lib/libsys/ptrace.2

.\"	$NetBSD: ptrace.2,v 1.2 1995/02/27 12:35:37 cgd Exp $
.\"
.\" This file is in the public domain.
.Dd June 19, 2025
.Dt PTRACE 2
.Os
.Sh NAME
.Nm ptrace
.Nd process tracing and debugging
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In sys/types.h
.In sys/ptrace.h
.Ft int
.Fn ptrace "int request" "pid_t pid" "caddr_t addr" "int data"
.Sh DESCRIPTION
The
.Fn ptrace
system call
provides tracing and debugging facilities.
It allows one process
(the
.Em tracing
process)
to control another
(the
.Em traced
process).
The tracing process must first attach to the traced process, and then
issue a series of
.Fn ptrace
system calls to control the execution of the process, as well as access
process memory and register state.
For the duration of the tracing session, the traced process will be
.Dq re-parented ,
with its parent process ID (and resulting behavior)
changed to the tracing process.
It is permissible for a tracing process to attach to more than one
other process at a time.
When the tracing process has completed its work, it must detach the
traced process; if a tracing process exits without first detaching all
processes it has attached, those processes will be killed.
.Pp
Most of the time, the traced process runs normally, but when it
receives a signal
(see
.Xr sigaction 2 ) ,
it stops.
The tracing process is expected to notice this via
.Xr wait 2
or the delivery of a
.Dv SIGCHLD
signal, examine the state of the stopped process, and cause it to
terminate or continue as appropriate.
The signal may be a normal process signal, generated as a result of
traced process behavior, or use of the
.Xr kill 2
system call; alternatively, it may be generated by the tracing facility
as a result of attaching, stepping by the tracing
process,
or an event in the traced process.
The tracing process may choose to intercept the signal, using it to
observe process behavior (such as
.Dv SIGTRAP ) ,
or forward the signal to the process if appropriate.
The
.Fn ptrace
system call
is the mechanism by which all this happens.
.Pp
A traced process may report additional signal stops corresponding to
events in the traced process.
These additional signal stops are reported as
.Dv SIGTRAP
or
.Dv SIGSTOP
signals.
The tracing process can use the
.Dv PT_LWPINFO
request to determine which events are associated with a
.Dv SIGTRAP
or
.Dv SIGSTOP
signal.
Note that multiple events may be associated with a single signal.
For example, events indicated by the
.Dv PL_FLAG_BORN ,
.Dv PL_FLAG_FORKED ,
and
.Dv PL_FLAG_EXEC
flags are also reported as a system call exit event
.Pq Dv PL_FLAG_SCX .
The signal stop for a new child process enabled via
.Dv PTRACE_FORK
will report a
.Dv SIGSTOP
signal.
All other additional signal stops use
.Dv SIGTRAP .
.Sh DETACH AND TERMINATION
.Pp
Normally, exiting tracing process should wait for all pending
debugging events and then detach from all alive traced processes
before exiting using
.Dv PT_DETACH
request.
If tracing process exits without detaching, for instance due to abnormal
termination, the destiny of the traced children processes is determined
by the
.Dv kern.kill_on_debugger_exit
sysctl control.
.Pp
If the control is set to the default value 1, such traced processes
are terminated.
If set to zero, kernel implicitly detaches traced processes.
Traced processes are un-stopped if needed, and then continue the execution
without tracing.
Kernel drops any
.Dv SIGTRAP
signals queued to the traced children, which could be either generated by
not yet consumed debug events, or sent by other means, the later should
not be done anyway.
.Sh SELECTING THE TARGET
The
.Fa pid
argument of the call specifies the target on which to perform
the requested operation.
For operations affecting the global process state, the process ID
is typically passed there.
Similarly, for operations affecting only a thread, the thread ID
needs to be passed.
.Pp
Still, for global operations, the ID of any thread can be used as the
target, and system will perform the request on the process owning
that thread.
If a thread operation got the process ID as
.Fa pid ,
the system randomly selects a thread from among the threads owned
by the process.
For single-threaded processes there is no difference between specifying
process or thread ID as the target.
.Sh DISABLING PTRACE
The
.Nm
subsystem provides rich facilities to manipulate other processes state.
Sometimes it may be desirable to disallow it either completely, or limit
its scope.
The following controls are provided for this:
.Bl -tag -width security.bsd.unprivileged_proc_debug
.It Va security.bsd.allow_ptrace
Setting this sysctl to zero makes
.Nm
return
.Er ENOSYS
always as if the syscall is not implemented by the kernel.
.It Va security.bsd.unprivileged_proc_debug
Setting this sysctl to zero disallows the use of
.Fn ptrace
by unprivileged processes.
.It Va security.bsd.see_other_uids
Setting this sysctl to zero prevents
.Fn ptrace
requests from targeting processes with a real user identifier different
from the caller's.
These requests will fail with error
.Er ESRCH .
.It Va security.bsd.see_other_gids
Setting this sysctl to zero disallows
.Fn ptrace
requests from processes that have no groups in common with the target process,
considering their sets of real and supplementary groups.
These requests will fail with error
.Er ESRCH .
