(I spent more time on it than I spent composing that last sentence, honest!
:-)
Thanks,
--Mike
-- Any sufficiently adverse technology is indistinguishable from Microsoft.
.\" Hey Emacs! This file is -*- nroff -*- source. .\" .\" Copyright (c) 1993 Michael Haardt .\" (michael@moria.de), .\" Fri Apr 2 11:32:09 MET DST 1993 .\" .\" changes Copyright 1999 Mike Coleman (mkc@acm.org) .\" -- major revision to increase usefulness and track recent Linux kernel (2.2.10) .\" and glibc (2.1.2) .\" Sun Nov 7 03:18:35 CST 1999 .\" .\" This is free documentation; you can redistribute it and/or .\" modify it under the terms of the GNU General Public License as .\" published by the Free Software Foundation; either version 2 of .\" the License, or (at your option) any later version. .\" .\" The GNU General Public License's references to "object code" .\" and "executables" are to be interpreted as the output of any .\" document formatting or typesetting system, including .\" intermediate and printed output. .\" .\" This manual is distributed in the hope that it will be useful, .\" but WITHOUT ANY WARRANTY; without even the implied warranty of .\" MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the .\" GNU General Public License for more details. .\" .\" You should have received a copy of the GNU General Public .\" License along with this manual; if not, write to the Free .\" Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111, .\" USA. .\" .\" Modified Fri Jul 23 23:47:18 1993 by Rik Faith <faith@cs.unc.edu> .\" Modified Fri Jan 31 16:46:30 1997 by Eric S. Raymond <esr@thyrsus.com> .\" Modified Thu Oct 7 17:28:49 1999 by Andries Brouwer <aeb@cwi.nl> .\" .\" .TH PTRACE 2 "7 November 1999" "Linux 2.2.10" "Linux Programmer's Manual" .SH NAME ptrace \- process trace .SH SYNOPSIS .B #include <sys/ptrace.h> .sp .BI "long int ptrace(enum __ptrace_request " request ", pid_t " pid ", void * " addr ", void * " data ) .sp .BI "long int ptrace(enum __ptrace_request " request ", ... ) .SH DESCRIPTION The .B ptrace system call provides a means by which a parent process may observe and control the execution of a child process, and examine and change its core image and registers. It is primarily used to implement breakpoint debugging and system call tracing. .LP The parent can initiate a trace by calling .BR fork (2) and having the resulting child do a PTRACE_TRACEME, followed (typically) by an .BR exec (2). Alternatively, the parent may commence trace of an existing process using PTRACE_ATTACH. .LP 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 .BR 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). .LP 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. .LP The value of \fIrequest\fP determines the action to be performed: .TP 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 .BR wait. Also, all subsequent calls to .BR 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. (\fIpid\fP, \fIaddr\fP, and \fIdata\fP are ignored.) .LP The above request is used only by the child process; the rest are used only by the parent. In the following requests, \fIpid\fP specifies the child process to be acted on. For requests other than PTRACE_KILL, the child process must be stopped. .TP PTRACE_PEEKTEXT, PTRACE_PEEKDATA Reads a word at the location .IR addr in the child's memory, returning the word as the result of the .B ptrace call. Linux does not have separate text and data address spaces, so the two requests are currently equivalent. (\fIdata\fP is ignored.) .TP PTRACE_PEEKUSR Reads a word at offset .I addr in the child's .B 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 .B ptrace call. Typically the offset must be word-aligned, though this might vary by architecture. (\fIdata\fP is ignored.) .TP PTRACE_POKETEXT, PTRACE_POKEDATA Copies a word from location .IR data in the parent's memory to location .IR addr in the child's memory. As above, the two requests are currently equivalent. .TP PTRACE_POKEUSR Copies a word from location .IR data in the parent's memory to offset .I addr in the child's .B USER area. As above, the offset must typically be word-aligned. In order to maintain the integrity of the kernel, some modifications to the .B USER area are disallowed. .TP PTRACE_GETREGS, PTRACE_GETFPREGS Copies the child's general purpose or floating-point registers, respectively, to location \fIdata\fP in the parent. See <linux/user.h> for information on the format of this data. (\fIaddr\fP is ignored.) .TP PTRACE_SETREGS, PTRACE_SETFPREGS Copies the child's general purpose or floating-point registers, respectively, from location \fIdata\fP in the parent. As for PTRACE_POKEUSR, some general purpose register modifications may be disallowed. (\fIaddr\fP is ignored.) .TP PTRACE_CONT Restarts the stopped child process. If \fIdata\fP 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. (\fIaddr\fP is ignored.) .TP 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. (\fIaddr\fP is ignored.) .TP PTRACE_KILL Sends the child a SIGKILL to terminate it. .TP PTRACE_ATTACH Attaches to the process specified in .IR 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 .BR ps (1) output as the child's parent), but a .BR 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 .BR wait to wait for the child to stop.) .TP 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.) .SH NOTES Although arguments to .B ptrace are interpreted according to the first prototype given, the second prototype is the actual GNU libc declaration. This means that unneeded trailing arguments may be omitted, but also that the trailing arguments will not be statically type-checked. .LP .BR init (8), the process with pid 1, may not be traced. .LP The layout of the contents of memory and the USER area are quite OS- and architecture-specific. .LP The size of a "word" is determined by the OS variant (e.g., for 32-bit Linux it's 32 bits, etc.). .LP 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 .BR wait when it stops, and there is no way for the new parent to effectively simulate this notification. .LP This page documents the way the .B ptrace call works currently in Linux. Its behavior differs noticeably on other flavors of Unix. In any case, use of .B ptrace is highly operating-system- and architecture-specific. .LP The SunOS man page describes it as "unique and arcane", which it is. The proc-based debugging interface present in Solaris 2 implements a superset of .B ptrace functionality in a more powerful and uniform way. .SH "RETURN VALUE" On success, PTRACE_PEEK* requests return the requested data, while other requests return zero. On error, all requests return \-1, and .I errno (3) is set appropriately. Since the value returned by a successful PTRACE_PEEK* request may be \-1, the caller must check .I errno after such requests to determine whether or not an error occurred. .SH ERRORS .TP 0.8i .B 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 .BR init (pid 1). .TP .B 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). .TP .B EIO \fIrequest\fP 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. .TP .B 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. .SH "CONFORMING TO" SVr4, SVID EXT, AT&T, X/OPEN, BSD 4.3 .SH "SEE ALSO" .BR exec (3), .BR wait (2), .BR signal (2), .BR fork (2), .BR gdb (1), .BR strace (1)
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