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AIX Version 4.3 Base Operating System and Extensions Technical Reference, Volume 1
ptrace, ptracex Subroutine
Purpose
Traces the execution of another process.
Library
Standard C Library (libc.a)
Syntax
#include <sys/reg.h>
#include <sys/ptrace.h>
#include <sys/ldr.h>
int ptrace (Request, Identifier,Address, Data, Buffer)
int Request;
int Identifier;
int *Address;
int Data;
int *Buffer;
int ptracex (request, identifier, long long addr, data, buff)
int request;
int identifier, long long addr;
int data;
int *buff;
Description
The ptrace subroutine allows a 32-bit process to trace the execution of another process. The ptrace subroutine is used to implement breakpoint debugging.
A debugged process executes normally until it encounters a signal. Then it enters a stopped state and its debugging process is notified with the wait subroutine. While the process is in the stopped state, the debugger examines and modifies its memory image by using the ptrace subroutine. For multi-threaded processes, the getthrds subroutine is used to identify each kernel thread in the debugged process. Also, the debugging process can cause the debugged process to terminate or continue, with the possibility of ignoring the signal that caused it to stop.
As a security measure, the ptrace subroutine inhibits the set-user-ID facility on subsequent exec subroutines.
(This paragraph only applies to AIX 4.3.1 and previous releases.) When a process
is executing under ptrace control, portions of the process's address space are
recopied after load, unload, and loadbind calls. For a 32-bit process, the main
program text (loaded in segment 1) and shared library modules (loaded in segment
13) is recopied. Any breakpoints or other modifications to these segments must
be reinserted after load, unload, or loadbind. Changes to privately loaded
modules persist. For a 64-bit process, shared library modules are recopied after
load and unload are called. (For AIX 4.3.0 and 4.3.1, these segments have a
virtual address of 0x09000000xxxxxxxx, where x denotes any value.) The
segments for the main programs and the segments containing privately loaded
modules are not recopied. When a 64-bit process calls loadbind, no segments
are recopied and the debugger is not notified.
(This paragraph only applies to AIX 4.3.2 and later releases.) When a process
executing under ptrace control calls load or unload, the debugger is notified
and the W_SLWTED flag is set in the status returned by wait. (A 32-bit process
calling loadbind is stopped as well.) If the process being debugged has added
modules in the shared library to its address space, the modules are added to the
process's private copy of the shared library segments. If shared library modules
are removed from a process's address space, the modules are deleted from the
process's private copy of the library text segment by freeing the pages that contain
the module. No other changes to the segment are made, and existing breakpoints
do not have to be reinserted.
When a process being traced forks, the child process is initialized with the
unmodified main program and shared library segment, effectively removing
breakpoints in these segments in the child process. If multiprocess debugging is
enabled, new copies of the main program and shared library segments are made.
Modifications to privately loaded modules, however, are not affected by a fork.
These breakpoints will remain in the child process, and if these breakpoints are
executed, a SIGTRAP signal will be generated and delivered to the process.
If a traced process initiates an exec subroutine, the process stops before executing the first instruction of the new image and returns the SIGTRAP signal.
Note: ptrace and ptracex are not supported in 64-bit mode.
For the 64-bit Process
Use ptracex where the debuggee is a 64-bit process and the operation requested uses the third (address) parameter to reference the debuggee's address space or is sensitive to register size.
If returning or passing an int doesn't work for a 64-bit debuggee (for example, PT_READ_GPR), the buffer parameter takes the address for the result. Thus, with the ptracex subroutine, PT_READ_GPR and PT_WRITE_GPR take a pointer to an 8 byte area representing the register value.
In general, ptracex supports all the calls that ptrace does when they are modified for any that are extended for 64-bit addresses (for example, GPRs, LR, CTR, IAR, and MSR). Anything whose size increases for 64-bit processes must be allowed for in the obvious way (for example, PT_REGSET must be an array of long longs for a 64-bit debuggee).
Parameters
| Request |
Determines the action to be taken by the ptrace subroutine and
has one of the following values:
| PT_ATTACH |
This request allows a debugging process to attach a current process and place it into trace mode for debugging. This request cannot be used if the target process is already being traced. The Identifier parameter is interpreted as the process ID of the traced process. The Address, Data, and Buffer parameters are ignored.
