NAME
ptrace - process trace SYNOPSIS
#include
long ptrace(enum ptracerequest request, pidt pid, void *addr, void *data); DESCRIPTION The ptrace() system call provides a means by which one process (the "tracer") may observe and control the execution of another process (the "tracee"), and examine and change the tracee's memory and registers. It is primarily used to implement breakpoint debugging and system call tracing. A tracee first needs to be attached to the tracer. Attachment and sub‐ sequent commands are per thread: in a multithreaded process, every thread can be individually attached to a (potentially different) tracer, or left not attached and thus not debugged. Therefore, "tracee" always means "(one) thread", never "a (possibly multithreaded) process". Ptrace commands are always sent to a specific tracee using a call of the form ptrace(PTRACEfoo, pid, ...) where pid is the thread ID of the corresponding Linux thread. (Note that in this page, a "multithreaded process" means a thread group consisting of threads created using the clone(2) CLONETHREAD flag.) A process can initiate a trace by calling fork(2) and having the resulting child do a PTRACETRACEME, followed (typically) by an execve(2). Alternatively, one process may commence tracing another process using PTRACEATTACH or PTRACESEIZE. While being traced, the tracee will stop each time a signal is deliv‐ ered, even if the signal is being ignored. (An exception is SIGKILL, which has its usual effect.) The tracer will be notified at its next call to waitpid(2) (or one of the related "wait" system calls); that call will return a status value containing information that indicates the cause of the stop in the tracee. While the tracee is stopped, the tracer can use various ptrace requests to inspect and modify the tracee. The tracer then causes the tracee to continue, optionally ignoring the delivered signal (or even delivering a different signal instead). If the PTRACEOTRACEEXEC option is not in effect, all successful calls to execve(2) by the traced process will cause it to be sent a SIGTRAP signal, giving the parent a chance to gain control before the new pro‐ gram begins execution. When the tracer is finished tracing, it can cause the tracee to con‐ tinue executing in a normal, untraced mode via PTRACEDETACH. The value of request determines the action to be performed: PTRACETRACEME Indicate that this process is to be traced by its parent. A process probably shouldn't make this request if its parent isn't expecting to trace it. (pid, addr, and data are ignored.) The PTRACETRACEME request is used only by the tracee; the remaining requests are used only by the tracer. In the following requests, pid specifies the thread ID of the tracee to be acted on. For requests other than PTRACEATTACH, PTRACESEIZE, PTRACEINTERRUPT and PTRACEKILL, the tracee must be stopped. PTRACEPEEKTEXT, PTRACEPEEKDATA Read a word at the address addr in the tracee's memory, return‐ ing the word as the result of the ptrace() call. Linux does not have separate text and data address spaces, so these two requests are currently equivalent. (data is ignored.) PTRACEPEEKUSER Read a word at offset addr in the tracee's USER area, which holds the registers and other information about the process (see ). The word is returned as the result of the ptrace() call. Typically, the offset must be word-aligned, though this might vary by architecture. See NOTES. (data is ignored.) PTRACEPOKETEXT, PTRACEPOKEDATA Copy the word data to the address addr in the tracee's memory. As for PTRACEPEEKTEXT and PTRACEPEEKDATA, these two requests are currently equivalent. PTRACEPOKEUSER Copy the word data to offset addr in the tracee's USER area. As
for PTRACEPEEKUSER, the offset must typically be word-aligned. In order to maintain the integrity of the kernel, some modifica‐ tions to the USER area are disallowed. PTRACEGETREGS, PTRACEGETFPREGS
Copy the tracee's general-purpose or floating-point registers, respectively, to the address data in the tracer. See
for information on the format of this data. (addr is ignored.) Note that SPARC systems have the meaning of data and addr reversed; that is, data is ignored and the registers are copied to the address addr. PTRACEGETREGS and PTRACEGETF‐ PREGS are not present on all architectures. PTRACEGETREGSET (since Linux 2.6.34) Read the tracee's registers. addr specifies, in an architec‐ ture-dependent way, the type of registers to be read. NTPRSTA‐ TUS (with numerical value 1) usually results in reading of gen‐
eral-purpose registers. If the CPU has, for example, floating- point and/or vector registers, they can be retrieved by setting addr to the corresponding NTfoo constant. data points to a struct iovec, which describes the destination buffer's location and length. On return, the kernel modifies iov.len to indicate the actual number of bytes returned. PTRACESETREGS, PTRACESETFPREGS
Modify the tracee's general-purpose or floating-point registers, respectively, from the address data in the tracer. As for
PTRACEPOKEUSER, some general-purpose register modifications may be disallowed. (addr is ignored.) Note that SPARC systems have the meaning of data and addr reversed; that is, data is ignored and the registers are copied from the address addr. PTRACESETREGS and PTRACESETFPREGS are not present on all architectures. PTRACESETREGSET (since Linux 2.6.34) Modify the tracee's registers. The meaning of addr and data is analogous to PTRACEGETREGSET.
PTRACEGETSIGINFO (since Linux 2.3.99-pre6) Retrieve information about the signal that caused the stop. Copy a siginfot structure (see sigaction(2)) from the tracee to the address data in the tracer. (addr is ignored.)
