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Date: Fri, 04 Apr 2008 02:21:00 -0000
From: Antony KING <antony.king@st.com>
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To: gdb@sourceware.org
Subject: Problem setting registers if stack point or frame pointer is 0
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Hi,

I have a problem trying to set a CPU register (using the GDB convenience 
variable mechanism) if the stack pointer (SP) or frame pointer (FP) CPU 
registers are 0. For example on an SH-4 device, where the FP register is 
R14 and the SP register is R15, I see the following error from GDB (6.7.1):

(gdb) <commands to connect to SH-4 target>
(gdb) set $r15 = 0
(gdb) set $pc = 0xa0000000
Value being assigned to is no longer active.
(gdb)

The error is being generated by value_assign() (valops.c) in the 
following code sequence:

      case lval_register:
        {
          struct frame_info *frame;
          int value_reg;

          /* Figure out which frame this is in currently.  */
          frame = frame_find_by_id (VALUE_FRAME_ID (toval));
          value_reg = VALUE_REGNUM (toval);

          if (!frame)
            error (_("Value being assigned to is no longer active."));

I can make the problem "go away" if I insert the following after the 
call to frame_find_by_id() in the above code:

          if (!frame)
            frame = get_current_frame();

This solution was "suggestion" by a source comment in the implementation 
of frame_find_by_id(). Is this the right solution do you think ?

(I am not familiar with the frame management of GDB so I thought I 
would ask those more in the know about this aspect of GDB internals :-).

Cheers,

Antony.