Provided by: freebsd-manpages_11.1-3_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, stepping by the tracing
     process, or an event in the traced 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.

     A traced process may report additional signal stops corresponding to events in the traced
     process.  These additional signal stops are reported as SIGTRAP or SIGSTOP signals.  The
     tracing process can use the PT_LWPINFO request to determine which events are associated with
     a SIGTRAP or SIGSTOP signal.  Note that multiple events may be associated with a single
     signal.  For example, events indicated by the PL_FLAG_BORN, PL_FLAG_FORKED, and PL_FLAG_EXEC
     flags are also reported as a system call exit event (PL_FLAG_SCX).  The signal stop for a
     new child process enabled via PTRACE_FORK will report a SIGSTOP signal.  All other
     additional signal stops use SIGTRAP.

     Each traced process has a tracing event mask.  An event in the traced process only reports a
     signal stop if the corresponding flag is set in the tracing event mask.  The current set of
     tracing event flags include:

     PTRACE_EXEC        Report a stop for a successful invocation of execve(2).  This event is
                        indicated by the PL_FLAG_EXEC flag in the pl_flags member of struct
                        ptrace_lwpinfo.

     PTRACE_SCE         Report a stop on each system call entry.  This event is indicated by the
                        PL_FLAG_SCE flag in the pl_flags member of struct ptrace_lwpinfo.

     PTRACE_SCX         Report a stop on each system call exit.  This event is indicated by the
                        PL_FLAG_SCX flag in the pl_flags member of struct ptrace_lwpinfo.

     PTRACE_SYSCALL     Report stops for both system call entry and exit.

     PTRACE_FORK        This event flag controls tracing for new child processes of a traced
                        process.

                        When this event flag is enabled, new child processes will enable tracing
                        and stop before executing their first instruction.  The new child process
                        will include the PL_FLAG_CHILD flag in the pl_flags member of struct
                        ptrace_lwpinfo.  The traced process will report a stop that includes the
                        PL_FLAG_FORKED flag.  The process ID of the new child process will also
                        be present in the pl_child_pid member of struct ptrace_lwpinfo.  If the
                        new child process was created via vfork(2), the traced process's stop
                        will also include the PL_FLAG_VFORKED flag.  Note that new child
                        processes will be attached with the default tracing event mask; they do
                        not inherit the event mask of the traced process.

                        When this event flag is not enabled, new child processes will execute
                        without tracing enabled.

     PTRACE_LWP         This event flag controls tracing of LWP (kernel thread) creation and
                        destruction.  When this event is enabled, new LWPs will stop and report
                        an event with PL_FLAG_BORN set before executing their first instruction,
                        and exiting LWPs will stop and report an event with PL_FLAG_EXITED set
                        before completing their termination.

                        Note that new processes do not report an event for the creation of their
                        initial thread, and exiting processes do not report an event for the
                        termination of the last thread.

     PTRACE_VFORK       Report a stop event when a parent process resumes after a vfork(2).

                        When a thread in the traced process creates a new child process via
                        vfork(2), the stop that reports PL_FLAG_FORKED and PL_FLAG_SCX occurs
                        just after the child process is created, but before the thread waits for
                        the child process to stop sharing process memory.  If a debugger is not
                        tracing the new child process, it must ensure that no breakpoints are
                        enabled in the shared process memory before detaching from the new child
                        process.  This means that no breakpoints are enabled in the parent
                        process either.

                        The PTRACE_VFORK flag enables a new stop that indicates when the new
                        child process stops sharing the process memory of the parent process.  A
                        debugger can reinsert breakpoints in the parent process and resume it in
                        response to this event.  This event is indicated by setting the
                        PL_FLAG_VFORK_DONE flag.

     The default tracing event mask when attaching to a process via PT_ATTACH, PT_TRACE_ME, or
     PTRACE_FORK includes only PTRACE_EXEC events.  All other event flags are disabled.