PTRACESETSIGINFO (since Linux 2.3.99-pre6) Set signal information: copy a siginfot structure from the address data in the tracer to the tracee. This will affect only signals that would normally be delivered to the tracee and were caught by the tracer. It may be difficult to tell these normal signals from synthetic signals generated by ptrace() itself. (addr is ignored.) PTRACESETOPTIONS (since Linux 2.4.6; see BUGS for caveats) Set ptrace options from data. (addr is ignored.) data is interpreted as a bit mask of options, which are specified by the following flags: PTRACEOEXITKILL (since Linux 3.8) If a tracer sets this flag, a SIGKILL signal will be sent to every tracee if the tracer exits. This option is use‐ ful for ptrace jailers that want to ensure that tracees can never escape the tracer's control. PTRACEOTRACECLONE (since Linux 2.5.46) Stop the tracee at the next clone(2) and automatically start tracing the newly cloned process, which will start with a SIGSTOP, or PTRACEEVENTSTOP if PTRACESEIZE was used. A waitpid(2) by the tracer will return a status value such that status>>8 == (SIGTRAP | (PTRACEEVENTCLONE<<8)) The PID of the new process can be retrieved with PTRACEGETEVENTMSG. This option may not catch clone(2) calls in all cases. If the tracee calls clone(2) with the CLONEVFORK flag, PTRACEEVENTVFORK will be delivered instead if PTRACEOTRACEVFORK is set; otherwise if the tracee calls clone(2) with the exit signal set to SIGCHLD, PTRACEEVENTFORK will be delivered if PTRACEOTRACEFORK is set. PTRACEOTRACEEXEC (since Linux 2.5.46) Stop the tracee at the next execve(2). A waitpid(2) by the tracer will return a status value such that status>>8 == (SIGTRAP | (PTRACEEVENTEXEC<<8)) If the execing thread is not a thread group leader, the thread ID is reset to thread group leader's ID before this stop. Since Linux 3.0, the former thread ID can be retrieved with PTRACEGETEVENTMSG. PTRACEOTRACEEXIT (since Linux 2.5.60) Stop the tracee at exit. A waitpid(2) by the tracer will return a status value such that status>>8 == (SIGTRAP | (PTRACEEVENTEXIT<<8)) The tracee's exit status can be retrieved with PTRACEGETEVENTMSG. The tracee is stopped early during process exit, when registers are still available, allowing the tracer to see where the exit occurred, whereas the normal exit notifi‐ cation is done after the process is finished exiting. Even though context is available, the tracer cannot pre‐ vent the exit from happening at this point. PTRACEOTRACEFORK (since Linux 2.5.46) Stop the tracee at the next fork(2) and automatically start tracing the newly forked process, which will start with a SIGSTOP, or PTRACEEVENTSTOP if PTRACESEIZE was used. A waitpid(2) by the tracer will return a status value such that status>>8 == (SIGTRAP | (PTRACEEVENTFORK<<8)) The PID of the new process can be retrieved with PTRACEGETEVENTMSG. PTRACEOTRACESYSGOOD (since Linux 2.4.6) When delivering system call traps, set bit 7 in the sig‐ nal number (i.e., deliver SIGTRAP|0x80). This makes it easy for the tracer to distinguish normal traps from those caused by a system call. (PTRACEOTRACESYSGOOD may not work on all architectures.) PTRACEOTRACEVFORK (since Linux 2.5.46) Stop the tracee at the next vfork(2) and automatically start tracing the newly vforked process, which will start with a SIGSTOP, or PTRACEEVENTSTOP if PTRACESEIZE was used. A waitpid(2) by the tracer will return a status value such that status>>8 == (SIGTRAP | (PTRACEEVENTVFORK<<8)) The PID of the new process can be retrieved with PTRACEGETEVENTMSG. PTRACEOTRACEVFORKDONE (since Linux 2.5.60) Stop the tracee at the completion of the next vfork(2). A waitpid(2) by the tracer will return a status value such that status>>8 == (SIGTRAP | (PTRACEEVENTVFORKDONE<<8)) The PID of the new process can (since Linux 2.6.18) be retrieved with PTRACEGETEVENTMSG. PTRACEGETEVENTMSG (since Linux 2.5.46) Retrieve a message (as an unsigned long) about the ptrace event that just happened, placing it at the address data in the tracer. For PTRACEEVENTEXIT, this is the tracee's exit sta‐ tus. For PTRACEEVENTFORK, PTRACEEVENTVFORK, PTRACEEVENTVFORKDONE, and PTRACEEVENTCLONE, this is the PID of the new process. (addr is ignored.) PTRACECONT Restart the stopped tracee process. If data is nonzero, it is interpreted as the number of a signal to be delivered to the tracee; otherwise, no signal is delivered. Thus, for example, the tracer can control whether a signal sent to the tracee is delivered or not. (addr is ignored.) PTRACESYSCALL, PTRACESINGLESTEP Restart the stopped tracee as for PTRACECONT, but arrange for the tracee to be stopped at the next entry to or exit from a system call, or after execution of a single instruction, respec‐ tively. (The tracee will also, as usual, be stopped upon receipt of a signal.) From the tracer's perspective, the tracee will appear to have been stopped by receipt of a SIGTRAP. So, for PTRACESYSCALL, for example, the idea is to inspect the arguments to the system call at the first stop, then do another PTRACESYSCALL and inspect the return value of the system call at the second stop. The data argument is treated as for PTRACECONT. (addr is ignored.) PTRACESYSEMU, PTRACESYSEMUSINGLESTEP (since Linux 2.6.14) For PTRACESYSEMU, continue and stop on entry to the next system call, which will not be executed. For PTRACESYSEMUSINGLESTEP, do the same but also singlestep if not a system call. This call is used by programs like User Mode Linux that want to emulate all the tracee's system calls. The data argument is treated as for PTRACECONT. The addr argument is ignored. These requests are currently supported only on x86. PTRACELISTEN (since Linux 3.4) Restart the stopped tracee, but prevent it from executing. The resulting state of the tracee is similar to a process which has been stopped by a SIGSTOP (or other stopping signal). See the
