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

NAME
     ptrace -- process tracing and debugging

LIBRARY
     Standard C Library (libc, -lc)

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

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

DESCRIPTION
     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 con-
     trol 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 permissi-
     ble 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 ter-
     minate 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 appro-
     priate.  The ptrace() system call is the mechanism by which all this happens.

     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 be:

     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 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 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 vfork(2) system call or rfork(2) call
		   with the RFMEM flag specified, the debugging events are reported to the parent
		   only after the execve(2) is executed.

     PT_READ_I, PT_READ_D
		   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: concep-
		   tually, 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().

     PT_WRITE_I, PT_WRITE_D
		   These requests parallel PT_READ_I and PT_READ_D, except that they write rather
		   than read.  The data argument supplies 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 spec-
		   ifying 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 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_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 otherwise 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 registers 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 specifies 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;
			   char    pl_tdname[MAXCOMLEN + 1];
			   int	   pl_child_pid;
		   };

		   The data argument is to be set to the size of the structure known to the call-
		   er.	This allows the structure to grow without affecting older programs.

		   The fields in the struct ptrace_lwpinfo have the following meaning:
		   pl_lwpid
			   LWP id of the thread
		   pl_event
			   Event that caused the stop.	Currently defined events are
			   PL_EVENT_NONE
				   No reason given
			   PL_EVENT_SIGNAL
				   Thread stopped due to the pending signal
		   pl_flags
			   Flags that specify additional details about observed stop.  Currently
			   defined flags are:
			   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.
			   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.
			   PL_FLAG_EXEC
				   When PL_FLAG_SCX is set, this flag may be additionally speci-
				   fied to inform that the program being executed by debuggee
				   process has been changed by successful execution of a system
				   call from the execve(2) family.
			   PL_FLAG_SI
				   Indicates that pl_siginfo member of struct ptrace_lwpinfo con-
				   tains valid information.
			   PL_FLAG_FORKED
				   Indicates that the process is returning from a call to fork(2)
				   that created a new child process.  The process identifier of
				   the new process is available in the pl_child_pid member of
				   struct ptrace_lwpinfo.
			   PL_FLAG_CHILD
				   The flag is set for first event reported from a new child,
				   which is automatically attached due to PT_FOLLOW_FORK enabled.
		   pl_sigmask
			   The current signal mask of the LWP
		   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.
		   pl_siginfo
			   The siginfo that accompanies the signal pending.  Only valid for
			   PL_EVENT_SIGNAL stop when PL_FLAG_SI is set in pl_flags.
		   pl_tdname
			   The name of the thread.
		   pl_child_pid
			   The process identifier of the new child process.  Only valid for a
			   PL_EVENT_SIGNAL stop when PL_FLAG_FORKED is set in pl_flags.

     PT_GETNUMLWPS
		   This request returns the number of kernel threads associated with the traced
		   process.

     PT_GETLWPLIST
		   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
		   in.

     PT_SETSTEP    This request will turn on single stepping of the specified process.

     PT_CLEARSTEP  This request will turn off single stepping of the specified process.

     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 system call entry.

     PT_TO_SCX	   This request will trace the specified process on each system call exit.

     PT_SYSCALL    This request will trace the specified process on each system call entry and
		   exit.

     PT_FOLLOW_FORK
		   This request controls tracing for new child processes of a traced process.  If
		   data is non-zero, then new child processes will enable tracing and stop before
		   executing their first instruction.  If 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 PL_FLAG_FORKED flag upon exit from a system call
		   that creates a new process.

     PT_VM_TIMESTAMP
		   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 tracing 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 follows:

		   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 unmodified from the value returned by previ-
		   ous 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 pathname 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.

RETURN VALUES
     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 afterwards.

ERRORS
     The ptrace() system call may fail if:

     [ESRCH]
			o   No process having the specified process ID exists.

     [EINVAL]
			o   A process attempted to use PT_ATTACH on itself.
			o   The request argument was not one of the legal requests.
			o   The signal number (in data) to PT_CONTINUE was neither 0 nor a legal
			    signal number.
			o   PT_GETREGS, PT_SETREGS, PT_GETFPREGS, PT_SETFPREGS, PT_GETDBREGS, or
			    PT_SETDBREGS was attempted on a process with no valid register set.
			    (This is normally true only of system processes.)
			o   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.
			o   The size (in data) provided to PT_LWPINFO was less than or equal to
			    zero, or larger than the ptrace_lwpinfo structure known to the ker-
			    nel.

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

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

     [ENOENT]
			o   PT_VM_ENTRY previously returned the last entry of the memory map.  No
			    more entries exist.

     [ENAMETOOLONG]
			o   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.

SEE ALSO
     execve(2), sigaction(2), wait(2), execv(3), i386_clr_watch(3), i386_set_watch(3)

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

BSD					  July 22, 2013 				      BSD
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