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.TH PTRACE 2 "2 July 1983"
.UC 4
ptrace \- process trace
.ft B
#include <signal.h>
.ft B
ptrace(request, pid, addr, data)
int request, pid, *addr, data;
.I 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
.I request
.I 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 \*(lqillegal instruction\*(rq or externally
generated like \*(lqinterrupt\*(rq.
.IR sigvec (2)
for the list.
Then the traced process enters a stopped state
and its parent is notified via
.IR  wait (2).
When the child is in the stopped state,
its core image can be examined and modified
.IR ptrace .
If desired, another
.I ptrace
request can then cause the child either to terminate
or to continue, possibly ignoring the signal.
The value of the
.I request
argument determines the precise
action of the call:
.TP 4
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.
.TP 4
word in the child process's address space
.I addr
is returned.
If I and D space are separated (e.g. historically
on a pdp-11), request 1 indicates I space,
2 D space.
.I Addr
must be even.
The child must be stopped.
The input
.I data
is ignored.
.TP 4
The word
of the system's per-process data area corresponding to
.I addr
is returned.
.I Addr
must be even and less than 512.
This space contains the registers and other information about
the process;
its layout corresponds to the
.I user
structure in the system.
.TP 4
.I data
is written at the word in the process's address space corresponding to
.I 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.
.TP 4
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.
.TP 4
.I data
argument is taken as a signal number
and the child's execution continues
at location
.I 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.
.I addr
is (int *)1 then execution continues from where it stopped.
.TP 4
The traced process terminates.
.TP 4
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
(On the VAX-11 the T-bit is used and just one instruction
is executed.)
This is part of the mechanism for implementing breakpoints.
As indicated,
these calls
(except for request 0)
can be used only when the subject process has stopped.
.I wait
call is used to determine
when a process stops;
in such a case the \*(lqtermination\*(rq status
returned by
.I wait
has the value 0177 to indicate stoppage rather
than genuine termination.
To forestall possible fraud,
.I ptrace
inhibits the set-user-id and set-group-id facilities
on subsequent
.IR  execve (2)
If a traced process calls
.IR execve ,
it will stop before executing the first instruction of the new image
showing signal SIGTRAP.
On a VAX-11, \*(lqword\*(rq also means a 32-bit integer,
but the \*(lqeven\*(rq
restriction does not apply.
A 0 value is returned if the call succeeds.  If the call fails
then a \-1 is returned and the global variable \fIerrno\fP is
set to indicate the error.
.TP 15
The request code is invalid.
.TP 15
The specified process does not exist.
.TP 15
The given signal number is invalid.
.TP 15
The specified address is out of bounds.
.TP 15
The specified process cannot be traced.
wait(2), sigvec(2), adb(1)
.I Ptrace
is unique and arcane; it should be replaced with a special file which
can be opened and read and written.  The control functions could then
be implemented with
.IR ioctl (2)
calls on this file.  This would be simpler to understand and have much
higher performance.
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 \*(lqillegal instruction\*(rq signals at a very high rate)
could be efficiently debugged.
The error indication, \-1, is a legitimate function value;
.I errno,
.IR intro (2),
can be used to disambiguate.
It should be possible to stop a process on occurrence of a system
in this way a completely controlled environment could
be provided.