Linux man pages : ptrace (2)
PTRACE(2) Linux Programmer's Manual PTRACE(2)
ptrace - process trace
long ptrace(enum __ptrace_request request, pid_t pid, void *addr, void
The ptrace system call provides a means by which a parent process may
observe and control the execution of another process, and examine and
change its core image and registers. It is primarily used to implement
breakpoint debugging and system call tracing.
The parent can initiate a trace by calling fork(2) and having the
resulting child do a PTRACE_TRACEME, followed (typically) by an
exec(3). Alternatively, the parent may commence trace of an existing
process using PTRACE_ATTACH.
While being traced, the child will stop each time a signal is deliv-
ered, even if the signal is being ignored. (The exception is SIGKILL,
which has its usual effect.) The parent will be notified at its next
wait(2) and may inspect and modify the child process while it is
stopped. The parent then causes the child to continue, optionally
ignoring the delivered signal (or even delivering a different signal
When the parent is finished tracing, it can terminate the child with
PTRACE_KILL or cause it to continue executing in a normal, untraced
mode via PTRACE_DETACH.
The value of request determines the action to be performed:
Indicates that this process is to be traced by its parent. Any
signal (except SIGKILL) delivered to this process will cause it
to stop and its parent to be notified via wait. Also, all sub-
sequent calls to exec by this process will cause a SIGTRAP to be
sent to it, giving the parent a chance to gain control before
the new program begins execution. A process probably shouldn't
make this request if its parent isn't expecting to trace it.
(pid, addr, and data are ignored.)
The above request is used only by the child process; the rest are used
only by the parent. In the following requests, pid specifies the child
process to be acted on. For requests other than PTRACE_KILL, the child
process must be stopped.
Reads a word at the location addr in the child's memory, return-
ing the word as the result of the ptrace call. Linux does not
have separate text and data address spaces, so the two requests
are currently equivalent. (The argument data is ignored.)
Reads a word at offset addr in the child's USER area, which
holds the registers and other information about the process (see
<linux/user.h> and <sys/user.h>). The word is returned as the
result of the ptrace call. Typically the offset must be word-
aligned, though this might vary by architecture. (data is
Copies the word data to location addr in the child's memory. As
above, the two requests are currently equivalent.
Copies the word data to offset addr in the child's USER area.
As above, the offset must typically be word-aligned. In order
to maintain the integrity of the kernel, some modifications to
the USER area are disallowed.
Copies the child's general purpose or floating-point registers,
respectively, to location data in the parent. See
<linux/user.h> for information on the format of this data.
(addr is ignored.)
Copies the child's general purpose or floating-point registers,
respectively, from location data in the parent. As for
PTRACE_POKEUSER, some general purpose register modifications may
be disallowed. (addr is ignored.)
Restarts the stopped child process. If data is non-zero and not
SIGSTOP, it is interpreted as a signal to be delivered to the
child; otherwise, no signal is delivered. Thus, for example,
the parent can control whether a signal sent to the child is
delivered or not. (addr is ignored.)
Restarts the stopped child as for PTRACE_CONT, but arranges for
the child to be stopped at the next entry to or exit from a sys-
tem call, or after execution of a single instruction, respec-
tively. (The child will also, as usual, be stopped upon receipt
of a signal.) From the parent's perspective, the child will
appear to have been stopped by receipt of a SIGTRAP. So, for
PTRACE_SYSCALL, for example, the idea is to inspect the argu-
ments to the system call at the first stop, then do another
PTRACE_SYSCALL and inspect the return value of the system call
at the second stop. (addr is ignored.)
Sends the child a SIGKILL to terminate it. (addr and data are
Attaches to the process specified in pid, making it a traced
"child" of the current process; the behavior of the child is as
if it had done a PTRACE_TRACEME. The current process actually
becomes the parent of the child process for most purposes (e.g.,
it will receive notification of child events and appears in
ps(1) output as the child's parent), but a getppid(2) by the
child will still return the pid of the original parent. The
child is sent a SIGSTOP, but will not necessarily have stopped
by the completion of this call; use wait to wait for the child
to stop. (addr and data are ignored.)
Restarts the stopped child as for PTRACE_CONT, but first
detaches from the process, undoing the reparenting effect of
PTRACE_ATTACH, and the effects of PTRACE_TRACEME. Although per-
haps not intended, under Linux a traced child can be detached in
this way regardless of which method was used to initiate trac-
ing. (addr is ignored.)
Although arguments to ptrace are interpreted according to the prototype
given, GNU libc currently declares ptrace as a variadic function with
only the request argument fixed. This means that unneeded trailing
arguments may be omitted, though doing so makes use of undocumented
init(8), the process with pid 1, may not be traced.
The layout of the contents of memory and the USER area are quite OS-
The size of a "word" is determined by the OS variant (e.g., for 32-bit
Linux it's 32 bits, etc.).
Tracing causes a few subtle differences in the semantics of traced pro-
cesses. For example, if a process is attached to with PTRACE_ATTACH,
its original parent can no longer receive notification via wait when it
stops, and there is no way for the new parent to effectively simulate
This page documents the way the ptrace call works currently in Linux.
Its behavior differs noticeably on other flavors of Unix. In any case,
use of ptrace is highly OS- and architecture-specific.
The SunOS man page describes ptrace as "unique and arcane", which it
is. The proc-based debugging interface present in Solaris 2 implements
a superset of ptrace functionality in a more powerful and uniform way.
On success, PTRACE_PEEK* requests return the requested data, while
other requests return zero. On error, all requests return -1, and
errno(3) is set appropriately. Since the value returned by a success-
ful PTRACE_PEEK* request may be -1, the caller must check errno after
such requests to determine whether or not an error occurred.
EPERM The specified process cannot be traced. This could be because
the parent has insufficient privileges; non-root processes can-
not trace processes that they cannot send signals to or those
running setuid/setgid programs, for obvious reasons. Alterna-
tively, the process may already be being traced, or be init (pid
ESRCH The specified process does not exist, or is not currently being
traced by the caller, or is not stopped (for requests that
EIO request is invalid, or an attempt was made to read from or write
to an invalid area in the parent's or child's memory, or there
was a word-alignment violation, or an invalid signal was speci-
fied during a restart request.
EFAULT There was an attempt to read from or write to an invalid area in
the parent's or child's memory, probably because the area wasn't
mapped or accessible. Unfortunately, under Linux, different
variations of this fault will return EIO or EFAULT more or less
SVr4, SVID EXT, AT&T, X/OPEN, BSD 4.3
gdb(1), strace(1), execve(2), fork(2), signal(2), wait(2), exec(3)
Linux 2.2.10 1999-11-07 PTRACE(2)