PTRACE(2) System Calls Manual PTRACE(2)
ptrace - process trace
ptrace(request, pid, addr, data)
Ptrace provides a means by which a parent process may control the exe-
cution 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. A process
being traced behaves normally until it encounters some signal whether
internally generated like `illegal instruction' or externally generated
like `interrupt.' See signal(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 value of the request argument determines the precise action of the
0 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.
1,2 The word in the child process's address space at addr is returned.
If I and D space are separated, request 1 indicates I space, 2 D
space. 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 corresponding 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 corresponds 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. If I and D space are separated, request 4 indicates I
space, 5 D space. Attempts to write in pure procedure fail if
another process is executing the same file.
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 regis-
ters, 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 exe-
cution continues at location addr as if it had incurred that sig-
nal. 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
8 The traced process terminates.
9 Execution continues as in request 7; however, as soon as possible
after execution of at least one instruction, execution stops again.
The signal number from the stop is SIGTRAP. (On the PDP-11 the T-
bit is used and just one instruction is executed; on the Interdata
the stop does not take place until a store instruction is exe-
cuted.) This is part of the mechanism for implementing break-
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
To forestall possible fraud, ptrace inhibits the set-user-id facility
on subsequent exec(2) calls. If a traced process calls exec, it will
stop before executing the first instruction of the new image showing
On the Interdata 8/32, `word' means a 32-bit word and `even' means 0
wait(2), signal(2), adb(1)
The value -1 is returned if request is invalid, pid is not a traceable
process, addr is out of bounds, or data specifies an illegal signal
On the Interdata 8/32, `as soon as possible' (request 7) means `as soon
as a store instruction has been executed.'
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, pro-
grams 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.
(ptrace = 26.)
(data in r0)
sys ptrace; pid; addr; request
(value in r0)