If this request is unsuccessful, -1 is returned and the errno global
variable is set to one the following codes:
| ESRCH |
Process ID is not valid; the traced process is a kernel process; the process is currently being traced; or, the debugger or traced process already exists. |
| EPERM |
Real or effective user ID of the debugger does not match that of the traced process, or the debugger does not have root authority. |
| EINVAL |
The debugger and the traced process are the same. |
|
| PT_CONTINUE |
This request allows the process to resume execution. If the Data parameter is 0, all pending signals, including the one that caused the process to stop, are concealed before the process resumes execution. If the data parameter is a valid signal number, the process resumes execution as if it had received that signal. If the Address parameter equals 1, the execution continues from where it stopped. If the Address parameter is not 1, it is assumed to be the address at which the process should resume execution. Upon successful completion, the value of the Data parameter is returned to the debugging process. The Identifier parameter is interpreted as the process ID of the traced process.
The Buffer parameter is ignored.
If this request is unsuccessful, -1 is returned and the errno global
variable is set to the following code:
| EIO |
The signal to be sent to the traced process is not a valid signal number. |
Note: For the PT_CONTINUE request, use ptracex with a 64-bit debuggee because the resume address needs 64 bits. |
| PTT_CONTINUE |
This request asks the scheduler to resume execution of the kernel thread
specified by Identifier. This kernel thread must be the one that caused the
exception. The Data parameter specifies how to handle
signals:
- If the Data parameter is zero, the kernel
thread which caused the exception will be resumed as if the signal never
occurred.
- If the Data parameter is a valid signal
number, the kernel thread which caused the exception will be resumed as if it had
received that signal.
The Address parameter specifies where to resume execution:
- If the Address parameter is one, execution
resumes from the address where it stopped.
- If the Address parameter contains an
address value other than one, execution resumes from that address.
The Buffer parameter should point to a PTTHREADS structure,
which contains a list of kernel thread identifiers to be started. This list should
be NULL terminated if it is smaller than the maximum allowed.
On successful completion, the value of the Data parameter is returned to
the debugging process. On unsuccessful completion, the value -1 is returned, and
the errno global variable is set as follows:
| EINVAL |
The Identifier parameter names the wrong kernel thread. |
| EIO |
The signal to be sent to the traced kernel thread is not a valid signal number. |
| ESRCH |
The Buffer parameter names an invalid kernel thread. Each kernel
thread in the list must be stopped and belong to the same process as the kernel
thread named by the Identifier parameter. |
Note: For the PTT_CONTINUE request, use ptracex with a 64-bit debuggee because the resume address needs 64 bits. |
| PT_DETACH |
This request allows a debugged process, specified by the Identifier parameter, to exit trace mode. The process then continues running, as if it had received the signal whose number is contained in the data parameter. The process is no longer traced and does not process any further ptrace calls. The
Address and Buffer parameters are ignored.
If this request is unsuccessful, -1 is returned and the errno global
variable is set to the following code:
| EIO |
Signal to be sent to the traced process is not a valid signal
number. |
|
| PT_KILL |
This request allows the process to terminate the same way it would with
an exit subroutine. |
| PT_LDINFO |
This request retrieves a description of the object modules that were
loaded by the debugged process.
The Identifier parameter is interpreted as the process ID of the traced
process.
The Buffer parameter is ignored. The Address parameter specifies the
location where the loader information is copied. The Data parameter
specifies the size of this area. The loader information is retrieved as a linked
list of ld_info structures. The ld_info structures are defined in the
/usr/include/sys/ldr.h file. The linked list is implemented so that the
ldinfo_nxt field of each element gives the offset of the next element
from this element. The ldinfo_nxt field of the last element has the
value 0.
If this request is unsuccessful, -1 is returned and the errno global
variable is set to the following code:
| ENOMEM |
Either the area is not large enough to accommodate the loader
information, or there is not enough memory to allocate an equivalent buffer in the
kernel. |
Note: For the PT_LDINFO request, use ptracex with a 64-bit debuggee because the source address needs 64 bits. |
| PT_MULTI |
This request turns multiprocess debugging mode on and off, to allow
debugging to continue across fork and exec subroutines. A 0 value for
the data parameter turns multiprocess debugging mode off, while all other values
turn it on. When multiprocess debugging mode is in effect, any fork subroutine allows both the traced process and its newly created process to trap
on the next instruction. If a traced process initiated an exec subroutine,
the process stops before executing the first instruction of the new image and
returns the SIGTRAP signal.
