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PTRACE(2) BSD System Calls Manual PTRACE(2)
ptrace -- process tracing and debugging
ptrace(int request, pid_t pid, caddr_t addr, int data);
ptrace() provides tracing and debugging facilities. It allows one
process (the tracing process) to control another (the traced process).
Most of the time, the traced process runs normally, but when it receives
a signal (see sigaction(2)), it stops. The tracing process is expected
to notice this via wait(2) or the delivery of a SIGCHLD signal, examine
the state of the stopped process, and cause it to terminate or continue
as appropriate. ptrace() is the mechanism by which all this happens.
The request argument specifies what operation is being performed; the
meaning of the rest of the arguments depends on the operation, but except
for one special case noted below, all ptrace() calls are made by the
tracing process, and the pid argument specifies the process ID of the
traced process. request can be:
PT_TRACE_ME This request is one of two used by the traced process; it
declares that the process expects to be traced by its par-parent.
ent. All the other arguments are ignored. (If the parent
process does not expect to trace the child, it will proba-probably
bly be rather confused by the results; once the traced
process stops, it cannot be made to continue except via
ptrace().) When a process has used this request and calls
execve(2) or any of the routines built on it (such as
execv(3)), it will stop before executing the first instruc-instruction
tion of the new image. Also, any setuid or setgid bits on
the executable being executed will be ignored.
This request is the other operation used by the traced
process; it allows a process that is not currently being
traced to deny future traces by its parent. All other
arguments are ignored. If the process is currently being
traced, it will exit with the exit status of ENOTSUP; oth-otherwise,
erwise, it sets a flag that denies future traces. An
attempt by the parent to trace a process which has set this
flag will result in a segmentation violation in the parent.
PT_CONTINUE The traced process continues execution. addr is an address
specifying the place where execution is to be resumed (a
new value for the program counter), or (caddr_t)1 to indi-indicate
cate that execution is to pick up where it left off. data
provides a signal number to be delivered to the traced
process as it resumes execution, or 0 if no signal is to be
PT_STEP The traced process continues execution for a single step.
The parameters are identical to those passed to
PT_KILL The traced process terminates, as if PT_CONTINUE had been
used with SIGKILL given as the signal to be delivered.
PT_ATTACH This request allows a process to gain control of an other-otherwise
wise unrelated process and begin tracing it. It does not
need any cooperation from the to-be-traced process. In
this case, pid specifies the process ID of the to-be-traced
process, and the other two arguments are ignored. This
request requires that the target process must have the same
real UID as the tracing process, and that it must not be
executing a setuid or setgid executable. (If the tracing
process is running as root, these restrictions do not
apply.) The tracing process will see the newly-traced
process stop and may then control it as if it had been
traced all along.
PT_DETACH This request is like PT_CONTINUE, except that it does not
allow specifying an alternate place to continue execution,
and after it succeeds, the traced process is no longer
traced and continues execution normally.
Some requests can cause ptrace() to return -1 as a non-error value; to
disambiguate, errno can be set to 0 before the call and checked after-afterwards.
wards. The possible errors are:
No process having the specified process ID exists.
oo A process attempted to use PT_ATTACH on itself.
oo The request was not one of the legal requests.
oo The signal number (in data) to PT_CONTINUE was neither 0 nor a
legal signal number.
oo PT_GETREGS, PT_SETREGS, PT_GETFPREGS, or PT_SETFPREGS was
attempted on a process with no valid register set. (This is
normally true only of system processes.)
oo PT_ATTACH was attempted on a process that was already being
oo A request attempted to manipulate a process that was being
traced by some process other than the one making the request.
oo A request (other than PT_ATTACH) specified a process that
oo A request (other than PT_ATTACH) attempted to manipulate a
process that wasn't being traced at all.
oo An attempt was made to use PT_ATTACH on a process in violation
of the requirements listed under PT_ATTACH above.
BSD November 7, 1994 BSD