"group-stop" subsection for additional information. PTRACELIS‐ TEN works only on tracees attached by PTRACESEIZE. PTRACEKILL Send the tracee a SIGKILL to terminate it. (addr and data are ignored.) This operation is deprecated; do not use it! Instead, send a SIGKILL directly using kill(2) or tgkill(2). The problem with
PTRACEKILL is that it requires the tracee to be in signal-
delivery-stop, otherwise it may not work (i.e., may complete successfully but won't kill the tracee). By contrast, sending a SIGKILL directly has no such limitation. PTRACEINTERRUPT (since Linux 3.4) Stop a tracee. If the tracee is running or sleeping in kernel space and PTRACESYSCALL is in effect, the system call is inter‐
rupted and syscall-exit-stop is reported. (The interrupted sys‐ tem call is restarted when the tracee is restarted.) If the tracee was already stopped by a signal and PTRACELISTEN was sent to it, the tracee stops with PTRACEEVENTSTOP and WSTOP‐
SIG(status) returns the stop signal. If any other ptrace-stop is generated at the same time (for example, if a signal is sent
to the tracee), this ptrace-stop happens. If none of the above applies (for example, if the tracee is running in userspace), it stops with PTRACEEVENTSTOP with WSTOPSIG(status) == SIGTRAP. PTRACEINTERRUPT only works on tracees attached by PTRACESEIZE. PTRACEATTACH Attach to the process specified in pid, making it a tracee of the calling process. The tracee is sent a SIGSTOP, but will not necessarily have stopped by the completion of this call; use waitpid(2) to wait for the tracee to stop. See the "Attaching and detaching" subsection for additional information. (addr and data are ignored.) PTRACESEIZE (since Linux 3.4) Attach to the process specified in pid, making it a tracee of the calling process. Unlike PTRACEATTACH, PTRACESEIZE does not stop the process. Only a PTRACESEIZEd process can accept PTRACEINTERRUPT and PTRACELISTEN commands. addr must be zero. data contains a bit mask of ptrace options to activate immedi‐ ately. PTRACEDETACH Restart the stopped tracee as for PTRACECONT, but first detach from it. Under Linux, a tracee can be detached in this way regardless of which method was used to initiate tracing. (addr is ignored.) Death under ptrace When a (possibly multithreaded) process receives a killing signal (one whose disposition is set to SIGDFL and whose default action is to kill the process), all threads exit. Tracees report their death to their tracer(s). Notification of this event is delivered via waitpid(2).
Note that the killing signal will first cause signal-delivery-stop (on one tracee only), and only after it is injected by the tracer (or after it was dispatched to a thread which isn't traced), will death from the signal happen on all tracees within a multithreaded process. (The term
"signal-delivery-stop" is explained below.)
SIGKILL does not generate signal-delivery-stop and therefore the tracer
can't suppress it. SIGKILL kills even within system calls (syscall-
exit-stop is not generated prior to death by SIGKILL). The net effect is that SIGKILL always kills the process (all its threads), even if some threads of the process are ptraced. When the tracee calls exit(2), it reports its death to its tracer. Other threads are not affected. When any thread executes exitgroup(2), every tracee in its thread group reports its death to its tracer. If the PTRACEOTRACEEXIT option is on, PTRACEEVENTEXIT will happen before actual death. This applies to exits via exit(2), exitgroup(2), and signal deaths (except SIGKILL), and when threads are torn down on execve(2) in a multithreaded process.
The tracer cannot assume that the ptrace-stopped tracee exists. There are many scenarios when the tracee may die while stopped (such as SIGKILL). Therefore, the tracer must be prepared to handle an ESRCH error on any ptrace operation. Unfortunately, the same error is
returned if the tracee exists but is not ptrace-stopped (for commands which require a stopped tracee), or if it is not traced by the process which issued the ptrace call. The tracer needs to keep track of the stopped/running state of the tracee, and interpret ESRCH as "tracee died unexpectedly" only if it knows that the tracee has been observed
to enter ptrace-stop. Note that there is no guarantee that wait‐ pid(WNOHANG) will reliably report the tracee's death status if a ptrace operation returned ESRCH. waitpid(WNOHANG) may return 0 instead. In other words, the tracee may be "not yet fully dead", but already refus‐ ing ptrace requests. The tracer can't assume that the tracee always ends its life by report‐ ing WIFEXITED(status) or WIFSIGNALED(status); there are cases where this does not occur. For example, if a thread other than thread group leader does an execve(2), it disappears; its PID will never be seen again, and any subsequent ptrace stops will be reported under the thread group leader's PID. Stopped states A tracee can be in two states: running or stopped. For the purposes of ptrace, a tracee which is blocked in a system call (such as read(2), pause(2), etc.) is nevertheless considered to be running, even if the tracee is blocked for a long time. The state of the tracee after
PTRACELISTEN is somewhat of a gray area: it is not in any ptrace-stop (ptrace commands won't work on it, and it will deliver waitpid(2) noti‐ fications), but it also may be considered "stopped" because it is not
executing instructions (is not scheduled), and if it was in group-stop before PTRACELISTEN, it will not respond to signals until SIGCONT is received. There are many kinds of states when the tracee is stopped, and in ptrace discussions they are often conflated. Therefore, it is impor‐ tant to use precise terms. In this manual page, any stopped state in which the tracee is ready to
accept ptrace commands from the tracer is called ptrace-stop. Ptrace-
stops can be further subdivided into signal-delivery-stop, group-stop,
syscall-stop, and so on. These stopped states are described in detail below.