The Identifier parameter is interpreted as the process ID of the traced
process.
The Address and Buffer parameters are ignored.
Also, when multiprocess debugging mode is enabled, the following values are
returned from the wait subroutine:
| W_SEWTED |
Process stopped during execution of the exec subroutine.
| | W_SFWTED |
Process stopped during execution of the fork subroutine. |
|
| PT_READ_BLOCK |
This request reads a block of data from the debugged process address
space. The Address parameter points to the block of data in the process
address space, and the Data parameter gives its length in bytes. The value
of the Data parameter must not be greater than 1024.
The Identifier parameter is interpreted as the process ID of the traced
process.
The Buffer parameter points to the location in the debugging process address
space where the data is copied. Upon successful completion, the ptrace
subroutine returns the value of the data parameter.
If this request is unsuccessful, -1 is returned and the errno global
variable is set to one of the following codes:
| EIO |
The Data parameter is less than 1 or greater than 1024. |
| EIO |
The Address parameter is not a valid pointer into the debugged process address space. |
| EFAULT |
The Buffer parameter does not point to a writable location in the
debugging process address space. |
|
Note: For the PT_READ_BLOCK request, use ptracex with a 64-bit debuggee because the source address needs 64 bits. |
| PT_READ_FPR |
This request stores the value of a floating-point register into the location pointed to by the Address parameter. The Data parameter specifies the floating-point register, defined in the sys/reg.h file for the machine type on which the process is executed.
The Identifier parameter is interpreted as the process ID of the traced
process.
The Buffer parameter is ignored.
If this request is unsuccessful, -1 is returned and the errno global
variable is set to the following code:
| EIO |
The Data parameter is not a valid floating-point register. The Data parameter must be in the range 256-287. |
|
|
| PTT_READ_FPRS |
This request writes the contents of the 32 floating point registers to the area specified by the Address parameter. This area must be at least 256 bytes long. The Identifier parameter specifies the traced kernel thread. The Data and Buffer parameters are ignored. |
| PT_READ_GPR |
This request returns the contents of one of the general-purpose or special-purpose registers of the debugged process. The Address parameter specifies the register whose value is returned. The value of the Address parameter is defined in the sys/reg.h file for the machine type on which the process is executed. The Identifier parameter is interpreted as the process ID of the traced process.
The Data and Buffer parameters are ignored. The buffer points to long long target area.
Note: If ptracex with a 64-bit debugee is used for this request, the register value is instead returned to the 8-byte area pointed to by the buffer pointer.
If this request is unsuccessful, -1 is returned and the errno global
variable is set to the following code:
| EIO |
The Address is not a valid general-purpose or special-purpose register. The Address parameter must be in the range 0-31 or 128-136. |
|
|
| PTT_READ_GPRS |
This request writes the contents of the 32 general purpose registers to the area specified by the Address parameter. This area must be at least 128 bytes long.
Note: If ptracex with a 64-bit debugee is used for the PTT_READ_GPRS request, there must be at least a 256 byte target area. The Identifier parameter specifies the traced kernel thread. The Data and Buffer parameters are ignored.
|
| PT_READ_I or PT_READ_D |
These requests return the word-aligned address in the debugged process
address space specified by the Address parameter.
On all machines currently supported by the Version
4 operating system, the PT_READ_I and PT_READ_D instruction and data
requests can be used with equal results.
The Identifier parameter is interpreted as the process ID of the traced
process.
The Data parameter is ignored.
If this request is unsuccessful, -1 is returned and the errno global
variable is set to the following code:
| EIO |
The Address is not word-aligned, or the Address is not valid. User blocks, kernel segments, and kernel extension segments are not
considered as valid addresses. |
|
Note: For the PT_READ_I or the PT_READ_D request, use ptracex with a 64-bit debuggee because the source address needs 64 bits.
|
| PTT_READ_SPRS |
This request writes the contents of the special purpose registers to the area specified by the Address parameter, which points to a ptsprs structure. The Identifier parameter specifies the traced kernel thread. The
Data and Buffer parameters are ignored.