.It Va security.bsd.see_jail_proc
Setting this sysctl to zero disallows
.Fn ptrace
requests from processes belonging to a different jail than that of the target
process, even if the requesting process' jail is an ancestor of the target
process'.
These requests will fail with error
.Er ESRCH .
.It Dv securelevel and init
The
.Xr init 1
process can only be traced with
.Nm
if securelevel is zero.
.It Dv procctl(2) PROC_TRACE_CTL
Process can deny attempts to trace itself with
.Xr procctl 2
.Dv PROC_TRACE_CTL
request.
In this case requests return
.Xr EPERM
error.
.El
.Sh TRACING EVENTS
.Pp
Each traced process has a tracing event mask.
An event in the traced process only reports a
signal stop if the corresponding flag is set in the tracing event mask.
The current set of tracing event flags include:
.Bl -tag -width "Dv PTRACE_SYSCALL"
.It Dv PTRACE_EXEC
Report a stop for a successful invocation of
.Xr execve 2 .
This event is indicated by the
.Dv PL_FLAG_EXEC
flag in the
.Va pl_flags
member of
.Vt "struct ptrace_lwpinfo" .
.It Dv PTRACE_SCE
Report a stop on each system call entry.
This event is indicated by the
.Dv PL_FLAG_SCE
flag in the
.Va pl_flags
member of
.Vt "struct ptrace_lwpinfo" .
.It Dv PTRACE_SCX
Report a stop on each system call exit.
This event is indicated by the
.Dv PL_FLAG_SCX
flag in the
.Va pl_flags
member of
.Vt "struct ptrace_lwpinfo" .
.It Dv PTRACE_SYSCALL
Report stops for both system call entry and exit.
.It Dv PTRACE_FORK
This event flag controls tracing for new child processes of a traced process.
.Pp
When this event flag is enabled,
new child processes will enable tracing and stop before executing their
first instruction.
The new child process will include the
.Dv PL_FLAG_CHILD
flag in the
.Va pl_flags
member of
.Vt "struct ptrace_lwpinfo" .
The traced process will report a stop that includes the
.Dv PL_FLAG_FORKED
flag.
The process ID of the new child process will also be present in the
.Va pl_child_pid
member of
.Vt "struct ptrace_lwpinfo" .
If the new child process was created via
.Xr vfork 2 ,
the traced process's stop will also include the
.Dv PL_FLAG_VFORKED
flag.
Note that new child processes will be attached with the default
tracing event mask;
they do not inherit the event mask of the traced process.
.Pp
When this event flag is not enabled,
new child processes will execute without tracing enabled.
.It Dv PTRACE_LWP
This event flag controls tracing of LWP
.Pq kernel thread
creation and destruction.
When this event is enabled,
new LWPs will stop and report an event with
.Dv PL_FLAG_BORN
set before executing their first instruction,
and exiting LWPs will stop and report an event with
.Dv PL_FLAG_EXITED
set before completing their termination.
.Pp
Note that new processes do not report an event for the creation of their
initial thread,
and exiting processes do not report an event for the termination of the
last thread.
.It Dv PTRACE_VFORK
Report a stop event when a parent process resumes after a
.Xr vfork 2 .
.Pp
When a thread in the traced process creates a new child process via
.Xr vfork 2 ,
the stop that reports
.Dv PL_FLAG_FORKED
and
.Dv PL_FLAG_SCX
occurs just after the child process is created,
but before the thread waits for the child process to stop sharing process
memory.
If a debugger is not tracing the new child process,
it must ensure that no breakpoints are enabled in the shared process
memory before detaching from the new child process.
This means that no breakpoints are enabled in the parent process either.
.Pp
The
.Dv PTRACE_VFORK
flag enables a new stop that indicates when the new child process stops
sharing the process memory of the parent process.
A debugger can reinsert breakpoints in the parent process and resume it
in response to this event.
This event is indicated by setting the
.Dv PL_FLAG_VFORK_DONE
flag.
.El
.Pp
The default tracing event mask when attaching to a process via
.Dv PT_ATTACH ,
.Dv PT_TRACE_ME ,
or
.Dv PTRACE_FORK
includes only
.Dv PTRACE_EXEC
events.
All other event flags are disabled.
.Sh PTRACE REQUESTS
.Pp
The
.Fa request
argument specifies what operation is being performed; the meaning of
the rest of the arguments depends on the operation, but except for one
special case noted below, all
.Fn ptrace
calls are made by the tracing process, and the
.Fa pid
argument specifies the process ID of the traced process
or a corresponding thread ID.
The
.Fa request
argument
can be:
.Bl -tag -width "Dv PT_GET_EVENT_MASK"
.It Dv PT_TRACE_ME
This request is the only one used by the traced process; it declares
that the process expects to be traced by its parent.
All the other arguments are ignored.
(If the parent process does not expect to trace the child, it will
probably be rather confused by the results; once the traced process
stops, it cannot be made to continue except via
.Fn ptrace . )
When a process has used this request and calls
.Xr execve 2
or any of the routines built on it
(such as
.Xr execv 3 ) ,
it will stop before executing the first instruction of the new image.
