PTRACE(2) System Calls Manual PTRACE(2)
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 argu-
ment. 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 exter-
nally 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 registers and other information about the process; its layout corresponds to the user structure in the
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 file.
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 gen-
eral 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
The traced process terminates.
Execution continues as in request PT_CONTINUE; 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 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 PT_TRACE_ME) 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 and set-group-id facilities on subsequent execve(2) calls. If a traced
process calls execve, 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.
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
[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 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
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.
4th Berkeley Distribution May 23, 1986 PTRACE(2)