Note: For the PTT_READ_SPRS request, use ptracex with the 64-bit debuggee because the new ptxsprs structure must be used.
|
| PT_REATT |
This request allows a new debugger, with the proper permissions, to trace a process that was already traced by another debugger. The Identifier parameter is interpreted as the process ID of the traced process. The Address, Data, and Buffer parameters are ignored.
If this request is unsuccessful, -1 is returned and the errno global
variable is set to one the following codes:
| ESRCH |
The Identifier is not valid; or the traced process is a kernel process. |
| EPERM |
Real or effective user ID of the debugger does not match that of the traced process, or the debugger does not have root authority. |
| EINVAL |
The debugger and the traced process are the same. |
|
|
| PT_REGSET |
This request writes the contents of all 32 general purpose registers to the area specified by the Address parameter. This area must be at least 128 bytes for the 32-bit debuggee or 256 bytes for the 64-bit debuggee. The Identifier parameter is interpreted as the process ID of the traced process. The Data and Buffer parameters are ignored.
If this request is unsuccessful, -1 is returned and the errno global
variable is set to the following code:
| EIO |
The Address parameter points to a location outside of the allocated address space of the process. |
|
Note: For the PT_REGSET request, use ptracex with the 64-bit debuggee because 64-bit registers requiring 256 bytes are returned. |
| PT_TRACE_ME |
This request must be issued by the debugged process to be traced. Upon receipt of a signal, this request sets the process trace flag, placing the process in a stopped state, rather than the action specified by the sigaction subroutine. The
Identifier, Address, Data, and Buffer parameters are ignored. Do not issue this request if the parent process does not expect to trace the debugged process.
As a security measure, the ptrace subroutine inhibits the set-user-ID
facility on subsequent exec subroutines, as shown in the following example:
if((childpid = fork()) == 0)
{ /* child process */
ptrace(PT_TRACE_ME,0,0,0,0);
execlp( )/* your favorite exec*/
}
else
{ /* parent */
/* wait for child to stop */
rc = wait(status)
Note: This is the only request that
should be performed by the child. The parent should perform all other requests when the child is in a stopped state.
If this request is unsuccessful, -1 is returned and the errno global
variable is set to the following code:
| ESRCH |
Process is debugged by a process that is not its parent. |
|
|
| PT_WRITE_BLOCK |
This request writes a block of data into the debugged process address space. The Address parameter points to the location in the process address space to be written into. The Data parameter gives the length of the block in bytes, and must not be greater than 1024. The Identifier parameter is interpreted as the process ID of the traced process. The Buffer parameter points to the location in the debugging process address
space where the data is copied. Upon successful completion, the value of the Data parameter is returned to the debugging process.
If this request is unsuccessful, -1 is returned and the errno global
variable is set to one of the following codes:
| EIO |
The Data parameter is less than 1 or greater than 1024. |
| EIO |
The Address parameter is not a valid pointer into the debugged process address space. |
| EFAULT |
The Buffer parameter does not point to a readable location in the debugging process address space. |
|
Note: For the PT_WRITE_BLOCK request, use ptracex with the 64-bit debuggee because 64-bit registers requiring 256 bytes are returned.
|
| PT_WRITE_FPR |
This request sets the floating-point register specified by the Data parameter to the value specified by the Address parameter.
The Identifier parameter is interpreted as the process ID of the traced process. The Buffer parameter is ignored.
If this request is unsuccessful, -1 is returned and the errno global
variable is set to the following code:
| EIO |
The Data parameter is not a valid floating-point register. The Data parameter must be in the range 256-287. |
|
|
| PTT_WRITE_FPRS |
This request updates the contents of the 32 floating point registers with the values specified in the area designated by the Address parameter. This area must be at least 256 bytes long. The Identifier parameter specifies the
traced kernel thread. The Data and Buffer parameters are ignored.
| | PT_WRITE_GPR |
This request stores the value of the Data parameter in one of the process general-purpose or special-purpose registers. The Address parameter specifies the register to be modified. Upon successful completion, the value of the Data parameter is returned to the debugging process. The Identifier parameter is interpreted as the process ID of the traced process. The Buffer parameter is ignored.
