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PTRACE(2)                 FreeBSD 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.  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 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;       /* LWP described. */
                           int     pl_event;       /* Event received. */

                   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.

     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:

                        +o   No process having the specified process ID exists.

                        +o   A process attempted to use PT_ATTACH on itself.
                        +o   The request argument was not one of the legal
                        +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_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.

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

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

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

FreeBSD 6.2                     August 11, 2003                    FreeBSD 6.2


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