MirBSD manpage: ptrace(2)

PTRACE(2)                  BSD Programmer's Manual                   PTRACE(2)


     ptrace - process tracing and debugging


     #include <sys/types.h>
     #include <sys/ptrace.h>

     ptrace(int request, pid_t pid, caddr_t addr, int data);


     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 ap-
     propriate. ptrace() is the mechanism by which all this happens. ptrace()
     is only available on kernels compiled with the PTRACE option.

     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:

          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.

          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
          OpenBSD implementation, these two requests are completely identical.
          The addr argument specifies the address (in the traced process' vir-
          tual 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 supplies the value to be

          The traced process continues execution. addr is an address specify-
          ing 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. 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.

          The traced process terminates, as if PT_CONTINUE had been used with
          SIGKILL given as the signal to be delivered.

          This request allows a process to gain control of an otherwise unre-
          lated process and begin tracing it. It does not need any cooperation
          from the to-be-traced process. In this case, pid specifies the pro-
          cess 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 exe-
          cuting a set-user-ID or set-group-ID 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.

          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 exe-
          cution normally.

          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 be made, piod_addr is the buffer in the parent
          and piod_len is the length of the I/O request. The piod_op member
          specifies what operation needs to be done. Possible values are:


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

          This request can be used to specify which events in the traced pro-
          cess should be reported to the tracing process. These events are
          specified in a "struct ptrace_event" defined as:

                typedef struct ptrace_event {
                        int     pe_set_event;
                } ptrace_event_t;

          Where pe_set_event is the set of events to be reported. This set is
          formed by OR'ing together the following values:

               Report fork(2).

          A pointer to this structure is passed in addr. The data argument
          should be set to sizeof(struct ptrace_event).

          This request can be used to determine which events in the traced
          process will be reported. The information is read into the "struct
          ptrace_event" pointed to by addr. The data argument should be set to
          sizeof(struct ptrace_event).

          This request reads the state information associated with the event
          that stopped the traced process. The information is reported in a
          "struct ptrace_state" defined as:

                typedef struct ptrace_state {
                        int     pe_report_event;
                        pid_t   pe_other_pid;
                } ptrace_state_t;

          Where pe_report_event is the event being reported. If the event be-
          ing reported is PTRACE_FORK, pe_other_pid will be set to the process
          ID of the other end of the fork. A pointer to this structure is
          passed in addr. The data argument should be set to sizeof(struct

     Additionally, machine-specific requests can exist. All OpenBSD platforms
     support the following requests:

          This request reads the traced process' machine registers into the
          "struct reg" (defined in <machine/reg.h>) pointed to by addr.

          This request is the converse of PT_GETREGS; it loads the traced pro-
          cess' machine registers from the "struct reg" (defined in
          <machine/reg.h>) pointed to by addr.

     In addition, all platforms but luna88k, mvme88k, sgi and vax support
     these additional requests:

          This request reads the traced process' floating-point registers into
          the "struct fpreg" (defined in <machine/reg.h>) pointed to by addr.

          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 following requests are available on i386:

          This request reads the traced process' XMM registers into the
          "struct xmmregs" (defined in <machine/reg.h>) pointed to by addr.

          This request is the converse of PT_GETXMMREGS; it loads the traced
          process' XMM registers from the "struct xmmregs" (defined in
          <machine/reg.h>) pointed to by addr.

     Finally, the following request is available on sparc and sparc64:

          This request reads the traced process' 'window cookie' into the int
          pointed to by addr. The window cookie needs to be 'XOR'ed' to
          stack-saved program counters.


     Some requests can cause ptrace() to return -1 as a non-error value; to
     disambiguate, errno is set to zero and this should be checked. The possi-
     ble errors are:

           No process having the specified process ID exists.

           •   A process attempted to use PT_ATTACH on itself.
           •   The request was not one of the legal requests.
           •   The signal number (in data) to PT_CONTINUE was neither 0 nor a
               legal signal number.
               tempted on a process with no valid register set. (This is nor-
               mally true only of system processes.)

           •   PT_ATTACH was attempted on a process that was already being
           •   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.

           •   A request (other than PT_ATTACH) attempted to manipulate a pro-
               cess that wasn't 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.
           •   An attempt was made to use PT_ATTACH on a system process.


     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.

     Single-stepping is not available.

MirBSD #10-current              March 2, 2004                                3

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