2 .\" $NetBSD: ptrace.2,v 1.2 1995/02/27 12:35:37 cgd Exp $
4 .\" This file is in the public domain.
10 .Nd process tracing and debugging
17 .Fn ptrace "int request" "pid_t pid" "caddr_t addr" "int data"
22 provides tracing and debugging facilities.
31 The tracing process must first attach to the traced process, and then
34 system calls to control the execution of the process, as well as access
35 process memory and register state.
36 For the duration of the tracing session, the traced process will be
38 with its parent process ID (and resulting behavior)
39 changed to the tracing process.
40 It is permissible for a tracing process to attach to more than one
41 other process at a time.
42 When the tracing process has completed its work, it must detach the
43 traced process; if a tracing process exits without first detaching all
44 processes it has attached, those processes will be killed.
46 Most of the time, the traced process runs normally, but when it
51 The tracing process is expected to notice this via
55 signal, examine the state of the stopped process, and cause it to
56 terminate or continue as appropriate.
57 The signal may be a normal process signal, generated as a result of
58 traced process behavior, or use of the
60 system call; alternatively, it may be generated by the tracing facility
61 as a result of attaching, system calls, or stepping by the tracing
63 The tracing process may choose to intercept the signal, using it to
64 observe process behavior (such as
66 or forward the signal to the process if appropriate.
70 is the mechanism by which all this happens.
74 argument specifies what operation is being performed; the meaning of
75 the rest of the arguments depends on the operation, but except for one
76 special case noted below, all
78 calls are made by the tracing process, and the
80 argument specifies the process ID of the traced process
81 or a corresponding thread ID.
88 This request is the only one used by the traced process; it declares
89 that the process expects to be traced by its parent.
90 All the other arguments are ignored.
91 (If the parent process does not expect to trace the child, it will
92 probably be rather confused by the results; once the traced process
93 stops, it cannot be made to continue except via
95 When a process has used this request and calls
97 or any of the routines built on it
100 it will stop before executing the first instruction of the new image.
101 Also, any setuid or setgid bits on the executable being executed will
103 If the child was created by
109 flag specified, the debugging events are reported to the parent
113 .It Dv PT_READ_I , Dv PT_READ_D
114 These requests read a single
116 of data from the traced process's address space.
119 has allowed for machines with distinct address spaces for instruction
120 and data, which is why there are two requests: conceptually,
122 reads from the instruction space and
124 reads from the data space.
127 implementation, these two requests are completely identical.
130 argument specifies the address
131 (in the traced process's virtual address space)
132 at which the read is to be done.
133 This address does not have to meet any alignment constraints.
134 The value read is returned as the return value from
136 .It Dv PT_WRITE_I , Dv PT_WRITE_D
137 These requests parallel
141 except that they write rather than read.
144 argument supplies the value to be written.
146 This request allows reading and writing arbitrary amounts of data in
147 the traced process's address space.
150 argument specifies a pointer to a
151 .Vt "struct ptrace_io_desc" ,
152 which is defined as follows:
154 struct ptrace_io_desc {
155 int piod_op; /* I/O operation */
156 void *piod_offs; /* child offset */
157 void *piod_addr; /* parent offset */
158 size_t piod_len; /* request length */
162 * Operations in piod_op.
164 #define PIOD_READ_D 1 /* Read from D space */
165 #define PIOD_WRITE_D 2 /* Write to D space */
166 #define PIOD_READ_I 3 /* Read from I space */
167 #define PIOD_WRITE_I 4 /* Write to I space */
173 The actual number of bytes read or written is stored in
177 The traced process continues execution.
181 is an address specifying the place where execution is to be resumed
182 (a new value for the program counter),
184 .Po Vt caddr_t Pc Ns 1
185 to indicate that execution is to pick up where it left off.
189 provides a signal number to be delivered to the traced process as it
190 resumes execution, or 0 if no signal is to be sent.
192 The traced process is single stepped one instruction.
197 .Po Vt caddr_t Pc Ns 1 .
201 provides a signal number to be delivered to the traced process as it
202 resumes execution, or 0 if no signal is to be sent.
204 The traced process terminates, as if
208 given as the signal to be delivered.
210 This request allows a process to gain control of an otherwise
211 unrelated process and begin tracing it.
212 It does not need any cooperation from the to-be-traced process.
216 specifies the process ID of the to-be-traced process, and the other
217 two arguments are ignored.
218 This request requires that the target process must have the same real
219 UID as the tracing process, and that it must not be executing a setuid
220 or setgid executable.
221 (If the tracing process is running as root, these restrictions do not
223 The tracing process will see the newly-traced process stop and may
224 then control it as if it had been traced all along.
226 This request is like PT_CONTINUE, except that it does not allow
227 specifying an alternate place to continue execution, and after it
228 succeeds, the traced process is no longer traced and continues
231 This request reads the traced process's machine registers into the
240 This request is the converse of
242 it loads the traced process's machine registers from the
251 This request reads the traced process's floating-point registers into
261 This request is the converse of
263 it loads the traced process's floating-point registers from the
272 This request reads the traced process's debug registers into
282 This request is the converse of
284 it loads the traced process's debug registers from the
293 This request can be used to obtain information about the kernel thread,
294 also known as light-weight process, that caused the traced process to stop.