When the running tracee enters ptrace-stop, it notifies its tracer using waitpid(2) (or one of the other "wait" system calls). Most of this manual page assumes that the tracer waits with: pid = waitpid(pidorminus1, &status, WALL);
Ptrace-stopped tracees are reported as returns with pid greater than 0 and WIFSTOPPED(status) true. The WALL flag does not include the WSTOPPED and WEXITED flags, but implies their functionality. Setting the WCONTINUED flag when calling waitpid(2) is not recommended:
the "continued" state is per-process and consuming it can confuse the real parent of the tracee. Use of the WNOHANG flag may cause waitpid(2) to return 0 ("no wait results available yet") even if the tracer knows there should be a notification. Example: errno = 0; ptrace(PTRACECONT, pid, 0L, 0L); if (errno == ESRCH) { /* tracee is dead */ r = waitpid(tracee, &status, WALL | WNOHANG); /* r can still be 0 here! */ }
The following kinds of ptrace-stops exist: signal-delivery-stops,
group-stops, PTRACEEVENT stops, syscall-stops. They all are reported by waitpid(2) with WIFSTOPPED(status) true. They may be differentiated by examining the value status>>8, and if there is ambiguity in that value, by querying PTRACEGETSIGINFO. (Note: the WSTOPSIG(status) macro can't be used to perform this examination, because it returns the value (status>>8) & 0xff.)
Signal-delivery-stop When a (possibly multithreaded) process receives any signal except SIGKILL, the kernel selects an arbitrary thread which handles the sig‐ nal. (If the signal is generated with tgkill(2), the target thread can be explicitly selected by the caller.) If the selected thread is
traced, it enters signal-delivery-stop. At this point, the signal is not yet delivered to the process, and can be suppressed by the tracer. If the tracer doesn't suppress the signal, it passes the signal to the tracee in the next ptrace restart request. This second step of signal delivery is called signal injection in this manual page. Note that if
the signal is blocked, signal-delivery-stop doesn't happen until the signal is unblocked, with the usual exception that SIGSTOP can't be blocked.
Signal-delivery-stop is observed by the tracer as waitpid(2) returning with WIFSTOPPED(status) true, with the signal returned by WSTOPSIG(sta‐ tus). If the signal is SIGTRAP, this may be a different kind of
ptrace-stop; see the "Syscall-stops" and "execve" sections below for details. If WSTOPSIG(status) returns a stopping signal, this may be a
group-stop; see below. Signal injection and suppression
After signal-delivery-stop is observed by the tracer, the tracer should restart the tracee with the call ptrace(PTRACErestart, pid, 0, sig) where PTRACErestart is one of the restarting ptrace requests. If sig is 0, then a signal is not delivered. Otherwise, the signal sig is delivered. This operation is called signal injection in this manual
page, to distinguish it from signal-delivery-stop. The sig value may be different from the WSTOPSIG(status) value: the tracer can cause a different signal to be injected. Note that a suppressed signal still causes system calls to return pre‐ maturely. In this case system calls will be restarted: the tracer will observe the tracee to reexecute the interrupted system call (or restartsyscall(2) system call for a few syscalls which use a different mechanism for restarting) if the tracer uses PTRACESYSCALL. Even sys‐ tem calls (such as poll(2)) which are not restartable after signal are restarted after signal is suppressed; however, kernel bugs exist which cause some syscalls to fail with EINTR even though no observable signal is injected to the tracee.
Restarting ptrace commands issued in ptrace-stops other than signal-
delivery-stop are not guaranteed to inject a signal, even if sig is nonzero. No error is reported; a nonzero sig may simply be ignored. Ptrace users should not try to "create a new signal" this way: use tgkill(2) instead. The fact that signal injection requests may be ignored when restarting
the tracee after ptrace stops that are not signal-delivery-stops is a cause of confusion among ptrace users. One typical scenario is that
the tracer observes group-stop, mistakes it for signal-delivery-stop, restarts the tracee with ptrace(PTRACErestart, pid, 0, stopsig) with the intention of injecting stopsig, but stopsig gets ignored and the tracee continues to run. The SIGCONT signal has a side effect of waking up (all threads of) a
group-stopped process. This side effect happens before signal-deliv‐
ery-stop. The tracer can't suppress this side effect (it can only sup‐ press signal injection, which only causes the SIGCONT handler to not be executed in the tracee, if such a handler is installed). In fact, wak‐
ing up from group-stop may be followed by signal-delivery-stop for sig‐ nal(s) other than SIGCONT, if they were pending when SIGCONT was deliv‐ ered. In other words, SIGCONT may be not the first signal observed by the tracee after it was sent.
Stopping signals cause (all threads of) a process to enter group-stop. This side effect happens after signal injection, and therefore can be suppressed by the tracer. In Linux 2.4 and earlier, the SIGSTOP signal can't be injected. PTRACEGETSIGINFO can be used to retrieve a siginfot structure which corresponds to the delivered signal. PTRACESETSIGINFO may be used to modify it. If PTRACESETSIGINFO has been used to alter siginfot, the sisigno field and the sig parameter in the restarting command must match, otherwise the result is undefined.
Group-stop When a (possibly multithreaded) process receives a stopping signal, all
threads stop. If some threads are traced, they enter a group-stop.
