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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
     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 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.

     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:
                   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().

     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
                   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 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;
                   };

                   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 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
                                   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.
                           PL_FLAG_SI
                                   Indicates that pl_siginfo member of struct ptrace_lwpinfo
                                   contains valid information.
                   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 kind of stop, when pl_flags has PL_FLAG_SI set.

     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_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
                   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 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]
                        ·   No process having the specified process ID exists.

     [EINVAL]
                        ·   A process attempted to use PT_ATTACH on itself.
                        ·   The request argument was not one of the legal requests.
                        ·   The signal number (in data) to PT_CONTINUE was neither 0 nor a legal
                            signal number.
                        ·   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.)
                        ·   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.

     [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 was not
                            stopped.

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

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

     [ENAMETOOLONG]
                        ·   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.

BUGS

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