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DosPtrace
Bindings: C, MASM
This call provides an interface into the OS/2 kernel to facilitate program
debugging.
DosPtrace (PtraceB)
PtraceB (PBYTE) - output
Address of the Ptrace command/data buffer. This buffer is used to
communicate between the debug program and the DosPtrace routines.
rc (USHORT) - return
Return code descriptions are:
0 NO_ERROR
1 ERROR_INVALID_FUNCTION
5 ERROR_ACCESS_DENIED
115 ERROR_PROTECTION_VIOLATION
303 ERROR_INVALID_PROCID
Remarks
DosPtrace allows a parent process to control the execution of another
process by implementing breakpoint debugging for a debugger. Both the
program under test and the program being debugged must be executing in
OS/2 mode.
DosPtrace supports debugging of a process with multiple threads by
allowing the debugger to read and write registers, freeze and resume
thread execution, and get status on the threads.
The debugger must be able to read and write the instructions, data, and
registers of the program being debugged to insert breakpoint
instructions. When a process runs in OS/2 mode, one process cannot
directly manipulate the address space of another process. OS/2 controls
this address space through the use of the trace flag facility in
DosExecPgm and the Ptrace buffer in DosPtrace.
The steps to program debugging in OS/2 mode follow:
1. The debug program issues DosExecPgm for the program to be debugged,
and specifies the trace option.
2. The debug program calls DosPtrace with the TRC_C_Stop command to
initialize the Ptrace Buffer.
3. The debug program sets up a Ptrace buffer with commands for inserting
the breakpoints and issues repeated DosPtrace requests as necessary.
4. The debug program sets up a Ptrace buffer with a command to begin
execution and issues DosPtrace. This may be a TRC_CS_Step, or
TRC_C_Go.
5. When the kernel DosPtrace program receives control from the program
being debugged, it returns to the debug program with the Ptrace
buffer set to the current register contents and with indicators of
the reason for return.
6. The kernel DosPtrace program receives control at a breakpoint
interrupt, at processor exceptions, or when the program ends.
To debug a process with multiple threads, set a field in the Ptrace
buffer (Ptrace_B.TID) to the thread ID of the thread of interest. This
causes the read/write register commands to receive only the register set
of the specified thread.
Note: For a process with multiple threads, the address space is the same
for all the threads in the process. When commands are issued to
read/write memory locations or set breakpoints, they affect all
the threads in the process, even when a command is issued with a
specific thread ID.
The debugger may suspend and resume specific threads through use of the
TRC_C_Freeze and TRC_C_Resume commands. Having only selected threads be
affected by the breakpoints is useful for manipulating them while other
threads are suspended.
When a process debugger terminates, the program being debugged also
terminates. To accomplish this, an internal link between the debugger
and the program being debugged is maintained. This link is established
as a result of the first successful DosPtrace command. Once established,
this link can not be reset.
The process being debugged does not need to be a direct child process. In
this situation, a small window of time exists between the DosExecPgm call
and the first DosPtrace call. If the debugger terminates during this
window, the program being debugged cannot be cleaned up. The system
automatically terminates the program that was to be debugged.
Specifying a trace option of 2 with DosStartSession enables a debugger
running in the parent session to trace all processes associated with an
application running in the child session, regardless of whether the
process was started by DosStartSession request or by DosExecPgm. See
DosStartSession for more information.
Contents of the Ptrace Buffer:
+----------+----+----+--------------------------------------------------+
| PTRACE_B | | | STRUCTURE |
+----------+----+----+--------------------------------------------------+
| PID | DW | 0 | ; Process ID of the process being debugged |
+----------+----+----+--------------------------------------------------+
| TID | DW | 0 | ; Thread ID of the process being debugged |
+----------+----+----+--------------------------------------------------+
| Cmd | DW | 0 | ; Request to DosPtrace, or DosPtrace result code |
+----------+----+----+--------------------------------------------------+
| Value | DW | ? | ; Data to DosPtrace, or DosPtrace error code |
+----------+----+----+--------------------------------------------------+
| OffV | DW | ? | ; Offset value |
+----------+----+----+--------------------------------------------------+
| SegV | DW | ? | ; Segment value |
+----------+----+----+--------------------------------------------------+
| MTE | DW | ? | ; Library Module handle |
+----------+----+----+--------------------------------------------------+
| Ptrace B | | | |
| ENDS | | | |
+----------+----+----+--------------------------------------------------+
Exceptions:
+-----------+----+----+----------------------------------+
| PTRACEREGS| | | STRUCTURE |
+-----------+----+----+----------------------------------+