Note that the stopping signal will first cause signal-delivery-stop (on one tracee only), and only after it is injected by the tracer (or after
it was dispatched to a thread which isn't traced), will group-stop be initiated on all tracees within the multithreaded process. As usual,
every tracee reports its group-stop separately to the corresponding tracer.
Group-stop is observed by the tracer as waitpid(2) returning with WIF‐ STOPPED(status) true, with the stopping signal available via WSTOP‐ SIG(status). The same result is returned by some other classes of
ptrace-stops, therefore the recommended practice is to perform the call ptrace(PTRACEGETSIGINFO, pid, 0, &siginfo) The call can be avoided if the signal is not SIGSTOP, SIGTSTP, SIGTTIN, or SIGTTOU; only these four signals are stopping signals. If the
tracer sees something else, it can't be a group-stop. Otherwise, the tracer needs to call PTRACEGETSIGINFO. If PTRACEGETSIGINFO fails
with EINVAL, then it is definitely a group-stop. (Other failure codes are possible, such as ESRCH ("no such process") if a SIGKILL killed the tracee.)
If tracee was attached using PTRACESEIZE, group-stop is indicated by PTRACEEVENTSTOP: status>>16 == PTRACEEVENTSTOP. This allows detec‐
tion of group-stops without requiring an extra PTRACEGETSIGINFO call.
As of Linux 2.6.38, after the tracer sees the tracee ptrace-stop and until it restarts or kills it, the tracee will not run, and will not send notifications (except SIGKILL death) to the tracer, even if the tracer enters into another waitpid(2) call. The kernel behavior described in the previous paragraph causes a prob‐ lem with transparent handling of stopping signals. If the tracer
restarts the tracee after group-stop, the stopping signal is effec‐ tively ignored—the tracee doesn't remain stopped, it runs. If the tracer doesn't restart the tracee before entering into the next wait‐ pid(2), future SIGCONT signals will not be reported to the tracer; this would cause the SIGCONT signals to have no effect on the tracee. Since Linux 3.4, there is a method to overcome this problem: instead of PTRACECONT, a PTRACELISTEN command can be used to restart a tracee in a way where it does not execute, but waits for a new event which it can report via waitpid(2) (such as when it is restarted by a SIGCONT). PTRACEEVENT stops If the tracer sets PTRACEOTRACE* options, the tracee will enter
ptrace-stops called PTRACEEVENT stops. PTRACEEVENT stops are observed by the tracer as waitpid(2) returning with WIFSTOPPED(status), and WSTOPSIG(status) returns SIGTRAP. An additional bit is set in the higher byte of the status word: the value status>>8 will be (SIGTRAP | PTRACEEVENTfoo << 8). The following events exist: PTRACEEVENTVFORK Stop before return from vfork(2) or clone(2) with the CLONEVFORK flag. When the tracee is continued after this stop, it will wait for child to exit/exec before continuing its execu‐ tion (in other words, the usual behavior on vfork(2)). PTRACEEVENTFORK Stop before return from fork(2) or clone(2) with the exit signal set to SIGCHLD. PTRACEEVENTCLONE Stop before return from clone(2). PTRACEEVENTVFORKDONE Stop before return from vfork(2) or clone(2) with the CLONEVFORK flag, but after the child unblocked this tracee by exiting or execing. For all four stops described above, the stop occurs in the parent (i.e., the tracee), not in the newly created thread. PTRACEGETEVENTMSG can be used to retrieve the new thread's ID. PTRACEEVENTEXEC Stop before return from execve(2). Since Linux 3.0, PTRACEGETEVENTMSG returns the former thread ID. PTRACEEVENTEXIT Stop before exit (including death from exitgroup(2)), signal death, or exit caused by execve(2) in a multithreaded process. PTRACEGETEVENTMSG returns the exit status. Registers can be examined (unlike when "real" exit happens). The tracee is still alive; it needs to be PTRACECONTed or PTRACEDETACHed to finish exiting. PTRACEEVENTSTOP
Stop induced by PTRACEINTERRUPT command, or group-stop, or ini‐
tial ptrace-stop when a new child is attached (only if attached using PTRACESEIZE). or PTRACEEVENTSTOP if PTRACESEIZE was used. PTRACEGETSIGINFO on PTRACEEVENT stops returns SIGTRAP in sisigno, with sicode set to (event<<8) | SIGTRAP.
Syscall-stops If the tracee was restarted by PTRACESYSCALL, the tracee enters
syscall-enter-stop just prior to entering any system call. If the tracer restarts the tracee with PTRACESYSCALL, the tracee enters
syscall-exit-stop when the system call is finished, or if it is inter‐
rupted by a signal. (That is, signal-delivery-stop never happens
between syscall-enter-stop and syscall-exit-stop; it happens after
syscall-exit-stop.) Other possibilities are that the tracee may stop in a PTRACEEVENT stop, exit (if it entered exit(2) or exitgroup(2)), be killed by SIGKILL, or die silently (if it is a thread group leader, the execve(2) happened in another thread, and that thread is not traced by the same tracer; this situation is discussed later).
Syscall-enter-stop and syscall-exit-stop are observed by the tracer as waitpid(2) returning with WIFSTOPPED(status) true, and WSTOPSIG(status) giving SIGTRAP. If the PTRACEOTRACESYSGOOD option was set by the tracer, then WSTOPSIG(status) will give the value (SIGTRAP | 0x80).
