how to set a watchpoint on an "unsigned short" parameter ?

Good day -

  I'd be most grateful if anyone could enlighten me as to how
  to set a watchpoint on an unsigned short parameter variable
  in lldb .

  I am trying to follow the instructions at :
    https://lldb.llvm.org/lldb-gdb.html
  but they do not work to set watchpoints.

  There seems to be no other documentation for LLDB commands -
  or if anyone knows of any , please let me know.

  I have a function like :
    void f ( unsigned short x )
    { .... }

  With the debugger stopped inside f, I have tried:

   (lldb) p &x
   (uint16_t *) $3 = 0x00000001001122c0
   (lldb) wa s v -s 2 -w write 0x00000001001122c0
   error: no variable named '0x00000001001122c0' found in this frame
   (lldb) wa s v -s 2 -w write x
   error: Watchpoint creation failed (addr=0xffffffffffffffff, size=0,
variable
   expression='x').
   error: cannot set a watchpoint with watch_size of 0
   (lldb) wa s e -s 2 -w write 0x00000001001122c0
   error: expression evaluation of address to watch failed
   expression evaluated: -s 2 -w write 0x00000001001122c0
   (lldb) wa s e -s 2 -w write *0x00000001001122c0
   error: expression evaluation of address to watch failed
   expression evaluated: -s 2 -w write *0x00000001001122c0
   (lldb) wa s e -s 2 -w write ((unsigned short*)0x00000001001122c0)
   error: expression evaluation of address to watch failed
   expression evaluated: -s 2 -w write ((unsigned short*)0x00000001001122c0)
   (lldb) wa s v -s 2 -w write &x
   error: 'x' doesn't have a valid address
   # ^- this error is really strange, particularly as I can do:
   (lldb) p &x
   (uint16_t *) $5 = 0x00000001001122c0

  It seems to me lldb's implementation of watch points is fundamentally broken -
  there is no way I've been able to get it to work .

  Unfortunately, I have to use MacOSX, so gdb is not available.

  Please, can anyone suggest how to successfully set a watchpoint on
  a parameter (stack) located variable value with lldb ?
  It does not seem to be possible.

  My next step, if no answers to this mail, would be to analyse the LLDB
  source code to see if I can figure out how watchpoints are meant to
  be set, seeing as there is no reference documentation for LLDB commands,
  either installed as manual pages or online. This to me makes LLDB unsuitable
  for production use, but unforunately I have to use it (I need to debug under
  MacOSX 10,14.3 ).

   The help output for is of no use either:
   (lldb) help watch set
   "Syntax: watchpoint set <subcommand> [<subcommand-options>]
    The following subcommands are supported:
      expression -- Set a watchpoint on an address by supplying an
expression. Use the
      '-w' option to specify the type of watchpoint and the '-s'
option to specify the
      byte size to watch for.
    "
    The above statement is provably false:
     (lldb) wa s v -s 2 x
     error: Watchpoint creation failed (addr=0xffffffffffffffff, size=0,
               variable expression='x').
     error: cannot set a watchpoint with watch_size of 0
     # maybe the -s option goes after the 'set' ? no:
    (lldb) wa s -s 2 v reader_id
    invalid command 'watchpoint set -s'.

    All attempts to
     "Use the '-w' option to specify the type of watchpoint and the
'-s' option to
      specify the byte size to watch for.
     "
    fail, so there must be alot missing from the help description.

    The help for the variable syntax is also vague, and provably false :
     "variable -- Set a watchpoint on a variable. Use the '-w'
option to specify the type
                     of watchpoint and the '-s' option to specify the
byte size to watch for.
                    If no '-w' option is specified, it defaults to
write. If no '-s' option is
                    specified, it defaults to the variable's byte
size. Note that there are
                    limited hardware resources for watchpoints. If
watchpoint setting
                    fails, consider disable/delete existing ones to
free up resources.

      (lldb) wa s v x
      error: Watchpoint creation failed (addr=0xffffffffffffffff,
size=0, variable
                expression='x').
      error: cannot set a watchpoint with watch_size of 0
      (lldb) wa s -s 2 v x
      invalid command 'watchpoint set -s'.
      (lldb) wa s v -s 2 x
      error: Watchpoint creation failed (addr=0xffffffffffffffff,
size=0, variable
                expression='x').
      error: cannot set a watchpoint with watch_size of 0

   So lldb's watch command can neither get the size from a '-s' option, nor can
   it get the size from sizeof(variable) . How then is one meant to
set the size ?

