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     cdb - C debugger

     cdb [ a.out [ core ] ]

     Cdb is a debugger for use with C programs.  It is useful for
     both post-mortem and interactive debugging.  An important
     feature of cdb is that even in the interactive case no
     advance planning is necessary to use it; in particular it is
     not necessary to compile or load the program in any special
     way nor to include any special routines in the object file.

     The first argument to cdb is an object program, preferably
     containing a symbol table; if not given ``a.out'' is used.
     The second argument is the name of a core-image file; if it
     is not given, ``core'' is used.  The core file need not be

     Commands to cdb consist of an address, followed by a single
     command character, possibly followed by a command modifier.
     Usually if no address is given the last-printed address is
     used.  An address may be followed by a comma and a number,
     in which case the command applies to the appropriate number
     of successive addresses.

     Addresses are expressions composed of names, decimal num-
     bers, and octal numbers (which begin with ``0'') and sepa-
     rated by ``+'' and ``-''.  Evaluation proceeds left-to-

     Names of external variables are written just as they are in
     C.  For various reasons the external names generated by C
     all begin with an underscore, which is automatically tacked
     on by cdb.  Currently it is not possible to suppress this
     feature, so symbols (defined in assembly-language programs)
     which do not begin with underscore are inaccessible.

     Variables local to a function (automatic, static, and argu-
     ments) are accessible by writing the name of the function, a
     colon ``:'', and the name of the local variable (e.g.
     ``main:argc'').  There is no notion of the ``current'' func-
     tion; its name must always be written explicitly.

     A number which begins with ``0'' is taken to be octal; oth-
     erwise numbers are decimal, just as in C.  There is no pro-
     vision for input of floating numbers.

     The construction ``name[expression]'' assumes that name is a
     pointer to an integer and is equivalent to the contents of
     the named cell plus twice the expression.  Notice that name
     has to be a genuine pointer and that arrays are not


     accessible in this way.  This is a consequence of the fact
     that types of variables are not currently saved in the sym-
     bol table.

     The command characters are:

     /m  print the addressed words.  m indicates the mode of
         printout; specifying a mode sets the mode until it is
         explicitly changed again:
         o   octal (default)
         i   decimal
         f   single-precision floating-point
         d   double-precision floating-point

     \   Print the specified bytes in octal.

     =   print the value of the addressed expression in octal.

     '   print the addressed bytes as characters.  Control and
         non-ASCII characters are escaped in octal.

     "   take the contents of the address as a pointer to a
         sequence of characters, and print the characters up to a
         null byte.  Control and non-ASCII characters are escaped
         as octal.

     &       Try to interpret the contents of the address as a
         pointer, and print symbolically where the pointer
         points.  The typeout contains the name of an external
         symbol and, if required, the smallest possible positive
         offset.  Only external symbols are considered.

     ?   Interpret the addressed location as a PDP-11 instruc-

     $m  If no m is given, print a stack trace of the terminated
         or stopped program.  The last call made is listed first;
         the actual arguments to each routine are given in octal.
         (If this is inappropriate, the arguments may be examined
         by name in the desired format using ``/''.)  If m is
         ``r'', the contents of the PDP-11 general registers are
         listed.  If m is ``f'', the contents of the floating-
         point registers are listed.  In all cases, the reason
         why the program stopped or terminated is indicated.

     %m  According to m, set or delete a breakpoint, or run or
         continue the program:

         b   An address within the program must be given; a
             breakpoint is set there.  Ordinarily, breakpoints
             will be set on the entry points of functions, but
             any location is possible as long as it is the first
             word of an instruction.  (Labels don't appear in the
             symbol table yet.)  Stopping at the actual first


             instruction of a function is undesirable because to
             make symbolic printouts work, the function's save
             sequence has to be completed; therefore cdb automat-
             ically moves breakpoints at the start of functions
             down to the first real code.

             It is impossible to set breakpoints on pure-
             procedure programs (-n flag on cc or ld) because the
             program text is write-protected.

         d   An address must be given; the breakpoint at that
             address is removed.

         r   Run the program being debugged.  Following the
             ``%r'', arguments may be given; they cannot specify
             I/O redirection (``>'', ``<'') or filters.    No
             address is permissible, and the program is restarted
             from scratch, not continued.  Breakpoints should
             have been set if any were desired.  The program will
             stop if any signal is generated, such as illegal
             instruction (including simulated floating point),
             bus error, or interrupt (see signal(II)); it will
             also stop when a breakpoint occurs and in any case
             announce the reason.  Then a stack trace can be
             printed, named locations examined, etc.

         c   Continue after a breakpoint.  It is possible but
             probably useless to continue after an error since
             there is no way to repair the cause of the error.

     cc (I), db (I), C Reference Manual

     Use caution in believing values of register variables at the
     lowest levels of the call stack; the value of a register is
     found by looking at the place where it was supposed to have
     been saved by the callee.

     Some things are still needed to make cdb uniformly better
     than db: non-C symbols, patching files, patching core images
     of programs being run.  It would be desirable to have the
     types of variables around to make the correct style printout
     more automatic.  Structure members should be available.

     Naturally, there are all sorts of neat features not handled,
     like conditional breakpoints, optional stopping on certain
     signals (like illegal instructions, to allow breakpointing
     of simulated floating-point programs).