Also, any setuid or setgid bits on the executable being executed will
be ignored.
If the child was created by
.Xr vfork 2
system call or
.Xr rfork 2
call with the
.Dv RFMEM
flag specified, the debugging events are reported to the parent
only after the
.Xr execve 2
is executed.
.It Dv PT_READ_I , Dv PT_READ_D
These requests read a single
.Vt int
of data from the traced process's address space.
Traditionally,
.Fn ptrace
has allowed for machines with distinct address spaces for instruction
and data, which is why there are two requests: conceptually,
.Dv PT_READ_I
reads from the instruction space and
.Dv PT_READ_D
reads from the data space.
In the current
.Fx
implementation, these two requests are completely identical.
The
.Fa addr
argument specifies the address
(in the traced process's virtual address space)
at which the read is to be done.
This address does not have to meet any alignment constraints.
The value read is returned as the return value from
.Fn ptrace .
.It Dv PT_WRITE_I , Dv PT_WRITE_D
These requests parallel
.Dv PT_READ_I
and
.Dv PT_READ_D ,
except that they write rather than read.
The
.Fa data
argument supplies the value to be written.
.It Dv PT_IO
This request allows reading and writing arbitrary amounts of data in
the traced process's address space.
The
.Fa addr
argument specifies a pointer to a
.Vt "struct ptrace_io_desc" ,
which is defined as follows:
.Bd -literal
struct ptrace_io_desc {
	int	piod_op;	/* I/O operation */
	void	*piod_offs;	/* child offset */
	void	*piod_addr;	/* parent offset */
	size_t	piod_len;	/* request length */
};

/*
 * Operations in piod_op.
 */
#define PIOD_READ_D	1	/* Read from D space */
#define PIOD_WRITE_D	2	/* Write to D space */
#define PIOD_READ_I	3	/* Read from I space */
#define PIOD_WRITE_I	4	/* Write to I space */
.Ed
.Pp
The
.Fa data
argument is ignored.
The actual number of bytes read or written is stored in
.Va piod_len
upon return.
.It Dv PT_CONTINUE
The traced process continues execution.
The
.Fa addr
argument
is an address specifying the place where execution is to be resumed
(a new value for the program counter),
or
.Po Vt caddr_t Pc Ns 1
to indicate that execution is to pick up where it left off.
The
.Fa data
argument
provides a signal number to be delivered to the traced process as it
resumes execution, or 0 if no signal is to be sent.
.It Dv PT_STEP
The traced process is single stepped one instruction.
The
.Fa addr
argument
should be passed
.Po Vt caddr_t Pc Ns 1 .
The
.Fa data
argument
provides a signal number to be delivered to the traced process as it
resumes execution, or 0 if no signal is to be sent.
.It Dv PT_KILL
The traced process terminates, as if
.Dv PT_CONTINUE
had been used with
.Dv SIGKILL
given as the signal to be delivered.
.It Dv PT_ATTACH
This request allows a process to gain control of an otherwise
unrelated process and begin tracing it.
It does not need any cooperation from the process to trace.
In
this case,
.Fa pid
specifies the process ID of the process to trace, and the other
two arguments are ignored.
This request requires that the target process must have the same real
UID as the tracing process, and that it must not be executing a setuid
or setgid executable.
(If the tracing process is running as root, these restrictions do not
apply.)
The tracing process will see the newly-traced process stop and may
then control it as if it had been traced all along.
.It Dv PT_DETACH
This request is like PT_CONTINUE, except that it does not allow
specifying an alternate place to continue execution, and after it
succeeds, the traced process is no longer traced and continues
execution normally.
.Pp
The parent of the traced process will be sent a
.Dv SIGCHLD
to indicate that the process has continued from a stopped state regardless of
whether the process was in a stopped state prior to the corresponding
.Dv PT_ATTACH
request.
A
.Xr wait 2
for the traced process would indicate that it had been continued.
.It Dv PT_GETREGS
This request reads the traced process's machine registers into the
.Do
.Vt "struct reg"
.Dc
(defined in
.In machine/reg.h )
pointed to by
.Fa addr .
.It Dv PT_SETREGS
This request is the converse of
.Dv PT_GETREGS ;
it loads the traced process's machine registers from the
.Do
.Vt "struct reg"
.Dc
(defined in
.In machine/reg.h )
pointed to by
.Fa addr .
.It Dv PT_GETFPREGS
This request reads the traced process's floating-point registers into
the
.Do
.Vt "struct fpreg"
.Dc
(defined in
.In machine/reg.h )
pointed to by
.Fa addr .
.It Dv PT_SETFPREGS
This request is the converse of
.Dv PT_GETFPREGS ;
it loads the traced process's floating-point registers from the
.Do
.Vt "struct fpreg"
.Dc
(defined in
.In machine/reg.h )
pointed to by
.Fa addr .
.It Dv PT_GETDBREGS
This request reads the traced process's debug registers into
the
.Do
.Vt "struct dbreg"
.Dc
(defined in
.In machine/reg.h )
pointed to by
.Fa addr .
