gdbint.texinfo

这个是LINUX下的GDB调度工具的源码

@cindex @code{longjmp} debugging
@value{GDBN} has support for figuring out that the target is doing a
@code{longjmp} and for stopping at the target of the jump, if we are
stepping.  This is done with a few specialized internal breakpoints,
which are visible in the output of the @samp{maint info breakpoint}
command.

@findex GET_LONGJMP_TARGET
To make this work, you need to define a macro called
@code{GET_LONGJMP_TARGET}, which will examine the @code{jmp_buf}
structure and extract the longjmp target address.  Since @code{jmp_buf}
is target specific, you will need to define it in the appropriate
@file{tm-@var{target}.h} file.  Look in @file{tm-sun4os4.h} and
@file{sparc-tdep.c} for examples of how to do this.

@section Watchpoints
@cindex watchpoints

Watchpoints are a special kind of breakpoints (@pxref{Algorithms,
breakpoints}) which break when data is accessed rather than when some
instruction is executed.  When you have data which changes without
your knowing what code does that, watchpoints are the silver bullet to
hunt down and kill such bugs.

@cindex hardware watchpoints
@cindex software watchpoints
Watchpoints can be either hardware-assisted or not; the latter type is
known as ``software watchpoints.''  @value{GDBN} always uses
hardware-assisted watchpoints if they are available, and falls back on
software watchpoints otherwise.  Typical situations where @value{GDBN}
will use software watchpoints are:

@itemize @bullet
@item
The watched memory region is too large for the underlying hardware
watchpoint support.  For example, each x86 debug register can watch up
to 4 bytes of memory, so trying to watch data structures whose size is
more than 16 bytes will cause @value{GDBN} to use software
watchpoints.

@item
The value of the expression to be watched depends on data held in
registers (as opposed to memory).

@item
Too many different watchpoints requested.  (On some architectures,
this situation is impossible to detect until the debugged program is
resumed.)  Note that x86 debug registers are used both for hardware
breakpoints and for watchpoints, so setting too many hardware
breakpoints might cause watchpoint insertion to fail.

@item
No hardware-assisted watchpoints provided by the target
implementation.
@end itemize

Software watchpoints are very slow, since @value{GDBN} needs to
single-step the program being debugged and test the value of the
watched expression(s) after each instruction.  The rest of this
section is mostly irrelevant for software watchpoints.

@value{GDBN} uses several macros and primitives to support hardware
watchpoints:

@table @code
@findex TARGET_HAS_HARDWARE_WATCHPOINTS
@item TARGET_HAS_HARDWARE_WATCHPOINTS
If defined, the target supports hardware watchpoints.

@findex TARGET_CAN_USE_HARDWARE_WATCHPOINT
@item TARGET_CAN_USE_HARDWARE_WATCHPOINT (@var{type}, @var{count}, @var{other})
Return the number of hardware watchpoints of type @var{type} that are
possible to be set.  The value is positive if @var{count} watchpoints
of this type can be set, zero if setting watchpoints of this type is
not supported, and negative if @var{count} is more than the maximum
number of watchpoints of type @var{type} that can be set.  @var{other}
is non-zero if other types of watchpoints are currently enabled (there
are architectures which cannot set watchpoints of different types at
the same time).

@findex TARGET_REGION_OK_FOR_HW_WATCHPOINT
@item TARGET_REGION_OK_FOR_HW_WATCHPOINT (@var{addr}, @var{len})
Return non-zero if hardware watchpoints can be used to watch a region
whose address is @var{addr} and whose length in bytes is @var{len}.

@findex TARGET_REGION_SIZE_OK_FOR_HW_WATCHPOINT
@item TARGET_REGION_SIZE_OK_FOR_HW_WATCHPOINT (@var{size})
Return non-zero if hardware watchpoints can be used to watch a region
whose size is @var{size}.  @value{GDBN} only uses this macro as a
fall-back, in case @code{TARGET_REGION_OK_FOR_HW_WATCHPOINT} is not
defined.

@findex TARGET_DISABLE_HW_WATCHPOINTS
@item TARGET_DISABLE_HW_WATCHPOINTS (@var{pid})
Disables watchpoints in the process identified by @var{pid}.  This is
used, e.g., on HP-UX which provides operations to disable and enable
the page-level memory protection that implements hardware watchpoints
on that platform.

@findex TARGET_ENABLE_HW_WATCHPOINTS
@item TARGET_ENABLE_HW_WATCHPOINTS (@var{pid})
Enables watchpoints in the process identified by @var{pid}.  This is
used, e.g., on HP-UX which provides operations to disable and enable
the page-level memory protection that implements hardware watchpoints
on that platform.

