PTRACE(II) 1/25/75 PTRACE(II)
NAME
ptrace - process trace
SYNOPSIS
(ptrace = 26.; not in assembler)
(data in r0)
sys ptrace; pid; addr; request
(value in r0)
ptrace(request, pid, addr, data);
DESCRIPTION
Ptrace 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 implemen-
tation of breakpoint debugging, but it should be adaptable
for simulation of non-UNIX environments. There are four ar-
guments 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. A process being traced behaves normally
until it encounters some signal whether internally generated
like ``illegal instruction'' or externally generated like
``interrupt.'' See signal(II) for the list. Then the
traced process enters a stopped state and its parent is no-
tified via wait(II). 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 ig-
noring the signal.
The value of the request argument determines the precise ac-
tion of the call:
0 This request is the only one used by the child process;
it declares that the process is to be traced by its par-
ent. All the other arguments are ignored. Peculiar re-
sults will ensue if the parent does not expect to trace
the child.
1,2 The word in the child process's address space at addr is
returned (in r0). Request 1 indicates the data space
(normally used); 2 indicates the instruction space (when
I and D space are separated). addr must be even. The
child must be stopped. The input data is ignored.
3 The word of the system's per-process data area corre-
sponding to addr is returned. Addr must be even and
less than 512. This space contains the registers and
other information about the process; its layout corre-
sponds to the user structure in the system.
4,5 The given data is written at the word in the process's
address space corresponding to addr, which must be even.
No useful value is returned. Request 4 specifies data
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PTRACE(II) 1/25/75 PTRACE(II)
space (normally used), 5 specifies instruction space.
Attempts to write in pure procedure result in termina-
tion of the child, instead of going through or causing
an error for the parent.
6 The process's system data is written, as it is read with
request 3. 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.
7 The data argument is taken as a signal number and the
child's execution continues as if it had incurred that
signal. Normally the signal number will be either 0 to
indicate that the signal which caused the stop should be
ignored, or that value fetched out of the process's im-
age indicating which signal caused the stop.
8 The traced process terminates.
As indicated, these calls (except for request 0) 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
facility on subsequent exec(II)
calls.
SEE ALSO
wait(II), signal(II), cdb(I)
DIAGNOSTICS
From assembler, the c-bit (error bit) is set on errors; from
C, -1 is returned and errno has the error code.
BUGS
The request 0 call should be able to specify signals which
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.
Also, it should be possible to stop a process on occurrence
of a system call; in this way a completely controlled envi-
ronment could be provided.
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