.It Dv PT_SETDBREGS
This request is the converse of
.Dv PT_GETDBREGS ;
it loads the traced process's debug registers from the
.Do
.Vt "struct dbreg"
.Dc
(defined in
.In machine/reg.h )
pointed to by
.Fa addr .
.It Dv PT_GETREGSET
This request reads the registers from the traced process.
The
.Fa data
argument specifies the register set to read, with the
.Fa addr
argument pointing at a
.Vt "struct iovec"
where the
.Va iov_base
field points to a register set specific structure to hold the registers,
and the
.Va iov_len
field holds the length of the structure.
.It Dv PT_SETREGSET
This request writes to the registers of the traced process.
The
.Fa data
argument specifies the register set to write to, with the
.Fa addr
argument pointing at a
.Vt "struct iovec"
where the
.Va iov_base
field points to a register set specific structure to hold the registers,
and the
.Va iov_len
field holds the length of the structure.
If
.Va iov_base
is NULL the kernel will return the expected length of the register set
specific structure in the
.Va iov_len
field and not change the target register set.
.It Dv PT_LWPINFO
This request can be used to obtain information about the kernel thread,
also known as light-weight process, that caused the traced process to stop.
The
.Fa addr
argument specifies a pointer to a
.Vt "struct ptrace_lwpinfo" ,
which is defined as follows:
.Bd -literal
struct ptrace_lwpinfo {
	lwpid_t pl_lwpid;
	int	pl_event;
	int	pl_flags;
	sigset_t pl_sigmask;
	sigset_t pl_siglist;
	siginfo_t pl_siginfo;
	char	pl_tdname[MAXCOMLEN + 1];
	pid_t	pl_child_pid;
	u_int	pl_syscall_code;
	u_int	pl_syscall_narg;
};
.Ed
.Pp
The
.Fa data
argument is to be set to the size of the structure known to the caller.
This allows the structure to grow without affecting older programs.
.Pp
The fields in the
.Vt "struct ptrace_lwpinfo"
have the following meaning:
.Bl -tag -width indent -compact
.It Va pl_lwpid
LWP id of the thread
.It Va pl_event
Event that caused the stop.
Currently defined events are:
.Bl -tag -width "Dv PL_EVENT_SIGNAL" -compact
.It Dv PL_EVENT_NONE
No reason given
.It Dv PL_EVENT_SIGNAL
Thread stopped due to the pending signal
.El
.It Va pl_flags
Flags that specify additional details about observed stop.
Currently defined flags are:
.Bl -tag -width indent -compact
.It Dv PL_FLAG_SCE
The thread stopped due to system call entry, right after the kernel is entered.
The debugger may examine syscall arguments that are stored in memory and
registers according to the ABI of the current process, and modify them,
if needed.
.It Dv PL_FLAG_SCX
The thread is stopped immediately before syscall is returning to the usermode.
The debugger may examine system call return values in the ABI-defined registers
and/or memory.
.It Dv PL_FLAG_EXEC
When
.Dv PL_FLAG_SCX
is set, this flag may be additionally specified to inform that the
program being executed by debuggee process has been changed by successful
execution of a system call from the
.Fn execve 2
family.
.It Dv PL_FLAG_SI
Indicates that
.Va pl_siginfo
member of
.Vt "struct ptrace_lwpinfo"
contains valid information.
.It Dv PL_FLAG_FORKED
Indicates that the process is returning from a call to
.Fn fork 2
that created a new child process.
The process identifier of the new process is available in the
.Va pl_child_pid
member of
.Vt "struct ptrace_lwpinfo" .
.It Dv PL_FLAG_CHILD
The flag is set for first event reported from a new child which is
automatically attached when
.Dv PTRACE_FORK
is enabled.
.It Dv PL_FLAG_BORN
This flag is set for the first event reported from a new LWP when
.Dv PTRACE_LWP
is enabled.
It is reported along with
.Dv PL_FLAG_SCX .
.It Dv PL_FLAG_EXITED
This flag is set for the last event reported by an exiting LWP when
.Dv PTRACE_LWP
is enabled.
Note that this event is not reported when the last LWP in a process exits.
The termination of the last thread is reported via a normal process exit
event.
.It Dv PL_FLAG_VFORKED
Indicates that the thread is returning from a call to
.Xr vfork 2
that created a new child process.
This flag is set in addition to
.Dv PL_FLAG_FORKED .
.It Dv PL_FLAG_VFORK_DONE
Indicates that the thread has resumed after a child process created via
.Xr vfork 2
has stopped sharing its address space with the traced process.
.El
.It Va pl_sigmask
The current signal mask of the LWP
.It Va pl_siglist
The current pending set of signals for the LWP.
Note that signals that are delivered to the process would not appear
on an LWP siglist until the thread is selected for delivery.
.It Va pl_siginfo
The siginfo that accompanies the signal pending.
Only valid for
.Dv PL_EVENT_SIGNAL
stop when
.Dv PL_FLAG_SI
is set in
.Va pl_flags .
.It Va pl_tdname
The name of the thread.
.It Va pl_child_pid
The process identifier of the new child process.
