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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 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:
|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 shouldnt make this request if its parent isnt 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 childs 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. (The argument data is ignored.)|
|Reads a word at offset addr in the childs 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. See NOTES. (data is ignored.)|
|Copies the word data to location addr in the childs memory. As above, the two requests are currently equivalent.|
|Copies the word data to offset addr in the childs 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 childs 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_GETSIGINFO (since Linux 2.3.99-pre6)|
|Retrieve information about the signal that caused the stop. Copies a siginfo_t structure (see sigaction(2)) from the child to location data in the parent. (addr is ignored.)|
|Copies the childs 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_SETSIGINFO (since Linux 2.3.99-pre6)|
|Set signal information. Copies a siginfo_t structure from location data in the parent to the child. This will only affect signals that would normally be delivered to the child 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.)|
|PTRACE_SETOPTIONS (since Linux 2.4.6; see BUGS for caveats)|
Sets ptrace options from data in the parent.
(addr is ignored.)
data is interpreted
as a bitmask of options, which are specified by the following flags:
|PTRACE_GETEVENTMSG (since Linux 2.5.46)|
|Retrieve a message (as an unsigned long) about the ptrace event that just happened, placing it in the location data in the parent. For PTRACE_EVENT_EXIT this is the childs exit status. For PTRACE_EVENT_FORK, PTRACE_EVENT_VFORK and PTRACE_EVENT_CLONE this is the PID of the new process. (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 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 parents 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_SYSEMU, PTRACE_SYSEMU_SINGLESTEP (since Linux 2.6.14)|
|For PTRACE_SYSEMU, continue and stop on entry to the next syscall, which will not be executed. For PTRACE_SYSEMU_SINGLESTEP, do the same but also singlestep if not a syscall. This call is used by programs like User Mode Linux that want to emulate all the the childs syscalls. (addr and data are ignored; not supported on all architectures.)|
|Sends the child a SIGKILL to terminate it. (addr and data are ignored.)|
|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 childs 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 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.)|
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 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 OS variant (e.g., for 32-bit Linux its 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.
|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 parents or childs memory, probably because the area wasnt mapped or accessible. Unfortunately, under Linux, different 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 parents or childs 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 parent has insufficient privileges (the required capability is CAP_SYS_PTRACE); non-root processes cannot trace processes that they cannot send signals to or those running set-user-ID/set-group-ID 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).|
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