Syscall-stops can be distinguished from signal-delivery-stop with SIG‐ TRAP by querying PTRACEGETSIGINFO for the following cases: sicode <= 0
SIGTRAP was delivered as a result of a user-space action, for example, a system call (tgkill(2), kill(2), sigqueue(3), etc.), expiration of a POSIX timer, change of state on a POSIX message queue, or completion of an asynchronous I/O request. sicode == SIKERNEL (0x80) SIGTRAP was sent by the kernel. sicode == SIGTRAP or sicode == (SIGTRAP|0x80)
This is a syscall-stop.
However, syscall-stops happen very often (twice per system call), and
performing PTRACEGETSIGINFO for every syscall-stop may be somewhat expensive. Some architectures allow the cases to be distinguished by examining
registers. For example, on x86, rax == -ENOSYS in syscall-enter-stop.
Since SIGTRAP (like any other signal) always happens after syscall-
exit-stop, and at this point rax almost never contains -ENOSYS, the
SIGTRAP looks like "syscall-stop which is not syscall-enter-stop"; in
other words, it looks like a "stray syscall-exit-stop" and can be detected this way. But such detection is fragile and is best avoided. Using the PTRACEOTRACESYSGOOD option is the recommended method to
distinguish syscall-stops from other kinds of ptrace-stops, since it is reliable and does not incur a performance penalty.
Syscall-enter-stop and syscall-exit-stop are indistinguishable from each other by the tracer. The tracer needs to keep track of the
sequence of ptrace-stops in order to not misinterpret syscall-enter-
stop as syscall-exit-stop or vice versa. The rule is that syscall-
enter-stop is always followed by syscall-exit-stop, PTRACEEVENT stop
or the tracee's death; no other kinds of ptrace-stop can occur in between.
If after syscall-enter-stop, the tracer uses a restarting command other
than PTRACESYSCALL, syscall-exit-stop is not generated.
PTRACEGETSIGINFO on syscall-stops returns SIGTRAP in sisigno, with sicode set to SIGTRAP or (SIGTRAP|0x80). PTRACESINGLESTEP, PTRACESYSEMU, PTRACESYSEMUSINGLESTEP stops [Details of these kinds of stops are yet to be documented.] Informational and restarting ptrace commands Most ptrace commands (all except PTRACEATTACH, PTRACESEIZE, PTRACETRACEME, PTRACEINTERRUPT, and PTRACEKILL) require the tracee
to be in a ptrace-stop, otherwise they fail with ESRCH.
When the tracee is in ptrace-stop, the tracer can read and write data to the tracee using informational commands. These commands leave the
tracee in ptrace-stopped state: ptrace(PTRACEPEEKTEXT/PEEKDATA/PEEKUSER, pid, addr, 0); ptrace(PTRACEPOKETEXT/POKEDATA/POKEUSER, pid, addr, longval); ptrace(PTRACEGETREGS/GETFPREGS, pid, 0, &struct); ptrace(PTRACESETREGS/SETFPREGS, pid, 0, &struct); ptrace(PTRACEGETREGSET, pid, NTfoo, &iov); ptrace(PTRACESETREGSET, pid, NTfoo, &iov); ptrace(PTRACEGETSIGINFO, pid, 0, &siginfo); ptrace(PTRACESETSIGINFO, pid, 0, &siginfo); ptrace(PTRACEGETEVENTMSG, pid, 0, &longvar); ptrace(PTRACESETOPTIONS, pid, 0, PTRACEOflags); Note that some errors are not reported. For example, setting signal
information (siginfo) may have no effect in some ptrace-stops, yet the call may succeed (return 0 and not set errno); querying PTRACEGETEVENTMSG may succeed and return some random value if current
ptrace-stop is not documented as returning a meaningful event message. The call ptrace(PTRACESETOPTIONS, pid, 0, PTRACEOflags); affects one tracee. The tracee's current flags are replaced. Flags
are inherited by new tracees created and "auto-attached" via active PTRACEOTRACEFORK, PTRACEOTRACEVFORK, or PTRACEOTRACECLONE options.
Another group of commands makes the ptrace-stopped tracee run. They have the form: ptrace(cmd, pid, 0, sig); where cmd is PTRACECONT, PTRACELISTEN, PTRACEDETACH, PTRACESYSCALL, PTRACESINGLESTEP, PTRACESYSEMU, or PTRACESYSEMUSINGLESTEP. If the
tracee is in signal-delivery-stop, sig is the signal to be injected (if it is nonzero). Otherwise, sig may be ignored. (When restarting a
tracee from a ptrace-stop other than signal-delivery-stop, recommended practice is to always pass 0 in sig.) Attaching and detaching A thread can be attached to the tracer using the call ptrace(PTRACEATTACH, pid, 0, 0); or ptrace(PTRACESEIZE, pid, 0, PTRACEOflags); PTRACEATTACH sends SIGSTOP to this thread. If the tracer wants this SIGSTOP to have no effect, it needs to suppress it. Note that if other signals are concurrently sent to this thread during attach, the tracer
may see the tracee enter signal-delivery-stop with other signal(s) first! The usual practice is to reinject these signals until SIGSTOP is seen, then suppress SIGSTOP injection. The design bug here is that a ptrace attach and a concurrently delivered SIGSTOP may race and the concurrent SIGSTOP may be lost. Since attaching sends SIGSTOP and the tracer usually suppresses it, this may cause a stray EINTR return from the currently executing system call in the tracee, as described in the "Signal injection and suppres‐ sion" section. Since Linux 3.4, PTRACESEIZE can be used instead of PTRACEATTACH. PTRACESEIZE does not stop the attached process. If you need to stop it after attach (or at any other time) without sending it any signals, use PTRACEINTERRUPT command. The request ptrace(PTRACETRACEME, 0, 0, 0); turns the calling thread into a tracee. The thread continues to run
(doesn't enter ptrace-stop). A common practice is to follow the PTRACETRACEME with raise(SIGSTOP);
and allow the parent (which is our tracer now) to observe our signal-
delivery-stop. If the PTRACEOTRACEFORK, PTRACEOTRACEVFORK, or PTRACEOTRACECLONE options are in effect, then children created by, respectively, vfork(2) or clone(2) with the CLONEVFORK flag, fork(2) or clone(2) with the exit signal set to SIGCHLD, and other kinds of clone(2), are automati‐ cally attached to the same tracer which traced their parent. SIGSTOP
is delivered to the children, causing them to enter signal-delivery- stop after they exit the system call which created them. Detaching of the tracee is performed by: ptrace(PTRACEDETACH, pid, 0, sig); PTRACEDETACH is a restarting operation; therefore it requires the