Only valid for a
.Dv PL_EVENT_SIGNAL
stop when
.Dv PL_FLAG_FORKED
is set in
.Va pl_flags .
.It Va pl_syscall_code
The ABI-specific identifier of the current system call.
Note that for indirect system calls this field reports the indirected
system call.
Only valid when
.Dv PL_FLAG_SCE
or
.Dv PL_FLAG_SCX
is set in
.Va pl_flags .
.It Va pl_syscall_narg
The number of arguments passed to the current system call not counting
the system call identifier.
Note that for indirect system calls this field reports the arguments
passed to the indirected system call.
Only valid when
.Dv PL_FLAG_SCE
or
.Dv PL_FLAG_SCX
is set in
.Va pl_flags .
.El
.It Dv PT_GETNUMLWPS
This request returns the number of kernel threads associated with the
traced process.
.It Dv PT_GETLWPLIST
This request can be used to get the current thread list.
A pointer to an array of type
.Vt lwpid_t
should be passed in
.Fa addr ,
with the array size specified by
.Fa data .
The return value from
.Fn ptrace
is the count of array entries filled in.
.It Dv PT_SETSTEP
This request will turn on single stepping of the specified process.
Stepping is automatically disabled when a single step trap is caught.
.It Dv PT_CLEARSTEP
This request will turn off single stepping of the specified process.
.It Dv PT_SUSPEND
This request will suspend the specified thread.
.It Dv PT_RESUME
This request will resume the specified thread.
.It Dv PT_TO_SCE
This request will set the
.Dv PTRACE_SCE
event flag to trace all future system call entries and continue the process.
The
.Fa addr
and
.Fa data
arguments are used the same as for
.Dv PT_CONTINUE .
.It Dv PT_TO_SCX
This request will set the
.Dv PTRACE_SCX
event flag to trace all future system call exits and continue the process.
The
.Fa addr
and
.Fa data
arguments are used the same as for
.Dv PT_CONTINUE .
.It Dv PT_SYSCALL
This request will set the
.Dv PTRACE_SYSCALL
event flag to trace all future system call entries and exits and continue
the process.
The
.Fa addr
and
.Fa data
arguments are used the same as for
.Dv PT_CONTINUE .
.It Dv PT_GET_SC_ARGS
For the thread which is stopped in either
.Dv PL_FLAG_SCE
or
.Dv PL_FLAG_SCX
state, that is, on entry or exit to a syscall,
this request fetches the syscall arguments.
.Pp
The arguments are copied out into the buffer pointed to by the
.Fa addr
pointer, sequentially.
Each syscall argument is stored as the machine word.
Kernel copies out as many arguments as the syscall accepts,
see the
.Va pl_syscall_narg
member of the
.Vt struct ptrace_lwpinfo ,
but not more than the
.Fa data
bytes in total are copied.
.It Dv PT_GET_SC_RET
Fetch the system call return values on exit from a syscall.
This request is only valid for threads stopped in a syscall
exit (the
.Dv PL_FLAG_SCX
state).
The
.Fa addr
argument specifies a pointer to a
.Vt "struct ptrace_sc_ret" ,
which is defined as follows:
.Bd -literal
struct ptrace_sc_ret {
	register_t	sr_retval[2];
	int		sr_error;
};
.Ed
.Pp
The
.Fa data
argument is set to the size of the structure.
.Pp
If the system call completed successfully,
.Va sr_error
is set to zero and the return values of the system call are saved in
.Va sr_retval .
If the system call failed to execute,
.Va sr_error
field is set to a positive
.Xr errno 2
value.
If the system call completed in an unusual fashion,
.Va sr_error
is set to a negative value:
.Bl -tag -width EJUSTRETURN -compact
.It Dv ERESTART
System call will be restarted.
.It Dv EJUSTRETURN
System call completed sucessfully but did not set a return value
.Po for example,
.Xr setcontext 2
and
.Xr sigreturn 2
.Pc .
.El
.It Dv PT_SET_SC_RET
Set the current system call return values.
This request is only valid for threads stopped in a syscall
entry (the
.Dv PL_FLAG_SCE
state).
The
.Fa addr
argument specifies a pointer to a
.Vt "struct ptrace_sc_ret" ,
the
.Fa data
argument is set to the size of the structure.
.Pp
The current system call handler is not executed.
After the thread is resumed, it returns to userspace with machine state
set as though the system call handler had returned the values passed via
the content of the
.Vt "struct ptrace_sc_ret"
argument.
.It Dv PT_FOLLOW_FORK
This request controls tracing for new child processes of a traced process.
If
.Fa data
is non-zero,
.Dv PTRACE_FORK
is set in the traced process's event tracing mask.
If
.Fa data
is zero,
.Dv PTRACE_FORK
is cleared from the traced process's event tracing mask.
.It Dv PT_LWP_EVENTS
This request controls tracing of LWP creation and destruction.
If
.Fa data
is non-zero,
.Dv PTRACE_LWP
is set in the traced process's event tracing mask.
If
.Fa data
is zero,
.Dv PTRACE_LWP
is cleared from the traced process's event tracing mask.