| rAX | DW | ? | ; Registers AX through SS |
+-----------+----+----+----------------------------------+
| rBX | DW | ? | |
+-----------+----+----+----------------------------------+
| rCX | DW | ? | |
+-----------+----+----+----------------------------------+
| rDX | DW | ? | |
+-----------+----+----+----------------------------------+
| rSI | DW | ? | |
+-----------+----+----+----------------------------------+
| rDI | DW | ? | |
+-----------+----+----+----------------------------------+
| rBP | DW | ? | |
+-----------+----+----+----------------------------------+
| rDS | DW | ? | |
+-----------+----+----+----------------------------------+
| rES | DW | ? | |
+-----------+----+----+----------------------------------+
| rIP | DW | ? | |
+-----------+----+----+----------------------------------+
| rCS | DW | ? | |
+-----------+----+----+----------------------------------+
| rF | DW | ? | |
+-----------+----+----+----------------------------------+
| rSP | DW | ? | |
+-----------+----+----+----------------------------------+
| rSS | DW | ? | |
+-----------+----+----+----------------------------------+
| Ptraceregs| | | |
| ENDS | | | |
+-----------+----+----+----------------------------------+
For the TRC_C_ReadMemB and TRC_C_WriteMemB commands, the remainder of the
Ptrace buffer contains:
+-----------+----+----+----------------------------------+
| PTRACEPTR | | | STRUCTURE |
+-----------+----+----+----------------------------------+
| OffB | DW | ? | ; Buffer Address |
+-----------+----+----+----------------------------------+
| SegB | DW | ? | |
+-----------+----+----+----------------------------------+
| Ptraceptr | | | |
| ENDS | | | |
+-----------+----+----+----------------------------------+
DosPtrace Commands: PTrace_B.Cmd must contain one of the following
commands
upon entrance to DosPtrace:
+-------------------+---------+-------------------+------------------+
| TRC_C_Null | EQU 0 | | ; Invalid |
+-------------------+---------+-------------------+------------------+
| TRC_C_ReadMem_I | EQU 1 | | ; Instruction |
+-------------------+---------+-------------------+------------------+
| TRC_C_ReadMem_D | EQU 2 | | ; Data |
+-------------------+---------+-------------------+------------------+
| TRC_C_ReadMem | EQU | TRC_C_ReadMem_I | |
+-------------------+---------+-------------------+------------------+
| TRC_C_ReadReg | EQU 3 | | |
+-------------------+---------+-------------------+------------------+
| TRC_C_WriteMem_I | EQU 4 | | ; Instruction |
+-------------------+---------+-------------------+------------------+
| TRC_C_WriteMem_D | EQU 5 | | ; Data |
+-------------------+---------+-------------------+------------------+
| TRC_C_WriteMem | EQU | TRC_C_WriteMem_I | |
+-------------------+---------+-------------------+------------------+
| TRC_C_WriteReg | EQU 6 | | |
+-------------------+---------+-------------------+------------------+
| TRC_C_Go | EQU 7 | | |
+-------------------+---------+-------------------+------------------+
| TRC_C_Term | EQU 8 | | |
+-------------------+---------+-------------------+------------------+
| TRC_C_SStep | EQU 9 | | |
+-------------------+---------+-------------------+------------------+
| TRC_C_Stop | EQU 10 | | ; Initialize |
+-------------------+---------+-------------------+------------------+
| TRC_C_Freeze | EQU 11 | | |
+-------------------+---------+-------------------+------------------+
| TRC_C_Resume | EQU 12 | | |
+-------------------+---------+-------------------+------------------+
| TRC_C_NumToSel | EQU 13 | | |
+-------------------+---------+-------------------+------------------+
| TRC_C_GetFPRegs | EQU 14 | | |
+-------------------+---------+-------------------+------------------+
| TRC_C_SetFPRegs | EQU 15 | | |
+-------------------+---------+-------------------+------------------+
| TRC_C_GetLibName | EQU 16 | | |
+-------------------+---------+-------------------+------------------+
| TRC_C_ThrdStat | EQU 17 | | |
+-------------------+---------+-------------------+------------------+
| TRC_C_ReadMem_B | EQU 18 | | ; Read Block |
+-------------------+---------+-------------------+------------------+
| TRC_C_WriteMem_B | EQU 19 | | ; Write Block |
+-------------------+---------+-------------------+------------------+
Commands and Required Input: A command is issued by placing the command
number in Ptrace buffer, and calling DosPtrace with that buffer.
All of the commands require that Ptrace_B.PID be the PID of the process
to debug.
TRC_C_Null Not a valid command
Memory Operations:
For the following commands, SegV:OffV is the affected location, and
Ptrace_B.Value contains the value to write to or that was read from
the debugger's memory. GDT segments cannot be written to: all except
privilege level 2 and 3 access is disallowed. A write to a shared code
segment makes that segment a non-shared segment before the write.
TRC_C_ReadMem_I Read instruction.
TRC_C_ReadMem_D Read data.
TRC_C_ReadMem Read any memory.
TRC_C_WriteMem_I Write instruction.
TRC_C_WriteMem_D Write data.
TRC_C_WriteMem Write to any memory.
Block operations:
For the block operations, SegV:OffV must point to the starting address
in the debugger's memory, and Value is the number of bytes to copy.
On return, Value contains the number of bytes actually copied.