tracee to be in ptrace-stop. If the tracee is in signal-delivery-stop, a signal can be injected. Otherwise, the sig parameter may be silently ignored. If the tracee is running when the tracer wants to detach it, the usual solution is to send SIGSTOP (using tgkill(2), to make sure it goes to
the correct thread), wait for the tracee to stop in signal-delivery- stop for SIGSTOP and then detach it (suppressing SIGSTOP injection). A design bug is that this can race with concurrent SIGSTOPs. Another
complication is that the tracee may enter other ptrace-stops and needs to be restarted and waited for again, until SIGSTOP is seen. Yet another complication is to be sure that the tracee is not already
ptrace-stopped, because no signal delivery happens while it is—not even SIGSTOP. If the tracer dies, all tracees are automatically detached and
restarted, unless they were in group-stop. Handling of restart from
group-stop is currently buggy, but the "as planned" behavior is to leave tracee stopped and waiting for SIGCONT. If the tracee is
restarted from signal-delivery-stop, the pending signal is injected. execve(2) under ptrace When one thread in a multithreaded process calls execve(2), the kernel destroys all other threads in the process, and resets the thread ID of the execing thread to the thread group ID (process ID). (Or, to put things another way, when a multithreaded process does an execve(2), at completion of the call, it appears as though the execve(2) occurred in the thread group leader, regardless of which thread did the execve(2).) This resetting of the thread ID looks very confusing to tracers: * All other threads stop in PTRACEEVENTEXIT stop, if the PTRACEOTRACEEXIT option was turned on. Then all other threads except the thread group leader report death as if they exited via exit(2) with exit code 0. * The execing tracee changes its thread ID while it is in the execve(2). (Remember, under ptrace, the "pid" returned from wait‐ pid(2), or fed into ptrace calls, is the tracee's thread ID.) That is, the tracee's thread ID is reset to be the same as its process ID, which is the same as the thread group leader's thread ID. * Then a PTRACEEVENTEXEC stop happens, if the PTRACEOTRACEEXEC option was turned on. * If the thread group leader has reported its PTRACEEVENTEXIT stop by this time, it appears to the tracer that the dead thread leader "reappears from nowhere". (Note: the thread group leader does not report death via WIFEXITED(status) until there is at least one other live thread. This eliminates the possibility that the tracer will see it dying and then reappearing.) If the thread group leader was still alive, for the tracer this may look as if thread group leader returns from a different system call than it entered, or even "returned from a system call even though it was not in any system call". If the thread group leader was not traced (or was traced by a different tracer), then during execve(2) it will appear as if it has become a tracee of the tracer of the execing tracee. All of the above effects are the artifacts of the thread ID change in the tracee. The PTRACEOTRACEEXEC option is the recommended tool for dealing with this situation. First, it enables PTRACEEVENTEXEC stop, which occurs before execve(2) returns. In this stop, the tracer can use PTRACEGETEVENTMSG to retrieve the tracee's former thread ID. (This feature was introduced in Linux 3.0). Second, the PTRACEOTRACEEXEC option disables legacy SIGTRAP generation on execve(2). When the tracer receives PTRACEEVENTEXEC stop notification, it is guaranteed that except this tracee and the thread group leader, no other threads from the process are alive. On receiving the PTRACEEVENTEXEC stop notification, the tracer should clean up all its internal data structures describing the threads of this process, and retain only one data structure—one which describes the single still running tracee, with thread ID == thread group ID == process ID. Example: two threads call execve(2) at the same time:
*** we get syscall-enter-stop in thread 1: ** PID1 execve("/bin/foo", "foo"
*** we issue PTRACESYSCALL for thread 1 ** *** we get syscall-enter-stop in thread 2: ** PID2 execve("/bin/bar", "bar"
*** we issue PTRACESYSCALL for thread 2 ** *** we get PTRACEEVENTEXEC for PID0, we issue PTRACESYSCALL ** *** we get syscall-exit-stop for PID0: ** PID0 <... execve resumed> ) = 0 If the PTRACEOTRACEEXEC option is not in effect for the execing tracee, the kernel delivers an extra SIGTRAP to the tracee after execve(2) returns. This is an ordinary signal (similar to one which
can be generated by kill -TRAP), not a special kind of ptrace-stop. Employing PTRACEGETSIGINFO for this signal returns sicode set to 0 (SIUSER). This signal may be blocked by signal mask, and thus may be delivered (much) later. Usually, the tracer (for example, strace(1)) would not want to show
this extra post-execve SIGTRAP signal to the user, and would suppress its delivery to the tracee (if SIGTRAP is set to SIGDFL, it is a killing signal). However, determining which SIGTRAP to suppress is not easy. Setting the PTRACEOTRACEEXEC option and thus suppressing this extra SIGTRAP is the recommended approach. Real parent The ptrace API (ab)uses the standard UNIX parent/child signaling over waitpid(2). This used to cause the real parent of the process to stop receiving several kinds of waitpid(2) notifications when the child process is traced by some other process. Many of these bugs have been fixed, but as of Linux 2.6.38 several still exist; see BUGS below. As of Linux 2.6.38, the following is believed to work correctly: * exit/death by signal is reported first to the tracer, then, when the tracer consumes the waitpid(2) result, to the real parent (to the real parent only when the whole multithreaded process exits). If the tracer and the real parent are the same process, the report is sent only once. RETURN VALUE On success, PTRACEPEEK* requests return the requested data, while other requests return zero. (On Linux, this is done in the libc wrap‐ per around ptrace system call. On the system call level, PTRACEPEEK* requests have a different API: they store the result at the address specified by data parameter, and return value is the error flag.)
