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.\" $FreeBSD$
.\"     $NetBSD: ptrace.2,v 1.2 1995/02/27 12:35:37 cgd Exp $
.\"
.\" This file is in the public domain.
.Dd February 19, 2012
.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, system calls, or stepping by the tracing
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
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 12n
.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.
.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 to-be-traced process.
In
this case,
.Fa pid
specifies the process ID of the to-be-traced process, 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.
.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_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];
        int     pl_child_pid;
};
.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 pl_lwpid
LWP id of the thread
.It pl_event
Event that caused the stop.
Currently defined events are
.Bl -tag -width indent -compact
.It PL_EVENT_NONE
No reason given
.It PL_EVENT_SIGNAL
Thread stopped due to the pending signal
.El
.It pl_flags
Flags that specify additional details about observed stop.
Currently defined flags are:
.Bl -tag -width indent -compact
.It 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 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 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 PL_FLAG_SI
Indicates that
.Va pl_siginfo
member of
.Vt "struct ptrace_lwpinfo"
contains valid information.
.It 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 PL_FLAG_CHILD
The flag is set for first event reported from a new child, which is
automatically attached due to
.Dv PT_FOLLOW_FORK
enabled.
.El
.It pl_sigmask
The current signal mask of the LWP
.It 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 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 pl_tdname
The name of the thread.
.It 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 .
.El
.It PT_GETNUMLWPS
This request returns the number of kernel threads associated with the
traced process.
.It 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 PT_SETSTEP
This request will turn on single stepping of the specified process.
.It PT_CLEARSTEP
This request will turn off single stepping of the specified process.
.It PT_SUSPEND
This request will suspend the specified thread.
.It PT_RESUME
This request will resume the specified thread.
.It PT_TO_SCE
This request will trace the specified process on each system call entry.
.It PT_TO_SCX
This request will trace the specified process on each system call exit.
.It PT_SYSCALL
This request will trace the specified process
on each system call entry and exit.
.It PT_FOLLOW_FORK
This request controls tracing for new child processes of a traced process.
If
.Fa data
is non-zero,
then new child processes will enable tracing and stop before executing their
first instruction.
If
.Fa data
is zero, then new child processes will execute without tracing enabled.
By default, tracing is not enabled for new child processes.
Child processes do not inherit this property.
The traced process will set the
.Dv PL_FLAG_FORKED
flag upon exit from a system call that creates a new process.
.It 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 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.
.El
.Pp
Additionally, machine-specific requests can exist.
.Sh RETURN VALUES
Some requests can cause
.Fn ptrace
to return
\-1
as a non-error value; to disambiguate,
.Va errno
can be set to 0 before the call and checked afterwards.
.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.
.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 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 v7 .