     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: 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;
                                   char    pl_tdname[MAXCOMLEN + 1];
                                   pid_t   pl_child_pid;
                                   u_int   pl_syscall_code;
                                   u_int   pl_syscall_narg;
                           };

                           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 successful
                                           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_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 when PTRACE_FORK
                                           is enabled.
                                   PL_FLAG_BORN
                                           This flag is set for the first event reported from a
                                           new LWP when PTRACE_LWP is enabled.  It is reported
                                           along with PL_FLAG_SCX.
                                   PL_FLAG_EXITED
                                           This flag is set for the last event reported by an
                                           exiting LWP when PTRACE_LWP is enabled.  Note that
                                           this event is not reported when the last LWP in a
                                           process exits.  The termination of the last thread is
                                           reported via a normal process exit event.
                                   PL_FLAG_VFORKED
                                           Indicates that the thread is returning from a call to
                                           vfork(2) that created a new child process.  This flag
                                           is set in addition to PL_FLAG_FORKED.
                                   PL_FLAG_VFORK_DONE
                                           Indicates that the thread has resumed after a child
                                           process created via vfork(2) has stopped sharing its
                                           address space with the traced process.
                           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.
                           pl_syscall_code
                                   The ABI-specific identifier of the current system call.  Note
                                   that for indirect system calls this field reports the
                                   indirected system call.  Only valid when PL_FLAG_SCE or
                                   PL_FLAG_SCX is set in pl_flags.
                           pl_syscall_narg
                                   The number of arguments passed to the current system call not
                                   counting the system call identifier.  Note that for indirect
                                   system calls this field reports the arguments passed to the
                                   indirected system call.  Only valid when PL_FLAG_SCE or
                                   PL_FLAG_SCX 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 set the PTRACE_SCE event flag to trace all future
                           system call entries and continue the process.  The addr and data
                           arguments are used the same as for PT_CONTINUE.

     PT_TO_SCX             This request will set the PTRACE_SCX event flag to trace all future
                           system call exits and continue the process.  The addr and data
                           arguments are used the same as for PT_CONTINUE.

     PT_SYSCALL            This request will set the PTRACE_SYSCALL event flag to trace all
                           future system call entries and exits and continue the process.  The
                           addr and data arguments are used the same as for PT_CONTINUE.

     PT_FOLLOW_FORK        This request controls tracing for new child processes of a traced
                           process.  If data is non-zero, PTRACE_FORK is set in the traced
                           process's event tracing mask.  If data is zero, PTRACE_FORK is cleared
                           from the traced process's event tracing mask.

     PT_LWP_EVENTS         This request controls tracing of LWP creation and destruction.  If
                           data is non-zero, PTRACE_LWP is set in the traced process's event
                           tracing mask.  If data is zero, PTRACE_LWP is cleared from the traced
                           process's event tracing mask.

     PT_GET_EVENT_MASK     This request reads the traced process's event tracing mask into the
                           integer pointed to by addr.  The size of the integer must be passed in
                           data.

     PT_SET_EVENT_MASK     This request sets the traced process's event tracing mask from the
                           integer pointed to by addr.  The size of the integer must be passed in
                           data.

     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.

x86 MACHINE-SPECIFIC REQUESTS

     PT_GETXMMREGS         Copy the XMM FPU state into the buffer pointed to by the argument
                           addr.  The buffer has the same layout as the 32-bit save buffer for
                           the machine instruction FXSAVE.

                           This request is only valid for i386 programs, both on native 32-bit
                           systems and on amd64 kernels.  For 64-bit amd64 programs, the XMM
                           state is reported as part of the FPU state returned by the
                           PT_GETFPREGS request.

                           The data argument is ignored.

     PT_SETXMMREGS         Load the XMM FPU state for the thread from the buffer pointed to by
                           the argument addr.  The buffer has the same layout as the 32-bit load
                           buffer for the machine instruction FXRSTOR.

                           As with PT_GETXMMREGS, this request is only valid for i386 programs.

                           The data argument is ignored.

