ptrace - process trace
ptrace(request, pid, addr, data)
int request, pid, *addr, data;
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. 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:
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.
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.
The word of the system's per-process data area corresponding to addr is returned.
Addr must be even on some machines and less than 512. This space contains the regis-
ters and other information about the process; its layout corresponds to the user
structure in the system.
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
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.
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.
The traced process terminates.
Execution continues as in request PT_CONTINUE; however, as soon as possible after exe-
cution 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 exe-
cuted.) 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 sub-
ject 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 stop-
page 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 exe-
cuting 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
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.
[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.
wait(2), sigvec(2), adb(1)
On the PDP-11 the PT_WRITE_U request may also write the child process's current overlay
number in the system data area; the T-bit is used to single step the processor and just
one instruction is executed for the PT_STEP request; a "word" means a 16-bit integer, and
the "even" restriction does apply.
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 nor-
mally 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
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 com-
pletely controlled environment could be provided.
4th Berkeley Distribution May 23, 1986 PTRACE(2)