Note: If ptracex with a 64-bit debugee is used for the PT_WRITE_GPR request, the new register value is NOT passed via the data parameter, but is instead passed via the 8-byte area pointed to by the buffer parameter.
If this request is unsuccessful, -1 is returned and the errno global
variable is set to the following code:
| EIO |
The Address parameter is not a valid general-purpose or special-purpose register. The Address parameter must be in the range 0-31 or 128-136. |
|
|
| PTT_WRITE_GPRS |
This request updates the contents of the 32 general purpose registers with the values specified in the area designated by the Address parameter. This area must be at least 128 bytes long. The Identifier parameter specifies the
traced kernel thread. The Data and Buffer parameters are ignored.
Note: For the PTT_WRITE_GPRS request, use ptracex with the 64-bit debuggee because 64-bit registers requiring 256 bytes are returned. The buffer points to long long source area. |
| PT_WRITE_I or PT_WRITE_D |
These requests write the value of the data parameter into the address space of the debugged process at the word-aligned address specified by the Address parameter. On all machines currently supported by the Version 4 operating system, instruction and data address spaces are not separated. The PT_WRITE_I and PT_WRITE_D instruction and data requests can be used with equal results. Upon successful completion, the value written into the address space of the debugged process is returned to the debugging process. The Identifier parameter is interpreted as the process ID of the traced process.
The Buffer parameter is ignored.
If this request is unsuccessful, -1 is returned and the errno global
variable is set to the following code:
| EIO |
The Address parameter points to a location in a pure procedure space and a copy cannot be made; the Address is not word-aligned; or, the
Address is not valid. User blocks, kernel segments, and kernel extension segments are not considered valid addresses. |
|
Note: For the or PT_WRITE_I or PT_WRITE_D request, use ptracex with a 64-bit debuggee because the target address needs 64 bits. |
| PTT_WRITE_SPRS |
This request updates the special purpose registers with the values in the area specified by the Address parameter, which points to a ptsprs structure. The Identifier parameter specifies the traced kernel thread. The Data and Buffer parameters are ignored.
|
Identifier |
Determined by the value of the Request parameter. |
| Address |
Determined by the value of the Request parameter. |
| Data |
Determined by the value of the Request parameter. |
| Buffer |
Determined by the value of the Request parameter. |
Note: For the PTT_READ_SPRS request, use ptracex with the 64-bit debuggee because the new ptxsprs structure must be used. |
|
Error Codes
The ptrace subroutine is unsuccessful when one of the following is true:
| EFAULT |
The Buffer parameter points to a location outside the debugging
process address space. |
| EINVAL |
The debugger and the traced process are the same; or the Identifier
parameter does not identify the thread that caused the exception. |
| EIO |
The Request parameter is not one of the values listed, or the
Request parameter is not valid for the machine type on which the process is executed. |
| ENOMEM |
Either the area is not large enough to accommodate the loader
information, or there is not enough memory to allocate an equivalent buffer in the kernel. |
| EPERM |
The Identifier parameter corresponds to a kernel thread which is
stopped in kernel mode and whose computational state cannot be read or written. |
| ESRCH |
The Identifier parameter identifies a process
or thread that does not exist, that has not executed a ptrace call with the PT_TRACE_ME request, or that is not stopped. |
For ptrace: If the debuggee is a 64-bit process, the options that refer to GPRs or SPRs fail with errno = EIO, and the options that specify addresses are limited to 32-bits.
For ptracex: If the debuggee is a 32-bit process, the options that refer to GPRs or SPRs fail with errno = EIO, and the options that specify addresses in the debuggee's address space that are larger than 2**32 - 1 fail with errno set to EIO.
Also, the options PT_READ_U and PT_WRITE_U are not supported if the debuggee is a 64-bit program (errno = ENOTSUP).
Implementation Specifics
This subroutine is part of Base Operating System
(BOS) Runtime.
Related Information
The exec, getprocs, getthrds, load, sigaction, unload, wait, waitpid, or
wait3 subroutine.
The dbx command.
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