297 argument specifies a pointer to a
298 .Vt "struct ptrace_lwpinfo" ,
299 which is defined as follows:
301 struct ptrace_lwpinfo {
307 siginfo_t pl_siginfo;
308 char pl_tdname[MAXCOMLEN + 1];
315 argument is to be set to the size of the structure known to the caller.
316 This allows the structure to grow without affecting older programs.
319 .Vt "struct ptrace_lwpinfo"
320 have the following meaning:
321 .Bl -tag -width indent -compact
325 Event that caused the stop.
326 Currently defined events are
327 .Bl -tag -width indent -compact
331 Thread stopped due to the pending signal
334 Flags that specify additional details about observed stop.
335 Currently defined flags are:
336 .Bl -tag -width indent -compact
338 The thread stopped due to system call entry, right after the kernel is entered.
339 The debugger may examine syscall arguments that are stored in memory and
340 registers according to the ABI of the current process, and modify them,
343 The thread is stopped immediately before syscall is returning to the usermode.
344 The debugger may examine system call return values in the ABI-defined registers
349 is set, this flag may be additionally specified to inform that the
350 program being executed by debuggee process has been changed by successful
351 execution of a system call from the
358 .Vt "struct ptrace_lwpinfo"
359 contains valid information.
361 Indicates that the process is returning from a call to
363 that created a new child process.
364 The process identifier of the new process is available in the
367 .Vt "struct ptrace_lwpinfo" .
369 The flag is set for first event reported from a new child, which is
370 automatically attached due to
375 The current signal mask of the LWP
377 The current pending set of signals for the LWP.
378 Note that signals that are delivered to the process would not appear
379 on an LWP siglist until the thread is selected for delivery.
381 The siginfo that accompanies the signal pending.
389 The name of the thread.
391 The process identifier of the new child process.
400 This request returns the number of kernel threads associated with the
403 This request can be used to get the current thread list.
404 A pointer to an array of type
408 with the array size specified by
410 The return value from
412 is the count of array entries filled in.
414 This request will turn on single stepping of the specified process.
416 This request will turn off single stepping of the specified process.
418 This request will suspend the specified thread.
420 This request will resume the specified thread.
422 This request will trace the specified process on each system call entry.
424 This request will trace the specified process on each system call exit.
426 This request will trace the specified process
427 on each system call entry and exit.
429 This request controls tracing for new child processes of a traced process.
433 then new child processes will enable tracing and stop before executing their
437 is zero, then new child processes will execute without tracing enabled.
438 By default, tracing is not enabled for new child processes.
439 Child processes do not inherit this property.
440 The traced process will set the
442 flag upon exit from a system call that creates a new process.
444 This request returns the generation number or timestamp of the memory map of
445 the traced process as the return value from
447 This provides a low-cost way for the tracing process to determine if the
448 VM map changed since the last time this request was made.
450 This request is used to iterate over the entries of the VM map of the traced
454 argument specifies a pointer to a
455 .Vt "struct ptrace_vm_entry" ,
456 which is defined as follows:
458 struct ptrace_vm_entry {
472 The first entry is returned by setting
475 Subsequent entries are returned by leaving
477 unmodified from the value returned by previous requests.
480 field can be used to detect changes to the VM map while iterating over the
482 The tracing process can then take appropriate action, such as restarting.
485 to a non-zero value on entry, the pathname of the backing object is returned
486 in the buffer pointed to by
488 provided the entry is backed by a vnode.
491 field is updated with the actual length of the pathname (including the
492 terminating null character).
495 field is the offset within the backing object at which the range starts.
496 The range is located in the VM space at
507 Additionally, machine-specific requests can exist.
509 Some requests can cause
513 as a non-error value; to disambiguate,
515 can be set to 0 before the call and checked afterwards.
519 system call may fail if:
524 No process having the specified process ID exists.
529 A process attempted to use
536 was not one of the legal requests.
543 was neither 0 nor a legal signal number.
552 was attempted on a process with no valid register set.
553 (This is normally true only of system processes.)
556 was given an invalid value for
558 This can also be caused by changes to the VM map of the process.
564 was less than or equal to zero, or larger than the
566 structure known to the kernel.
572 was attempted on a process that was already being traced.
574 A request attempted to manipulate a process that was being traced by
575 some process other than the one making the request.
580 specified a process that was not stopped.
588 attempted to manipulate a process that was not being traced at all.
590 An attempt was made to use
592 on a process in violation of the requirements listed under
600 previously returned the last entry of the memory map.
601 No more entries exist.
603 .It Bq Er ENAMETOOLONG
607 cannot return the pathname of the backing object because the buffer is not big
610 holds the minimum buffer size required on return.
618 .Xr i386_clr_watch 3 ,