TRC_C_ReadMem_B Read memory block.
TRC_C_WriteMem_B Write memory block.
Register / Thread Operations:
For the following commands, Ptrace_B.TID must contain the thread ID of
the thread in question. If the Ptrace_B.TID field is zero:
o TRC_C_ThrdStat returns the number of threads in the process,
(PTrace_B.Value).
o TRC_C_Freeze and TRC_C_Resume affects all of the debugger's threads.
TRC_C_ReadReg Examine thread's registers.
TRC_C_WriteReg Write thread's registers.
TRC_C_Freeze Suspend a thread.
TRC_C_Resume Resume a suspended thread.
TRC_C_ThrdStat Get thread status.
Command Operations:
For the following commands, the Ptrace_B.PID must be valid. The
Ptrace_B registers are ignored for these commands. For TRC_C_Go and
TRC_C_SStep, any thread may gain control first. The TRC_C_Term
command terminates the program being debugged.
TRC_C_Go Run the program being debugged.
TRC_C_Term Terminate the program being debugged.
TRC_C_SStep Run one instruction.
TRC_C_Stop Initialize PTrace buffer.
Library Support:
For TRC_C_NumToSel, Ptrace_B.Value should be set to the segment number
on entrance, and a valid selector on exit. Also, Ptrace_B.MTE should
be set to the module's handle. The MTE identifies the different
library files in the program being debugged.
For TRC_C_GetLibName, SegV:OffV should point to a buffer where the
name of the library is returned. PTrace_B.Value should hold the
library's module handle (MTE).
TRC_C_NumToSel Convert Segment number to selector.
TRC_C_GetLibName Return name of module.
Floating Point Support:
For the following two commands, SegV:OffV must contain a pointer to a
94 byte buffer to be used to read/write the floating point registers
from/to.
The layout of this area is described in the NPX287 manual under the
heading FSAVE/FRSTOR memory layout.
TRC_C_GetFPRegs Read floating point registers.
TRC_C_SetFPRegs Write floating point registers.
DosPtrace Return Codes:
When DosPtrace returns to the debug program, the result is placed in
Ptrace_B.Cmd, and reflects the reason for the return.
The values returned are:
+----------------+----------+----------------+
| TRC_C_SUC_ret | EQU 0 | ; Success |
+----------------+----------+----------------+
| TRC_C_ERR_ret | EQU -1 | ; Error |
+----------------+----------+----------------+
| TRC_C_SIG_ret | EQU -2 | ; Signal |
+----------------+----------+----------------+
| TRC_C_TBT_ret | EQU -3 | ; Single Step |
+----------------+----------+----------------+
| TRC_C_BPT_ret | EQU -4 | ; Breakpoint |
+----------------+----------+----------------+
| TRC_C_NMI_ret | EQU -5 | ; Parity Error |
+----------------+----------+----------------+
| TRC_C_KIL_ret | EQU -6 | ; Process |
| | | dying |
+----------------+----------+----------------+
| TRC_C_GPF_ret | EQU -7 | ; GP fault |
+----------------+----------+----------------+
| TRC_C_LIB_ret | EQU -8 | ; Library load |
+----------------+----------+----------------+
| TRC_C_FPE_ret | EQU -9 | ; FP error |
+----------------+----------+----------------+
| TRC_C_THD_ret | EQU -10 | ; Thread |
| | | ending |
+----------------+----------+----------------+
| TRC_C_STP_ret | EQU -11 | ; Async. Stop. |
+----------------+----------+----------------+
If Ptrace_B.Cmd is returned as TRC_C_ERR_ret, Ptrace_B.Value is set to
one of the following:
TRACE_BAD_COMMAND EQU 1
TRACE_CHILD_NOT_FOUND EQU 2
TRACE_CHILD_UNTRACEABLE EQU 5
If Ptrace_B.Cmd is returned as TRC_C_SIG_ret, the process is about to
receive a signal.
If Ptrace_B.Cmd is returned as TRC_C_KIL_ret, the process is about to
terminate.
If Ptrace_B.Cmd returns as TRC_C_GPF_ret, the process creates a
General Protection fault. The fault type is returned in
PTrace_B.Value, and SegV:OffV contains the reference that generated
the fault.
If Ptrace_B.Cmd is returned as TRC_C_LIB_ret, a library module has
been loaded. The new module table entry (MTE) is returned in
Ptrace_B.Value. This can be used with the library support commands to
identify the library module. The program module's MTE is returned in
PTrace_B.MTE. In this case, the initial TRC_C_Stop command should be
re-issued until TRC_C_SUC_ret is returned.
If Ptrace_B.Cmd is returned as TRC_C_FPE_ret, the process has
generated a floating point error.
If Ptrace_B.Cmd is returned as TRC_C_THD_ret, the Ptrace_b.TID field
contains the thread ID of the terminating thread.
If Ptrace_B.Cmd is returned as TRC_C_STP_ret, then the asynchronous
stop caused the debugger to stop.
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