PTRACE(2)

NAME
ptrace - process trace

SYNOPSIS
#include <sys/signal.h>
#include <sys/ptrace.h>
#include <sys/user.h>

ptrace(request, pid, addr, data)
int request, pid, *addr, data;

DESCRIPTION
Ptrace provides a means by which a parent process may control the execution of a child process, and examine and change its core image. Its primary use is for the implementation of breakpoint debugging. There are four arguments whose interpretation depends on a request argument. Generally, pid is the process ID of the traced process, which must be a child (no more distant descendant) of the tracing process. addr must in all cases be aligned to a 4-byte boundary.
A process being traced behaves normally until it encounters some signal whether internally generated like “illegal instruction” or externally generated like “interrupt”. See sigvec(2) for the list. Then the traced process enters a stopped state and its parent is notified via wait(2). When the child is in the stopped state, its core image can be examined and modified using ptrace. If desired, another ptrace request can then cause the child either to terminate or to continue, possibly ignoring the signal.

The value of the request argument determines the precise action of the call:

PT_TRACE_ME
    This request is the only one used by the child process; it declares that the process is to be traced by its parent. All the other arguments are ignored. Peculiar results will ensue if the parent does not expect to trace the child.

PT_READ_I, PT_READ_D
    The word in the child process's address space at addr is returned. If I and D space are separated (e.g. historically on a pdp-11), request PT_READ_I indicates I space, PT_READ_D D space. Addr must be even on some machines. The child must be stopped. The input data is ignored.

PT_READ_U
    The word of the system's per-process data area (or user page) with offset addr is returned. Addr must be aligned and less than the system pagesize returned by getpagesize(2). Values of addr that are between 0 and sizeof(struct user) refer to the user structure in the system which contains the floating point registers and other information about the process.
    To access the CPU registers the value of u_ar0 should be read (with PT_READ_U) and then this value anded with pagesize-1 to give an offset within the user page. This offset is the base offset of the 16 CPU registers.

PT_WRITE_I, PT_WRITE_D
    The given data is written at the word in the process's address space corresponding to addr, which must be even on some machines. No useful value is returned. If I and D space are separated, request PT_WRITE_I indicates I space, PT_WRITE_D D space. Attempts to write in pure procedure fail if another process is executing the same file.

PT_WRITE_U
    The process's system data is written, as it is read with request PT_READ_U. Only a few locations can be written in this way: the general registers, the floating point status and registers, and certain bits of the processor status word.

PT_CONTINUE
    The data argument is taken as a signal number and the child's execution continues at location addr as if it had incurred that signal. Normally the signal number will be either 0 to indicate that the signal that caused the stop should be ignored, or that value fetched out of the process's image indicating which signal caused the stop. If addr is (int *)1 then execution continues from where it stopped.

PT_KILL
    The traced process terminates.

PT_STEP
    Execution continues as in request PT_CONTINUE; however, as soon as possible after execution of at least one instruction, execution stops again. The signal number from the stop is SIGTRAP. This is part of the mechanism for implementing breakpoints.

As indicated, these calls (except for request PT_TRACE_ME) can be used only when the subject process has stopped. The wait call is used to determine when a process stops; in such a case the “termination” status returned by wait has the value 0177 to indicate stoppage rather than genuine termination.

To forestall possible fraud, ptrace inhibits the set-user-id and set-group-id facilities on subsequent execve(2) calls. If a traced process calls execve, it will stop before executing the first instruction of the new image showing signal SIGTRAP.

RETURN VALUE
A 0 value is returned if the call succeeds. If the call fails then a -1 is returned and the global variable errno is set to indicate the error.

ERRORS

[EIO]
    The request code is invalid.

[ESRCH]
    The specified process does not exist.

[EIO]
    The given signal number is invalid.

[EIO]
    The specified address is out of bounds.

[EPERM]
    The specified process cannot be traced.

SEE ALSO
wait(2), sigvec(2), adb(1)

BUGS
Ptrace is unique and arcane; it should be replaced with a special file that can be opened and read and written. The control functions could then be implemented with ioctl(2) calls on this file. This would be simpler to understand and have much higher performance.

The request PT_TRACE_ME call should be able to specify signals that are to be treated normally and not cause a stop. In this way, for example, programs with simulated floating point (which use “illegal instruction” signals at a very high rate) could be efficiently debugged.

The error indication, -1, is a legitimate function value; errno, (see intro(2)), can be used to disambiguate.

It should be possible to stop a process on occurrence of a system call; in this way a completely controlled environment could be provided.