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PTRACE(2)		    BSD System Calls Manual		     PTRACE(2)

     ptrace — process tracing and debugging

     Standard C Library (libc, -lc)

     #include <sys/types.h>
     #include <sys/ptrace.h>

     ptrace(int request, pid_t pid, caddr_t addr, int data);

     The ptrace() system call provides tracing and debugging facilities.  It
     allows one process (the tracing process) to control another (the traced
     process).	The tracing process must first attach to the traced process,
     and then issue a series of 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
     “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.

     Most of the time, the traced process runs normally, but when it receives
     a signal (see sigaction(2)), it stops.  The tracing process is expected
     to notice this via wait(2) or the delivery of a 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 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 SIGTRAP), or forward the signal to the process if
     appropriate.  The ptrace() system call is the mechanism by which all this

     The 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 ptrace() calls are made by the
     tracing process, and the pid argument specifies the process ID of the
     traced process or a corresponding thread ID.  The request argument can

     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 par‐
		   ent.	 All the other arguments are ignored.  (If the parent
		   process does not expect to trace the child, it will proba‐
		   bly be rather confused by the results; once the traced
		   process stops, it cannot be made to continue except via
		   ptrace().)  When a process has used this request and calls
		   execve(2) or any of the routines built on it (such as
		   execv(3)), it will stop before executing the first instruc‐
		   tion of the new image.  Also, any setuid or setgid bits on
		   the executable being executed will be ignored.

		   These requests read a single int of data from the traced
		   process's address space.  Traditionally, ptrace() has
		   allowed for machines with distinct address spaces for
		   instruction and data, which is why there are two requests:
		   conceptually, PT_READ_I reads from the instruction space
		   and PT_READ_D reads from the data space.  In the current
		   FreeBSD implementation, these two requests are completely
		   identical.  The 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 ptrace().

		   These requests parallel PT_READ_I and PT_READ_D, except
		   that they write rather than read.  The data argument sup‐
		   plies the value to be written.

     PT_IO	   This request allows reading and writing arbitrary amounts
		   of data in the traced process's address space.  The addr
		   argument specifies a pointer to a struct ptrace_io_desc,
		   which is defined as follows:

		   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 */

		   The data argument is ignored.  The actual number of bytes
		   read or written is stored in piod_len upon return.

     PT_CONTINUE   The traced process continues execution.  The addr argument
		   is an address specifying the place where execution is to be
		   resumed (a new value for the program counter), or
		   (caddr_t)1 to indicate that execution is to pick up where
		   it left off.	 The 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.

     PT_STEP	   The traced process is single stepped one instruction.  The
		   addr argument should be passed (caddr_t)1.  The data argu‐
		   ment provides a signal number to be delivered to the traced
		   process as it resumes execution, or 0 if no signal is to be

     PT_KILL	   The traced process terminates, as if PT_CONTINUE had been
		   used with SIGKILL given as the signal to be delivered.

     PT_ATTACH	   This request allows a process to gain control of an other‐
		   wise unrelated process and begin tracing it.	 It does not
		   need any cooperation from the to-be-traced process.	In
		   this case, 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.

     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.

     PT_GETREGS	   This request reads the traced process's machine registers
		   into the “struct reg” (defined in <machine/reg.h>) pointed
		   to by addr.

     PT_SETREGS	   This request is the converse of PT_GETREGS; it loads the
		   traced process's machine registers from the “struct reg”
		   (defined in <machine/reg.h>) pointed to by addr.

     PT_GETFPREGS  This request reads the traced process's floating-point reg‐
		   isters into the “struct fpreg” (defined in <machine/reg.h>)
		   pointed to by addr.

     PT_SETFPREGS  This request is the converse of PT_GETFPREGS; it loads the
		   traced process's floating-point registers from the “struct
		   fpreg” (defined in <machine/reg.h>) pointed to by addr.

     PT_GETDBREGS  This request reads the traced process's debug registers
		   into the “struct dbreg” (defined in <machine/reg.h>)
		   pointed to by addr.

     PT_SETDBREGS  This request is the converse of PT_GETDBREGS; it loads the
		   traced process's debug registers from the “struct dbreg”
		   (defined in <machine/reg.h>) pointed to by addr.

     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 addr argument spec‐
		   ifies a pointer to a struct ptrace_lwpinfo, which is
		   defined as follows:

		   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;

		   The 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.

