long int ptrace(enum __ptrace_request request, pid_t pid, void * addr, void * data)
DESCRIPTION
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(2).
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 delivered, 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 instead).
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:
PTRACE_TRACEME
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 subsequent 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.
PTRACE_PEEKTEXT, PTRACE_PEEKDATA
Reads a word at the location
addr
in the child's memory, returning 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. (data is ignored.)
PTRACE_PEEKUSER
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 ignored.)
PTRACE_POKETEXT, PTRACE_POKEDATA
Copies a word from location
data
in the parent's memory to location
addr
in the child's memory. As above, the two requests are currently equivalent.
PTRACE_POKEUSER
Copies a word from location
data
in the parent's memory 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.
PTRACE_GETREGS, PTRACE_GETFPREGS
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.)
PTRACE_SETREGS, PTRACE_SETFPREGS
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.)
PTRACE_CONT
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.)
PTRACE_SYSCALL, PTRACE_SINGLESTEP
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 system call, or after execution
of a single instruction, respectively. (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 arguments 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.)
PTRACE_KILL
Sends the child a SIGKILL to terminate it. (addr and data are
ignored.)
PTRACE_ATTACH
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
getpid(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.)
PTRACE_DETACH
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 perhaps not intended, under Linux a traced child
can be detached in this way regardless of which method was used to initiate
tracing. (addr is ignored.)
NOTES
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
gcc(1)
behavior.
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- and
architecture-specific.
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 processes.
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 notification.
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.
RETURN VALUE
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 successful PTRACE_PEEK*
request may be -1, the caller must check
errno
after such requests to determine whether or not an error occurred.
ERRORS
EPERM
The specified process cannot be traced. This could be because the parent has
insufficient privileges; non-root processes cannot trace processes that they
cannot send signals to or those running setuid/setgid programs, for obvious
reasons. Alternatively, the process may already be being traced, or be
init
(pid 1).
ESRCH
The specified process does not exist, or is not currently being traced by the
caller, or is not stopped (for requests that require 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 specified 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 arbitrarily.