@findex target_insert_watchpoint
@findex target_remove_watchpoint
@item target_insert_watchpoint (@var{addr}, @var{len}, @var{type})
@itemx target_remove_watchpoint (@var{addr}, @var{len}, @var{type})
Insert or remove a hardware watchpoint starting at @var{addr}, for
@var{len} bytes.  @var{type} is the watchpoint type, one of the
possible values of the enumerated data type @code{target_hw_bp_type},
defined by @file{breakpoint.h} as follows:

@smallexample
 enum target_hw_bp_type
   @{
     hw_write   = 0, /* Common (write) HW watchpoint */
     hw_read    = 1, /* Read    HW watchpoint */
     hw_access  = 2, /* Access (read or write) HW watchpoint */
     hw_execute = 3  /* Execute HW breakpoint */
   @};
@end smallexample

@noindent
These two macros should return 0 for success, non-zero for failure.

@cindex insert or remove hardware breakpoint
@findex target_remove_hw_breakpoint
@findex target_insert_hw_breakpoint
@item target_remove_hw_breakpoint (@var{addr}, @var{shadow})
@itemx target_insert_hw_breakpoint (@var{addr}, @var{shadow})
Insert or remove a hardware-assisted breakpoint at address @var{addr}.
Returns zero for success, non-zero for failure.  @var{shadow} is the
real contents of the byte where the breakpoint has been inserted; it
is generally not valid when hardware breakpoints are used, but since
no other code touches these values, the implementations of the above
two macros can use them for their internal purposes.

@findex target_stopped_data_address
@item target_stopped_data_address (@var{addr_p})
If the inferior has some watchpoint that triggered, place the address
associated with the watchpoint at the location pointed to by
@var{addr_p} and return non-zero.  Otherwise, return zero.

@findex HAVE_STEPPABLE_WATCHPOINT
@item HAVE_STEPPABLE_WATCHPOINT
If defined to a non-zero value, it is not necessary to disable a
watchpoint to step over it.

@findex HAVE_NONSTEPPABLE_WATCHPOINT
@item HAVE_NONSTEPPABLE_WATCHPOINT
If defined to a non-zero value, @value{GDBN} should disable a
watchpoint to step the inferior over it.

@findex HAVE_CONTINUABLE_WATCHPOINT
@item HAVE_CONTINUABLE_WATCHPOINT
If defined to a non-zero value, it is possible to continue the
inferior after a watchpoint has been hit.

@findex CANNOT_STEP_HW_WATCHPOINTS
@item CANNOT_STEP_HW_WATCHPOINTS
If this is defined to a non-zero value, @value{GDBN} will remove all
watchpoints before stepping the inferior.

@findex STOPPED_BY_WATCHPOINT
@item STOPPED_BY_WATCHPOINT (@var{wait_status})
Return non-zero if stopped by a watchpoint.  @var{wait_status} is of
the type @code{struct target_waitstatus}, defined by @file{target.h}.
@end table

@subsection x86 Watchpoints
@cindex x86 debug registers
@cindex watchpoints, on x86

The 32-bit Intel x86 (a.k.a.@: ia32) processors feature special debug
registers designed to facilitate debugging.  @value{GDBN} provides a
generic library of functions that x86-based ports can use to implement
support for watchpoints and hardware-assisted breakpoints.  This
subsection documents the x86 watchpoint facilities in @value{GDBN}.

To use the generic x86 watchpoint support, a port should do the
following:

@itemize @bullet
@findex I386_USE_GENERIC_WATCHPOINTS
@item
Define the macro @code{I386_USE_GENERIC_WATCHPOINTS} somewhere in the
target-dependent headers.

@item
Include the @file{config/i386/nm-i386.h} header file @emph{after}
defining @code{I386_USE_GENERIC_WATCHPOINTS}.

@item
Add @file{i386-nat.o} to the value of the Make variable
@code{NATDEPFILES} (@pxref{Native Debugging, NATDEPFILES}) or
@code{TDEPFILES} (@pxref{Target Architecture Definition, TDEPFILES}).

@item
Provide implementations for the @code{I386_DR_LOW_*} macros described
below.  Typically, each macro should call a target-specific function
which does the real work.
@end itemize

The x86 watchpoint support works by maintaining mirror images of the
debug registers.  Values are copied between the mirror images and the
real debug registers via a set of macros which each target needs to
provide:

@table @code
@findex I386_DR_LOW_SET_CONTROL
@item I386_DR_LOW_SET_CONTROL (@var{val})
Set the Debug Control (DR7) register to the value @var{val}.

@findex I386_DR_LOW_SET_ADDR
@item I386_DR_LOW_SET_ADDR (@var{idx}, @var{addr})
Put the address @var{addr} into the debug register number @var{idx}.

@findex I386_DR_LOW_RESET_ADDR
@item I386_DR_LOW_RESET_ADDR (@var{idx})
Reset (i.e.@: zero out) the address stored in the debug register
number @var{idx}.