.It Dv PT_GET_EVENT_MASK
This request reads the traced process's event tracing mask into the
integer pointed to by
.Fa addr .
The size of the integer must be passed in
.Fa data .
.It Dv PT_SET_EVENT_MASK
This request sets the traced process's event tracing mask from the
integer pointed to by
.Fa addr .
The size of the integer must be passed in
.Fa data .
.It Dv PT_VM_TIMESTAMP
This request returns the generation number or timestamp of the memory map of
the traced process as the return value from
.Fn ptrace .
This provides a low-cost way for the tracing process to determine if the
VM map changed since the last time this request was made.
.It Dv PT_VM_ENTRY
This request is used to iterate over the entries of the VM map of the traced
process.
The
.Fa addr
argument specifies a pointer to a
.Vt "struct ptrace_vm_entry" ,
which is defined as follows:
.Bd -literal
struct ptrace_vm_entry {
	int		pve_entry;
	int		pve_timestamp;
	u_long		pve_start;
	u_long		pve_end;
	u_long		pve_offset;
	u_int		pve_prot;
	u_int		pve_pathlen;
	long		pve_fileid;
	uint32_t	pve_fsid;
	char		*pve_path;
};
.Ed
.Pp
The first entry is returned by setting
.Va pve_entry
to zero.
Subsequent entries are returned by leaving
.Va pve_entry
unmodified from the value returned by previous requests.
The
.Va pve_timestamp
field can be used to detect changes to the VM map while iterating over the
entries.
The tracing process can then take appropriate action, such as restarting.
By setting
.Va pve_pathlen
to a non-zero value on entry, the pathname of the backing object is returned
in the buffer pointed to by
.Va pve_path ,
provided the entry is backed by a vnode.
The
.Va pve_pathlen
field is updated with the actual length of the pathname (including the
terminating null character).
The
.Va pve_offset
field is the offset within the backing object at which the range starts.
The range is located in the VM space at
.Va pve_start
and extends up to
.Va pve_end
(inclusive).
.Pp
The
.Fa data
argument is ignored.
.It Dv PT_COREDUMP
This request creates a coredump for the stopped program.
The
.Fa addr
argument specifies a pointer to a
.Vt "struct ptrace_coredump" ,
which is defined as follows:
.Bd -literal
struct ptrace_coredump {
	int		pc_fd;
	uint32_t	pc_flags;
	off_t		pc_limit;
};
.Ed
The fields of the structure are:
.Bl -tag -width pc_flags
.It Dv pc_fd
File descriptor to write the dump to.
It must refer to a regular file, opened for writing.
.It Dv pc_flags
Flags.
The following flags are defined:
.Bl -tag -width PC_COMPRESS
.It Dv PC_COMPRESS
Request compression of the dump.
.It Dv PC_ALL
Include non-dumpable entries into the dump.
The dumper ignores
.Dv MAP_NOCORE
flag of the process map entry, but device mappings are not dumped even with
.Dv PC_ALL
set.
.El
.It Dv pc_limit
Maximum size of the coredump.
Specify zero for no limit.
.El
.Pp
The size of
.Vt "struct ptrace_coredump"
must be passed in
.Fa data .
.It Dv PT_SC_REMOTE
Request to execute a syscall in the context of the traced process,
in the specified thread.
The
.Fa addr
argument must point to the
.Vt "struct ptrace_sc_remote" ,
which describes the requested syscall and its arguments, and receives
the result.
The size of
.Vt "struct ptrace_sc_remote"
must be passed in
.Fa data.
.Bd -literal
struct ptrace_sc_remote {
	struct ptrace_sc_ret pscr_ret;
	u_int	pscr_syscall;
	u_int	pscr_nargs;
	u_long	*pscr_args;
};
.Ed
The
.Dv pscr_syscall
contains the syscall number to execute, the
.Dv pscr_nargs
is the number of supplied arguments, which are supplied in the
.Dv pscr_args
array.
Result of the execution is returned in the
.Dv pscr_ret
member.
Note that the request and its result do not affect the returned value from
the currently executed syscall, if any.
.El
.Sh PT_COREDUMP and PT_SC_REMOTE usage
The process must be stopped before dumping or initiating a remote system call.
A single thread in the target process is temporarily unsuspended
in the kernel to perform the action.
If the
.Nm
call fails before a thread is unsuspended, there is no event to
.Xr waitpid 2
for.
If a thread was unsuspended, it will stop again before the
.Nm
call returns, and the process must be waited upon using
.Xr waitpid 2
to consume the new stop event.
Since it is hard to deduce whether a thread was unsuspended before
an error occurred, it is recommended to unconditionally perform
.Xr waitpid 2
with
.Dv WNOHANG
flag after
.Dv PT_COREDUMP
and
.Dv PT_SC_REMOTE ,
and silently accept zero result from it.
.Pp
For
.Dv PT_SC_REMOTE ,
the selected thread must be stopped in the safe place, which is
currently defined as a syscall exit, or a return from kernel to
user mode (basically, a signal handler call place).
Kernel returns
.Er EBUSY
status if attempt is made to execute remote syscall at unsafe stop.