   Obviously, lldb's watchpoint implementation needs much further work - has
   anyone been able to get it to do anything useful ? If so, please
let me know how,
   and what.

   If there is any reference documentation for lldb, besides the false
and misleading
   'https://lldb.llvm.org/lldb-gdb.html’, which actually describes in detail and
   correctly the syntax of every lldb command, please let me know.

   Any ideas gratefully received.

Thank You & Best Regards,
Jason Vas Dias

"w s v x" would be the command you want.

(lldb) b f
Breakpoint 1: where = watch`f + 12 at watch.c:5:4, address = 0x000050ec
(lldb) r
hexagon-sim INFO: The rev_id used in the simulation is 0x00004060 (v60a_512)
hexagon-sim INFO: Setting up debug server on port 57824
Process 1 launched: '/usr2/tedwood/lldb_test/watch' (hexagon)
Process 1 stopped
* thread #1, name = 'T1', stop reason = breakpoint 1.1
    frame #0: 0x000050ec watch`f(i=2) at watch.c:5:4
   2
   3 unsigned short f(unsigned short i)
   4 {
-> 5 i++;
   6 return i;
   7 }
   8
(lldb) w s v i
Watchpoint created: Watchpoint 1: addr = 0x0410eec6 size = 2 state = enabled type = w
    declare @ '/usr2/tedwood/lldb_test/watch.c:3'
    watchpoint spec = 'i'
    new value: 2
(lldb) c
Process 1 resuming

Watchpoint 1 hit:
old value: 2
new value: 3
Process 1 stopped
* thread #1, name = 'T1', stop reason = watchpoint 1
    frame #0: 0x000050f8 watch`f(i=3) at watch.c:6:10
   3 unsigned short f(unsigned short i)
   4 {
   5 i++;
-> 6 return i;
   7 }
   8
   9 int main(int argc, char **argv)

Good day Ted -

Thanks for responding - but I did try that one:

      (lldb) wa s v x
      error: Watchpoint creation failed (addr=0xffffffffffffffff,
size=0, variable
                expression='x').
      error: cannot set a watchpoint with watch_size of 0
      (lldb) wa s -s 2 v x
      invalid command 'watchpoint set -s'.
      (lldb) wa s v -s 2 x
      error: Watchpoint creation failed (addr=0xffffffffffffffff,
size=0, variable
                expression='x').
      error: cannot set a watchpoint with watch_size of 0

  I can't seem to get lldb to recognize the '-s' / '--size' options no matter
  where I put them . And the documentation, such as it is,
  (on https://lldb.llvm.org/lldb-gdb.html), is very vague and incomplete.

  I guess my problems are because I am compiling with GCC , and trying
  to debug with LLDB .
  But since the program I am debugging is targetted mainly for the
Linux platform,
  (I am just using MacOSX for testing) I wanted to compile with GCC .

  I guess it is not possible to debug GCC compiled programs with LLDB ?

  The problem I am trying to track down is stack corruption caused by
  fprintf() :

        void f ( void *r, unsigned short x )
        { ...
           fprintf(stderr, "some string %lu %c\n",
                    r==(void*)0UL)? 255UL : A_MACRO(r),
                    x? '0' : '1'
                  );
             // after this fprintf, the value of x changes from 12 to 8630 .
             // why ? It would be nice to be able to use LLDB to find out,
            // but this does not work.
        }

     I've just had to comment out the fprintf , so the problem does not occur.

     Coming from a background of using GDB for the past 25 years, I find
     this lack of watchpoint support in LLDB very difficult to accept.

Thanks & Best Regards,
Jason

I don't see anything on this line that would change x:
           fprintf(stderr, "some string %lu %c\n",
                    r==(void*)0UL)? 255UL : A_MACRO(r),
                    x? '0' : '1'
                  );

I suggest you set a breakpoint on the line and a breakpoint on the next line. Verify that x is the wrong value using "frame variable x" when you hit the second breakpoint.
Make sure this breakpoint is on a source line that does something, not in the middle of the function return.

If the value changes, disassemble the line and set a breakpoint on the call instruction for printf. See if the value changes when you hit that breakpoint.
If it does, look at the assembly and see if anything is changing the data at the address of x. You can get the address with "frame variable &x".
If the call to printf is changing the value of x, you've probably got stack corruption going on.