     PT_GETXSTATE_INFO     Report which XSAVE FPU extensions are supported by the CPU and allowed
                           in userspace programs.  The addr argument must point to a variable of
                           type struct ptrace_xstate_info, which contains the information on the
                           request return.  struct ptrace_xstate_info is defined as follows:

                           struct ptrace_xstate_info {
                                   uint64_t        xsave_mask;
                                   uint32_t        xsave_len;
                           };
                           The xsave_mask field is a bitmask of the currently enabled extensions.
                           The meaning of the bits is defined in the Intel and AMD processor
                           documentation.  The xsave_len field reports the length of the XSAVE
                           area for storing the hardware state for currently enabled extensions
                           in the format defined by the x86 XSAVE machine instruction.

                           The data argument value must be equal to the size of the struct
                           ptrace_xstate_info.

     PT_GETXSTATE          Return the content of the XSAVE area for the thread.  The addr
                           argument points to the buffer where the content is copied, and the
                           data argument specifies the size of the buffer.  The kernel copies out
                           as much content as allowed by the buffer size.  The buffer layout is
                           specified by the layout of the save area for the XSAVE machine
                           instruction.

     PT_SETXSTATE          Load the XSAVE state for the thread from the buffer specified by the
                           addr pointer.  The buffer size is passed in the data argument.  The
                           buffer must be at least as large as the struct savefpu (defined in
                           x86/fpu.h) to allow the complete x87 FPU and XMM state load.  It must
                           not be larger than the XSAVE state length, as reported by the
                           xsave_len field from the struct ptrace_xstate_info of the
                           PT_GETXSTATE_INFO request.  Layout of the buffer is identical to the
                           layout of the load area for the XRSTOR machine instruction.

     PT_GETFSBASE          Return the value of the base used when doing segmented memory
                           addressing using the %fs segment register.  The addr argument points
                           to an unsigned long variable where the base value is stored.

                           The data argument is ignored.

     PT_GETGSBASE          Like the PT_GETFSBASE request, but returns the base for the %gs
                           segment register.

     PT_SETFSBASE          Set the base for the %fs segment register to the value pointed to by
                           the addr argument.  addr must point to the unsigned long variable
                           containing the new base.

                           The data argument is ignored.

     PT_SETGSBASE          Like the PT_SETFSBASE request, but sets the base for the %gs segment
                           register.

PowerPC MACHINE-SPECIFIC REQUESTS

     PT_GETVRREGS     Return the thread's ALTIVEC machine state in the buffer pointed to by addr.

                      The data argument is ignored.

     PT_SETVRREGS     Set the thread's ALTIVEC machine state from the buffer pointed to by addr.

                      The data argument is ignored.

     Additionally, other machine-specific requests can exist.

RETURN VALUES

     Most requests return 0 on success and -1 on error.  Some requests can cause ptrace() to
     return -1 as a non-error value, among them are PT_READ_I and PT_READ_D, which return the
     value read from the process memory on success.  To disambiguate, errno can be set to 0
     before the call and checked afterwards.

     The current ptrace() implementation always sets errno to 0 before calling into the kernel,
     both for historic reasons and for consistency with other operating systems.  It is
     recommended to assign zero to errno explicitly for forward compatibility.

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.
                        ·   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
                            kernel.
                        ·   The size (in data) provided to the x86-specific PT_GETXSTATE_INFO
                            request was not equal to the size of the struct ptrace_xstate_info.
                        ·   The size (in data) provided to the x86-specific PT_SETXSTATE request
                            was less than the size of the x87 plus the XMM save area.
                        ·   The size (in data) provided to the x86-specific PT_SETXSTATE request
                            was larger than returned in the xsave_len member of the struct
                            ptrace_xstate_info from the PT_GETXSTATE_INFO request.
                        ·   The base value, provided to the amd64-specific requests PT_SETFSBASE
                            or PT_SETGSBASE, pointed outside of the valid user address space.
                            This error will not occur in 32-bit programs.

     [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.