PTRACE(2)                 NetBSD 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, void *addr, int data);

DESCRIPTION
     ptrace() provides tracing and debugging facilities.  It allows one
     process (the tracing process) to control another (the traced process).
     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.  ptrace() 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.  request 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 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.

     PT_READ_I, PT_READ_D
                   These requests read a single int of data from the traced
                   process' 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
                   NetBSD implementation, these two requests are completely
                   identical.  The addr argument specifies the address (in the
                   traced process' virtual address space) at which the read is
                   to be done.  This address does not have to meet any align-
                   ment 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 sup-
                   plies the value to be written.

     PT_CONTINUE   The traced process continues execution.  addr is an address
                   specifying the place where execution is to be resumed (a
                   new value for the program counter), or (caddr_t)1 to indi-
                   cate that execution is to pick up where it left off.  data
                   provides a signal number to be delivered to the traced
                   process as it resumes execution, or 0 if no signal is to be
                   sent.  If a negative value is supplied, that is the nega-
                   tive of the LWP ID of the thread to be resumed, and only
                   that thread executes.

     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.

                   Three other restrictions apply to all tracing processes,
                   even those running as root.  First, no process may trace a
                   system process.  Second, no process may trace the process
                   running init(8).  Third, if a process has its root direc-
                   tory set with chroot(2), it may not trace another process
                   unless that process's root directory is at or below the
                   tracing process's root.

     PT_DETACH     This request is like PT_CONTINUE, except that after it suc-
                   ceeds, the traced process is no longer traced and continues
                   execution normally.

     PT_IO         This request is a more general interface that can be used
                   instead of PT_READ_D, PT_WRITE_D, PT_READ_I, and
                   PT_WRITE_I.  The I/O request is encoded in a ``struct
                   ptrace_io_desc'' defined as:

                         struct ptrace_io_desc {
                                 int     piod_op;
                                 void    *piod_offs;
                                 void    *piod_addr;
                                 size_t  piod_len;
                         };

                   where piod_offs is the offset within the traced process
                   where the I/O operation should take place, piod_addr is the
                   buffer in the tracing process, and piod_len is the length
                   of the I/O request.  The piod_op field specifies which type
                   of I/O operation to perform.  Possible values are:

                         PIOD_READ_D
                         PIOD_WRITE_D
                         PIOD_READ_I
                         PIOD_WRITE_I

                   See the description of PT_READ_I for the difference between
                   I and D spaces.  A pointer to the I/O descriptor is passed
                   in the addr argument to ptrace().  On return, the piod_len
                   field in the I/O descriptor will be updated with the actual
                   number of bytes transferred.  If the requested I/O could
                   not be successfully performed, ptrace() will return -1 and
                   set errno.

     PT_DUMPCORE   Makes the process specified in the pid pid generate a core
                   dump.  The addr argument should contain the name of the
                   core file to be generated and the data argument should con-
                   tain the length of the core filename.  This ptrace call
                   currently does not stop the child process so it can gener-
                   ate inconsistent data.

     PT_LWPINFO    Returns information about a thread from the list of threads
                   for the process specified in the pid argument.  The addr
                   argument should contain a ``struct ptrace_lwpinfo'' defined
                   as:

                         struct ptrace_lwpinfo {
                                 lwpid_t pl_lwpid;
                                 int pl_event;
                         };

                   where pl_lwpid contains a thread LWP ID.  Information is
                   returned for the thread following the one with the speci-
                   fied ID in the process thread list, or for the first thread
                   if pl_lwpid is 0.  Upon return pl_lwpid contains the LWP ID
                   of the thread that was found, or 0 if there is no thread
                   after the one whose LWP ID was supplied in the call.
                   pl_event contains the event that stopped the thread.  Pos-
                   sible values are:

                         PL_EVENT_NONE
                         PL_EVENT_SIGNAL

                   The data argument should contain ``sizeof(struct
                   ptrace_lwpinfo)''.

     PT_SYSCALL    Stops a process before and after executing each system
                   call.

     PT_SYSCALLEMU
                   Intercept and ignore a system call before it has been exe-
                   cuted, for use with PT_SYSCALL.

     Additionally, the following requests exist but are not available on all
     machine architectures.  The file <machine/ptrace.h> lists which requests
     exist on a given machine.

     PT_STEP       Execution continues as in request PT_CONTINUE; however as
                   soon as possible after execution of at least one instruc-
                   tion, execution stops again.  If the data argument is
                   greater than 0, it contains the LWP ID of the thread to be
                   stepped, and any other threads are continued.  If the data
                   argument is less than zero, it contains the negative of the
                   LWP ID of the thread to be stepped, and only that thread
                   executes.

     PT_GETREGS    This request reads the traced process' machine registers
                   into the ``struct reg'' (defined in <machine/reg.h>)
                   pointed to by addr.  The data argument contains the LWP ID
                   of the thread whose registers are to be read.  If zero is
                   supplied, the first thread of the process is read.

     PT_SETREGS    This request is the converse of PT_GETREGS; it loads the
                   traced process' machine registers from the ``struct reg''
                   (defined in <machine/reg.h>) pointed to by addr.  The data
                   argument contains the LWP ID of the thread whose registers
                   are to be written.  If zero is supplied, the first thread
                   of the process is written.

     PT_GETFPREGS  This request reads the traced process' floating-point reg-
                   isters into the ``struct fpreg'' (defined in
                   <machine/reg.h>) pointed to by addr.  The data argument
                   contains the LWP ID of the thread whose registers are to be
                   read.  If zero is supplied, the first thread of the process
                   is read.

     PT_SETFPREGS  This request is the converse of PT_GETFPREGS; it loads the
                   traced process' floating-point registers from the ``struct
                   fpreg'' (defined in <machine/reg.h>) pointed to by addr.
                   The data argument contains the LWP ID of the thread whose
                   registers are to be written.  If zero is supplied, the
                   first thread of the process is written.

     PT_DUMPCORE   Cause the traced process to dump core.  If the addr argu-
                   ment is not NULL it is taken to be the pathname of the core
                   file to be generated and the data argument should contain
                   the length of the pathname.  The pathname may contain %
                   patterns that are expanded as described in sysctl(8).  If
                   the data argument is NULL, the default core file path gen-
                   eration rules are followed.

ERRORS
     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-
     wards.  The possible errors are:

     [EAGAIN]  Process is currently exec'ing and cannot be traced.

     [EBUSY]
                  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
                   wasn't stopped.

     [EINVAL]
                  A process attempted to use PT_ATTACH on itself.
                  The request was not a legal request on this machine archi-
                   tecture.
                  The signal number (in data) to PT_CONTINUE was neither 0
                   nor a legal signal number.
                  PT_GETREGS, PT_SETREGS, PT_GETFPREGS, or PT_SETFPREGS was
                   attempted on a process with no valid register set.  (This
                   is normally true only of system processes.)

     [EPERM]
                  A request (other than PT_ATTACH) attempted to manipulate a
                   process that wasn't being traced at all.
                  An attempt was made to use PT_ATTACH on a process in viola-
                   tion of the requirements listed under PT_ATTACH above.

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

SEE ALSO
     sigaction(2), signal(7)

BUGS
     On the SPARC, the PC is set to the provided PC value for PT_CONTINUE and
     similar calls, but the NPC is set willy-nilly to 4 greater than the PC
     value.  Using PT_GETREGS and PT_SETREGS to modify the PC, passing
     (caddr_t)1 to ptrace(), should be able to sidestep this.

NetBSD 6.0                      August 31, 2011                     NetBSD 6.0