.Pp
Note that neither
.Dv kern.trap_enotcap
sysctl setting, nor the corresponding
.Xr procctl 2
flag
.Dv PROC_TRAPCAP_CTL_ENABLE
are obeyed during the execution of the syscall by
.Dv PT_SC_REMOTE .
In other words,
.Dv SIGTRAP
signal is not sent to a process executing in capability mode,
which violated a mode access restriction.
.Pp
Note that due to the mode of execution for the remote syscall, in
particular, the setting where only one thread is allowed to run,
the syscall might block on resources owned by suspended threads.
This might result in the target process deadlock.
In this situation, the only way out is to kill the target.
.Sh ARM MACHINE-SPECIFIC REQUESTS
.Bl -tag -width "Dv PT_SETVFPREGS"
.It Dv PT_GETVFPREGS
Return the thread's
.Dv VFP
machine state in the buffer pointed to by
.Fa addr .
.Pp
The
.Fa data
argument is ignored.
.It Dv PT_SETVFPREGS
Set the thread's
.Dv VFP
machine state from the buffer pointed to by
.Fa addr .
.Pp
The
.Fa data
argument is ignored.
.El
.Sh x86 MACHINE-SPECIFIC REQUESTS
.Bl -tag -width "Dv PT_GETXSTATE_INFO"
.It Dv PT_GETXMMREGS
Copy the XMM FPU state into the buffer pointed to by the
argument
.Fa addr .
The buffer has the same layout as the 32-bit save buffer for the
machine instruction
.Dv FXSAVE .
.Pp
This request is only valid for i386 programs, both on native 32-bit
systems and on amd64 kernels.
For 64-bit amd64 programs, the XMM state is reported as part of
the FPU state returned by the
.Dv PT_GETFPREGS
request.
.Pp
The
.Fa data
argument is ignored.
.It Dv PT_SETXMMREGS
Load the XMM FPU state for the thread from the buffer pointed to
by the argument
.Fa addr .
The buffer has the same layout as the 32-bit load buffer for the
machine instruction
.Dv FXRSTOR .
.Pp
As with
.Dv PT_GETXMMREGS ,
this request is only valid for i386 programs.
.Pp
The
.Fa data
argument is ignored.
.It Dv PT_GETXSTATE_INFO
Report which XSAVE FPU extensions are supported by the CPU
and allowed in userspace programs.
The
.Fa addr
argument must point to a variable of type
.Vt struct ptrace_xstate_info ,
which contains the information on the request return.
.Vt struct ptrace_xstate_info
is defined as follows:
.Bd -literal
struct ptrace_xstate_info {
	uint64_t	xsave_mask;
	uint32_t	xsave_len;
};
.Ed
The
.Dv xsave_mask
field is a bitmask of the currently enabled extensions.
The meaning of the bits is defined in the Intel and AMD
processor documentation.
The
.Dv xsave_len
field reports the length of the XSAVE area for storing the hardware
state for currently enabled extensions in the format defined by the x86
.Dv XSAVE
machine instruction.
.Pp
The
.Fa data
argument value must be equal to the size of the
.Vt struct ptrace_xstate_info .
.It Dv PT_GETXSTATE
Return the content of the XSAVE area for the thread.
The
.Fa addr
argument points to the buffer where the content is copied, and the
.Fa data
argument specifies the size of the buffer.
The kernel copies out as much content as allowed by the buffer size.
The buffer layout is specified by the layout of the save area for the
.Dv XSAVE
machine instruction.
.It Dv PT_SETXSTATE
Load the XSAVE state for the thread from the buffer specified by the
.Fa addr
pointer.
The buffer size is passed in the
.Fa data
argument.
The buffer must be at least as large as the
.Vt struct savefpu
(defined in
.Pa x86/fpu.h )
to allow the complete x87 FPU and XMM state load.
It must not be larger than the XSAVE state length, as reported by the
.Dv xsave_len
field from the
.Vt struct ptrace_xstate_info
of the
.Dv PT_GETXSTATE_INFO
request.
Layout of the buffer is identical to the layout of the load area for the
.Dv XRSTOR
machine instruction.
.It Dv PT_GETFSBASE
Return the value of the base used when doing segmented
memory addressing using the %fs segment register.
The
.Fa addr
argument points to an
.Vt unsigned long
variable where the base value is stored.
.Pp
The
.Fa data
argument is ignored.
.It Dv PT_GETGSBASE
Like the
.Dv PT_GETFSBASE
request, but returns the base for the %gs segment register.
.It Dv PT_SETFSBASE
Set the base for the %fs segment register to the value pointed to
by the
.Fa addr
argument.
.Fa addr
must point to the
.Vt unsigned long
variable containing the new base.
.Pp
The
.Fa data
argument is ignored.
.It Dv PT_SETGSBASE
Like the
.Dv PT_SETFSBASE
request, but sets the base for the %gs segment register.
.El
.Sh PowerPC MACHINE-SPECIFIC REQUESTS
.Bl -tag -width "Dv PT_SETVRREGS"
.It Dv PT_GETVRREGS
Return the thread's
.Dv ALTIVEC
machine state in the buffer pointed to by
.Fa addr .
