Provided by: freebsd-manpages_8.2-1_all bug

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]
                        +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.

     [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.

BUGS

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