a handful of commands areenough 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 onemacro definition within another stop working. In the following short m4 session, we define a macrofoo which expands to 0000; we then use the m4 built-in defn to define bar as the same thing. How-ever, 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$ ./m4define(foo,0000)foo0000define(bar,defn(‘foo’))bar0000changequote( QUOTE , UNQUOTE )define(baz,defn( QUOTE foo UNQUOTE ))bazC-dm4: End of input: 0: fatal error: EOF in stringLet us use gdb to try to see what is going on.$ gdb m4gdb is free software and you are welcome to distribute copiesof it under certain conditions; type "show copying" to seethe conditions.There is absolutely no warranty for gdb; type "show warranty"for details.gdb 2003-07-22-cvs, 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 firstprompt comes up very quickly. We now tell gdb to use a narrower display width than usual, so thatexamples fit in this manual.(gdb) set width 70We need to see how the m4 built-in changequote works. Having looked at the source, we know therelevant subroutine is m4_changequote, so we set a breakpoint there with the gdb break command.