.Pp
The
.Fa data
argument is ignored.
.It Dv PT_SETVRREGS
Set the thread's
.Dv ALTIVEC
machine state from the buffer pointed to by
.Fa addr .
.Pp
The
.Fa data
argument is ignored.
.It Dv PT_GETVSRREGS
Return doubleword 1 of the thread's
.Dv VSX
registers VSR0-VSR31 in the buffer pointed to by
.Fa addr .
.Pp
The
.Fa data
argument is ignored.
.It Dv PT_SETVSRREGS
Set doubleword 1 of the thread's
.Dv VSX
registers VSR0-VSR31 from the buffer pointed to by
.Fa addr .
.Pp
The
.Fa data
argument is ignored.
.El
.Pp
Additionally, other machine-specific requests can exist.
.Sh RETURN VALUES
Most requests return 0 on success and \-1 on error.
Some requests can cause
.Fn ptrace
to return
\-1
as a non-error value, among them are
.Dv PT_READ_I
and
.Dv PT_READ_D ,
which return the value read from the process memory on success.
To disambiguate,
.Va errno
can be set to 0 before the call and checked afterwards.
.Pp
The current
.Fn ptrace
implementation always sets
.Va errno
to 0 before calling into the kernel, both for historic reasons and for
consistency with other operating systems.
It is recommended to assign zero to
.Va errno
explicitly for forward compatibility.
.Sh ERRORS
The
.Fn ptrace
system call may fail if:
.Bl -tag -width Er
.It Bq Er ESRCH
.Bl -bullet -compact
.It
No process having the specified process ID exists.
.El
.It Bq Er EINVAL
.Bl -bullet -compact
.It
A process attempted to use
.Dv PT_ATTACH
on itself.
.It
The
.Fa request
argument
was not one of the legal requests.
.It
The signal number
(in
.Fa data )
to
.Dv PT_CONTINUE
was neither 0 nor a legal signal number.
.It
.Dv PT_GETREGS ,
.Dv PT_SETREGS ,
.Dv PT_GETFPREGS ,
.Dv PT_SETFPREGS ,
.Dv PT_GETDBREGS ,
or
.Dv PT_SETDBREGS
was attempted on a process with no valid register set.
(This is normally true only of system processes.)
.It
.Dv PT_VM_ENTRY
was given an invalid value for
.Fa pve_entry .
This can also be caused by changes to the VM map of the process.
.It
The size (in
.Fa data )
provided to
.Dv PT_LWPINFO
was less than or equal to zero, or larger than the
.Vt ptrace_lwpinfo
structure known to the kernel.
.It
The size (in
.Fa data )
provided to the x86-specific
.Dv PT_GETXSTATE_INFO
request was not equal to the size of the
.Vt struct ptrace_xstate_info .
.It
The size (in
.Fa data )
provided to the x86-specific
.Dv PT_SETXSTATE
request was less than the size of the x87 plus the XMM save area.
.It
The size (in
.Fa data )
provided to the x86-specific
.Dv PT_SETXSTATE
request was larger than returned in the
.Dv xsave_len
member of the
.Vt struct ptrace_xstate_info
from the
.Dv PT_GETXSTATE_INFO
request.
.It
The base value, provided to the amd64-specific requests
.Dv PT_SETFSBASE
or
.Dv PT_SETGSBASE ,
pointed outside of the valid user address space.
This error will not occur in 32-bit programs.
.El
.It Bq Er EBUSY
.Bl -bullet -compact
.It
.Dv PT_ATTACH
was attempted on a process that was already being traced.
.It
A request attempted to manipulate a process that was being traced by
some process other than the one making the request.
.It
A request
(other than
.Dv PT_ATTACH )
specified a process that was not stopped.
.El
.It Bq Er EPERM
.Bl -bullet -compact
.It
A request
(other than
.Dv PT_ATTACH )
attempted to manipulate a process that was not being traced at all.
.It
An attempt was made to use
.Dv PT_ATTACH
on a process in violation of the requirements listed under
.Dv PT_ATTACH
above.
.El
.It Bq Er ENOENT
.Bl -bullet -compact
.It
.Dv PT_VM_ENTRY
previously returned the last entry of the memory map.
No more entries exist.
.El
.It Bq Er ENOMEM
.Bl -bullet -compact
.It
A
.Dv PT_READ_I,
.Dv PT_READ_D,
.Dv PT_WRITE_I, or
.Dv PT_WRITE_D
request attempted to access an invalid address, or a memory allocation failure
occurred when accessing process memory.
.El
.It Bq Er ENAMETOOLONG
.Bl -bullet -compact
.It
.Dv PT_VM_ENTRY
cannot return the pathname of the backing object because the buffer is not big
enough.
.Fa pve_pathlen
holds the minimum buffer size required on return.
.El
.El
.Sh SEE ALSO
.Xr execve 2 ,
.Xr sigaction 2 ,
.Xr wait 2 ,
.Xr execv 3 ,
.Xr i386_clr_watch 3 ,
.Xr i386_set_watch 3
.Sh HISTORY
The
.Fn ptrace
function appeared in
.At v6 .