On error, all requests return -1, and errno is set appropriately. Since the value returned by a successful PTRACEPEEK* request may be
-1, the caller must clear errno before the call, and then check it afterward to determine whether or not an error occurred. ERRORS EBUSY (i386 only) There was an error with allocating or freeing a debug register. EFAULT There was an attempt to read from or write to an invalid area in the tracer's or the tracee's memory, probably because the area wasn't mapped or accessible. Unfortunately, under Linux, dif‐ ferent variations of this fault will return EIO or EFAULT more or less arbitrarily. EINVAL An attempt was made to set an invalid option. EIO request is invalid, or an attempt was made to read from or write to an invalid area in the tracer's or the tracee's memory, or
there was a word-alignment violation, or an invalid signal was specified during a restart request. EPERM The specified process cannot be traced. This could be because the tracer has insufficient privileges (the required capability is CAPSYSPTRACE); unprivileged processes cannot trace pro‐
cesses that they cannot send signals to or those running set-
user-ID/set-group-ID programs, for obvious reasons. Alterna‐ tively, the process may already be being traced, or (on kernels before 2.6.26) be init(8) (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 a stopped tracee). CONFORMING TO SVr4, 4.3BSD. NOTES Although arguments to ptrace() are interpreted according to the proto‐ type given, glibc currently declares ptrace() as a variadic function with only the request argument fixed. It is recommended to always sup‐ ply four arguments, even if the requested operation does not use them, setting unused/ignored arguments to 0L or (void *) 0. In Linux kernels before 2.6.26, init(8), the process with PID 1, may not be traced. The layout of the contents of memory and the USER area are quite oper‐
ating-system- and architecture-specific. The offset supplied, and the data returned, might not entirely match with the definition of struct user.
The size of a "word" is determined by the operating-system variant
(e.g., for 32-bit Linux it is 32 bits). 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 specific to the operating system and archi‐ tecture. BUGS On hosts with 2.6 kernel headers, PTRACESETOPTIONS is declared with a different value than the one for 2.4. This leads to applications com‐ piled with 2.6 kernel headers failing when run on 2.4 kernels. This can be worked around by redefining PTRACESETOPTIONS to PTRACEOLDSE‐ TOPTIONS, if that is defined.
Group-stop notifications are sent to the tracer, but not to real par‐ ent. Last confirmed on 2.6.38.6. If a thread group leader is traced and exits by calling exit(2), a PTRACEEVENTEXIT stop will happen for it (if requested), but the sub‐ sequent WIFEXITED notification will not be delivered until all other threads exit. As explained above, if one of other threads calls execve(2), the death of the thread group leader will never be reported. If the execed thread is not traced by this tracer, the tracer will never know that execve(2) happened. One possible workaround is to PTRACEDETACH the thread group leader instead of restarting it in this case. Last confirmed on 2.6.38.6. A SIGKILL signal may still cause a PTRACEEVENTEXIT stop before actual signal death. This may be changed in the future; SIGKILL is meant to always immediately kill tasks even under ptrace. Last confirmed on 2.6.38.6. Some system calls return with EINTR if a signal was sent to a tracee, but delivery was suppressed by the tracer. (This is very typical oper‐ ation: it is usually done by debuggers on every attach, in order to not introduce a bogus SIGSTOP). As of Linux 3.2.9, the following system calls are affected (this list is likely incomplete): epollwait(2), and read(2) from an inotify(7) file descriptor. The usual symptom of this bug is that when you attach to a quiescent process with the command
strace -p
then, instead of the usual and expected one-line output such as restartsyscall(<... resuming interrupted call ...> or select(6, [5], NULL, [5], NULL ('' denotes the cursor position), you observe more than one line. For example: clockgettime(CLOCKMONOTONIC, {15370, 690928118}) = 0 epollwait(4, What is not visible here is that the process was blocked in epollwait(2) before strace(1) has attached to it. Attaching caused epollwait(2) to return to user space with the error EINTR. In this particular case, the program reacted to EINTR by checking the current time, and then executing epollwait(2) again. (Programs which do not expect such "stray" EINTR errors may behave in an unintended way upon an strace(1) attach.) SEE ALSO gdb(1), strace(1), clone(2), execve(2), fork(2), gettid(2), sigac‐ tion(2), tgkill(2), vfork(2), waitpid(2), exec(3), capabilities(7), signal(7) COLOPHON
This page is part of release 3.53 of the Linux man-pages project. A description of the project, and information about reporting bugs, can
be found at http://www.kernel.org/doc/man-pages/.
Linux 2013-07-11 PTRACE(2)