PTRACE(2) BSD System Calls Manual PTRACE(2)
NAME
ptrace -- process tracing and debugging
SYNOPSIS
#include <sys/types.h>
#include <sys/ptrace.h>
int
ptrace(int request, pid_t pid, caddr_t addr, int data);
DESCRIPTION
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 argu-ments arguments
ments 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 parent. All the other arguments are ignored. (If the parent
process does not expect to trace the child, it will probably 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 rou-tines routines
tines built on it (such as execv(3)), it will stop before executing the first instruction
of the new image. Also, any setuid or setgid bits on the executable being executed will
be ignored.
PT_DENY_ATTACH
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; otherwise, it sets a flag that denies future traces. An attempt by the par-ent parent
ent 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-cate 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 sent.
PT_STEP The traced process continues execution for a single step. The parameters are identical
to those passed to PT_CONTINUE.
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 otherwise 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 argu-ments arguments
ments 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 exe-cutable. executable.
cutable. (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.
ERRORS
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 afterwards. The possible errors are:
[ESRCH]
No process having the specified process ID exists.
[EINVAL]
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.)
[EBUSY]
oo PT_ATTACH was attempted on a process that was already being traced.
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 wasn't stopped.
[EPERM]
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
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