4.4BSD/usr/share/man/cat2/ptrace.0
PTRACE(2) BSD Programmer's Manual PTRACE(2)
NNAAMMEE
ptrace - process trace
SSYYNNOOPPSSIISS
##iinncclluuddee <<ssyyss//ppaarraamm..hh>>
##iinncclluuddee <<ssyyss//ppttrraaccee..hh>>
##iinncclluuddee <<ssiiggnnaall..hh>>
ppttrraaccee((rreeqquueesstt,, ppiidd,, aaddddrr,, ddaattaa))
iinntt rreeqquueesstt,, ppiidd,, **aaddddrr,, ddaattaa;;
DDEESSCCRRIIPPTTIIOONN
_P_t_r_a_c_e provides a means by which a parent process may con-
trol the execution of a child process, and examine and
change its core image. Its primary use is for the imple-
mentation of breakpoint debugging. There are four argu-
ments whose interpretation depends on a _r_e_q_u_e_s_t argument.
Generally, _p_i_d is the process ID of the traced process,
which must be a child (no more distant descendant) of the
tracing process. A process being traced behaves normally
until it encounters some signal whether internally gener-
ated like "illegal instruction" or externally generated
like "interrupt". See _s_i_g_v_e_c(2) for the list. Then the
traced process enters a stopped state and its parent is
notified via _w_a_i_t(2). When the child is in the stopped
state, its core image can be examined and modified using
_p_t_r_a_c_e. If desired, another _p_t_r_a_c_e request can then cause
the child either to terminate or to continue, possibly
ignoring the signal.
The value of the _r_e_q_u_e_s_t argument determines the precise
action of the call:
PT_TRACE_ME
This request is the only one used by the child pro-
cess; 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 _a_d_d_r
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. _A_d_d_r must be even on some
machines. The child must be stopped. The input _d_a_t_a
is ignored.
PT_READ_U
The word of the system's per-process data area corre-
sponding to _a_d_d_r is returned. _A_d_d_r must be even on
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some machines and less than 512. This space contains
the registers and other information about the process;
its layout corresponds to the _u_s_e_r structure in the
system.
PT_WRITE_I, PT_WRITE_D
The given _d_a_t_a is written at the word in the process's
address space corresponding to _a_d_d_r_, 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 float-
ing point status and registers, and certain bits of
the processor status word.
PT_CONTINUE
The _d_a_t_a argument is taken as a signal number and the
child's execution continues at location _a_d_d_r 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 _a_d_d_r is (int *)1 then exe-
cution continues from where it stopped.
PT_KILL
The traced process terminates.
PT_STEP
Execution continues as in request PT_CONTINUE; how-
ever, as soon as possible after execution of at least
one instruction, execution stops again. The signal
number from the stop is SIGTRAP. (On the VAX-11 the
T-bit is used and just one instruction is executed.)
This is part of the mechanism for implementing break-
points.
PT_ATTACH
The process indicated by _p_i_d is re-parented to the
calling process and delivered a SIGSTOP signal. The
child process may then be traced by the parent, as in
PT_TRACE_ME. A process already being traced cannot be
attached to.
If the calling process is not owned by root, it must
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have the same real user ID as the target process and
not have used set user or group priviledges.
PT_DETACH
The process indicated by _p_i_d is detached from tracing
and continues its execution. The process, which must
be a traced child of the caller, is re-parented with
the parent it had before tracing began. The _d_a_t_a and
_a_d_d_r arguments behave as in PT_CONTINUE.
As indicated, these calls (except for request PT_TRACE_ME)
can be used only when the subject process has stopped.
The _w_a_i_t call is used to determine when a process stops;
in such a case the "termination" status returned by _w_a_i_t
has the value 0177 to indicate stoppage rather than gen-
uine termination.
To forestall possible fraud, _p_t_r_a_c_e inhibits the set-user-
id and set-group-id facilities on subsequent _e_x_e_c_v_e(2)
calls. If a traced process calls _e_x_e_c_v_e, it will stop
before executing the first instruction of the new image
showing signal SIGTRAP.
On a VAX-11, "word" also means a 32-bit integer, but the
"even" restriction does not apply.
RREETTUURRNN VVAALLUUEE
A 0 value is returned if the call succeeds. If the call
fails then a -1 is returned and the global variable _e_r_r_n_o
is set to indicate the error.
EERRRROORRSS
[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.
SSEEEE AALLSSOO
wait(2), sigvec(2), adb(1)
BBUUGGSS
_P_t_r_a_c_e 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 _i_o_c_t_l(2)
calls on this file. This would be simpler to understand
and have much better performance.
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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;
_e_r_r_n_o_, (see _i_n_t_r_o(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 environ-
ment could be provided.
PT_STEP is not supported on all architectures. For exam-
ple, the SPARC architecture does not have a trace bit,
which complicates single instruction stepping. Debuggers
and the like can emulate PT_STEP by placing breakpoints at
all possible locations of the next instruction.
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