		   The fields in the struct ptrace_lwpinfo have the following
			   LWP id of the thread
			   Event that caused the stop.	Currently defined
			   events are
				   No reason given
				   Thread stopped due to the pending signal
			   Flags that specify additional details about
			   observed stop.  Currently defined flags are:
				   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.
				   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
				   When 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 succesful execution of
				   a system call from the execve(2) family.
				   Indicates that pl_siginfo member of struct
				   ptrace_lwpinfo contains valid information.
			   The current signal mask of the LWP
			   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.
			   The siginfo that accompanies the signal pending.
			   Only valid for PL_EVENT_SIGNAL kind of stop, when
			   pl_flags has PL_FLAG_SI set.

		   This request returns the number of kernel threads associ‐
		   ated with the traced process.

		   This request can be used to get the current thread list.  A
		   pointer to an array of type lwpid_t should be passed in
		   addr, with the array size specified by data.	 The return
		   value from ptrace() is the count of array entries filled

     PT_SETSTEP	   This request will turn on single stepping of the specified

     PT_CLEARSTEP  This request will turn off single stepping of the specified

     PT_SUSPEND	   This request will suspend the specified thread.

     PT_RESUME	   This request will resume the specified thread.

     PT_TO_SCE	   This request will trace the specified process on each sys‐
		   tem call entry.

     PT_TO_SCX	   This request will trace the specified process on each sys‐
		   tem call exit.

     PT_SYSCALL	   This request will trace the specified process on each sys‐
		   tem call entry and exit.

		   This request returns the generation number or timestamp of
		   the memory map of the traced process as the return value
		   from ptrace().  This provides a low-cost way for the trac‐
		   ing process to determine if the VM map changed since the
		   last time this request was made.

     PT_VM_ENTRY   This request is used to iterate over the entries of the VM
		   map of the traced process.  The addr argument specifies a
		   pointer to a struct ptrace_vm_entry, which is defined as

		   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;

		   The first entry is returned by setting pve_entry to zero.
		   Subsequent entries are returned by leaving pve_entry unmod‐
		   ified from the value returned by previous requests.	The
		   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 pve_pathlen to a non-zero value on entry, the path‐
		   name of the backing object is returned in the buffer
		   pointed to by pve_path, provided the entry is backed by a
		   vnode.  The pve_pathlen field is updated with the actual
		   length of the pathname (including the terminating null
		   character).	The pve_offset field is the offset within the
		   backing object at which the range starts.  The range is
		   located in the VM space at pve_start and extends up to
		   pve_end (inclusive).

		   The data argument is ignored.

     Additionally, machine-specific requests can exist.

     Some requests can cause ptrace() to return -1 as a non-error value; to
     disambiguate, errno can be set to 0 before the call and checked after‐

     The ptrace() system call may fail if:

			·   No process having the specified process ID exists.

			·   A process attempted to use PT_ATTACH on itself.
			·   The request argument was not one of the legal
			·   The signal number (in data) to PT_CONTINUE was
			    neither 0 nor a legal signal number.
			    attempted on a process with no valid register set.
			    (This is normally true only of system processes.)
			·   PT_VM_ENTRY was given an invalid value for
			    pve_entry.	This can also be caused by changes to
			    the VM map of the process.

			·   PT_ATTACH was attempted on a process that was
			    already being traced.
			·   A request attempted to manipulate a process that
			    was being traced by some process other than the
			    one making the request.
			·   A request (other than PT_ATTACH) specified a
			    process that was not stopped.

			·   A request (other than PT_ATTACH) attempted to
			    manipulate a process that was not being traced at
			·   An attempt was made to use PT_ATTACH on a process
			    in violation of the requirements listed under
			    PT_ATTACH above.

			·   PT_VM_ENTRY previously returned the last entry of
			    the memory map.  No more entries exist.

			·   PT_VM_ENTRY cannot return the pathname of the
			    backing object because the buffer is not big
			    enough.  pve_pathlen holds the minimum buffer size
			    required on return.

     execve(2), sigaction(2), wait(2), execv(3), i386_clr_watch(3),

     The ptrace() function appeared in Version 7 AT&T UNIX.

     The PL_FLAG_SCE, PL_FLAG_SCX and PL_FLAG_EXEC are not implemented for
     MIPS and ARM architectures.

BSD				 July 10, 2010				   BSD

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