Scientific Operations



CDB(I)                        2/8/75                       CDB(I)



NAME
     cdb - C debugger

SYNOPSIS
     cdb [ a.out [ core ] ]

DESCRIPTION
     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 ad-
     vance 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
     present.

     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-
     right.

     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 accessi-
     ble in this way.  This is a consequence  of  the  fact  that
     types  of  variables  are  not currently saved in the symbol


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CDB(I)                        2/8/75                       CDB(I)


     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 se-
         quence 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 point-
         er,  and  print  symbolically  where the pointer points.
         The typeout contains the name of an external symbol and,
         if required, the smallest possible positive offset.  On-
         ly external symbols are considered.

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

     $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 in-
             struction 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


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CDB(I)                        2/8/75                       CDB(I)


             down to the first real code.

             It is impossible to set breakpoints  on  pure-proce-
             dure programs (-n flag on cc or ld) because the pro-
             gram text is write-protected.

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

         r   Run  the  program  being  debugged.   Following  the
             ``%r'', arguments may be given; they cannot  specify
             I/O  redirection  (``>'', ``<'') or filters.  No ad-
             dress 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 in-
             struction (including simulated floating point),  bus
             error,  or  interrupt (see signal(II)); it will also
             stop when a breakpoint occurs and in  any  case  an-
             nounce the reason.  Then a stack trace can be print-
             ed, 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.


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

BUGS
     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).












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