@findex I386_DR_LOW_GET_STATUS
@item I386_DR_LOW_GET_STATUS
Return the value of the Debug Status (DR6) register.  This value is
used immediately after it is returned by
@code{I386_DR_LOW_GET_STATUS}, so as to support per-thread status
register values.
@end table

For each one of the 4 debug registers (whose indices are from 0 to 3)
that store addresses, a reference count is maintained by @value{GDBN},
to allow sharing of debug registers by several watchpoints.  This
allows users to define several watchpoints that watch the same
expression, but with different conditions and/or commands, without
wasting debug registers which are in short supply.  @value{GDBN}
maintains the reference counts internally, targets don't have to do
anything to use this feature.

The x86 debug registers can each watch a region that is 1, 2, or 4
bytes long.  The ia32 architecture requires that each watched region
be appropriately aligned: 2-byte region on 2-byte boundary, 4-byte
region on 4-byte boundary.  However, the x86 watchpoint support in
@value{GDBN} can watch unaligned regions and regions larger than 4
bytes (up to 16 bytes) by allocating several debug registers to watch
a single region.  This allocation of several registers per a watched
region is also done automatically without target code intervention.

The generic x86 watchpoint support provides the following API for the
@value{GDBN}'s application code:

@table @code
@findex i386_region_ok_for_watchpoint
@item i386_region_ok_for_watchpoint (@var{addr}, @var{len})
The macro @code{TARGET_REGION_OK_FOR_HW_WATCHPOINT} is set to call
this function.  It counts the number of debug registers required to
watch a given region, and returns a non-zero value if that number is
less than 4, the number of debug registers available to x86
processors.

@findex i386_stopped_data_address
@item i386_stopped_data_address (@var{addr_p})
The target function
@code{target_stopped_data_address} is set to call this function.
This
function examines the breakpoint condition bits in the DR6 Debug
Status register, as returned by the @code{I386_DR_LOW_GET_STATUS}
macro, and returns the address associated with the first bit that is
set in DR6.

@findex i386_stopped_by_watchpoint
@item i386_stopped_by_watchpoint (void)
The macro @code{STOPPED_BY_WATCHPOINT}
is set to call this function.  The
argument passed to @code{STOPPED_BY_WATCHPOINT} is ignored.  This
function examines the breakpoint condition bits in the DR6 Debug
Status register, as returned by the @code{I386_DR_LOW_GET_STATUS}
macro, and returns true if any bit is set.  Otherwise, false is
returned.

@findex i386_insert_watchpoint
@findex i386_remove_watchpoint
@item i386_insert_watchpoint (@var{addr}, @var{len}, @var{type})
@itemx i386_remove_watchpoint (@var{addr}, @var{len}, @var{type})
Insert or remove a watchpoint.  The macros
@code{target_insert_watchpoint} and @code{target_remove_watchpoint}
are set to call these functions.  @code{i386_insert_watchpoint} first
looks for a debug register which is already set to watch the same
region for the same access types; if found, it just increments the
reference count of that debug register, thus implementing debug
register sharing between watchpoints.  If no such register is found,
the function looks for a vacant debug register, sets its mirrored
value to @var{addr}, sets the mirrored value of DR7 Debug Control
register as appropriate for the @var{len} and @var{type} parameters,
and then passes the new values of the debug register and DR7 to the
inferior by calling @code{I386_DR_LOW_SET_ADDR} and
@code{I386_DR_LOW_SET_CONTROL}.  If more than one debug register is
required to cover the given region, the above process is repeated for
each debug register.

@code{i386_remove_watchpoint} does the opposite: it resets the address
in the mirrored value of the debug register and its read/write and
length bits in the mirrored value of DR7, then passes these new
values to the inferior via @code{I386_DR_LOW_RESET_ADDR} and
@code{I386_DR_LOW_SET_CONTROL}.  If a register is shared by several
watchpoints, each time a @code{i386_remove_watchpoint} is called, it
decrements the reference count, and only calls
@code{I386_DR_LOW_RESET_ADDR} and @code{I386_DR_LOW_SET_CONTROL} when
the count goes to zero.

@findex i386_insert_hw_breakpoint
@findex i386_remove_hw_breakpoint
@item i386_insert_hw_breakpoint (@var{addr}, @var{shadow}
@itemx i386_remove_hw_breakpoint (@var{addr}, @var{shadow})
These functions insert and remove hardware-assisted breakpoints.  The
macros @code{target_insert_hw_breakpoint} and
@code{target_remove_hw_breakpoint} are set to call these functions.
These functions work like @code{i386_insert_watchpoint} and
@code{i386_remove_watchpoint}, respectively, except that they set up
the debug registers to watch instruction execution, and each
hardware-assisted breakpoint always requires exactly one debug
register.

@findex i386_stopped_by_hwbp
@item i386_stopped_by_hwbp (void)
This function returns non-zero if the inferior has some watchpoint or
hardware breakpoint that triggered.  It works like
@code{i386_stopped_data_address}, except that it doesn't record the
address whose watchpoint triggered.

@findex i386_cleanup_dregs
@item i386_cleanup_dregs (void)
This function clears all the reference counts, addresses, and control