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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.

Return Code

rc (USHORT) - return
Return code descriptions are:
  • 0 NO_ERROR


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:

  • The debug program issues DosExecPgm for the program to be debugged, and specifies the trace option.
  • The debug program calls DosPtrace with the TRC_C_Stop command to initialize the Ptrace Buffer.
  • The debug program sets up a Ptrace buffer with commands for inserting the breakpoints and issues repeated DosPtrace requests as necessary.
  • 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.
  • 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.
  • 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:

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


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 ?

For the TRC_C_ReadMemB and TRC_C_WriteMemB commands, the remainder of the Ptrace buffer contains:

OffB DW ?  ; Buffer Address
SegB DW ?

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_Term EQU 8
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.

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.

Read instruction.
Read data.
Read any memory.
Write instruction.
Write data.
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.
Read memory block.
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:
  • TRC_C_ThrdStat returns the number of threads in the process, (PTrace_B.Value).
  • TRC_C_Freeze and TRC_C_Resume affects all of the debugger's threads.
Examine thread's registers.
Write thread's registers.
Suspend a thread.
Resume a suspended thread.
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.
Run the program being debugged.
Terminate the program being debugged.
Run one instruction.
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).
Convert Segment number to selector.
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.
Read floating point registers.
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:


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.




USHORT  rc = DosPtrace(PtraceB);
PBYTE   PtraceB;   /* Ptrace buffer (returned) */

USHORT  rc;        /* return code */


EXTRN  DosPtrace:FAR

PUSH@  OTHER   Ptrace_B      ;Ptrace buffer (returned)
CALL   DosPtrace

Returns WORD