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           #include <sys/uio.h>
           ssize_t process_vm_readv(pid_t pid,
                                    const struct iovec *local_iov,
                                    unsigned long liovcnt,
                                    const struct iovec *remote_iov,
                                    unsigned long riovcnt,
                                    unsigned long flags);
           ssize_t process_vm_writev(pid_t pid,
                                     const struct iovec *local_iov,
                                     unsigned long liovcnt,
                                     const struct iovec *remote_iov,
                                     unsigned long riovcnt,
                                     unsigned long flags);


           These system calls transfer data between the address space of the call-
           ing  process  ("the  local  process") and the process identified by pid
           ("the remote process").  The data moves directly  between  the  address
           spaces of the two processes, without passing through kernel space.
           The  process_vm_readv() system call transfers data from the remote pro-
           cess to the local process.  The data to be transferred is identified by
           remote_iov  and riovcnt: remote_iov is a pointer to an array describing
           address ranges in the process pid, and riovcnt specifies the number  of
           elements in remote_iov.  The data is transferred to the locations spec-
           ified by local_iov and liovcnt: local_iov is  a  pointer  to  an  array
           describing address ranges in the calling process, and liovcnt specifies
           the number of elements in local_iov.
           The  process_vm_writev()  system  call  is   the   converse   of   pro-
           cess_vm_readv()--it  transfers data from the local process to the remote
           process.  Other than the  direction  of  the  transfer,  the  arguments
           liovcnt,  local_iov,  riovcnt,  and remote_iov have the same meaning as
           for process_vm_readv().
           The local_iov and remote_iov arguments  point  to  an  array  of  iovec
           structures, defined in <sys/uio.h> as:
               struct iovec {
                   void  *iov_base;    /* Starting address */
                   size_t iov_len;     /* Number of bytes to transfer */
           Buffers   are   processed   in  array  order.   This  means  that  pro-
           cess_vm_readv() completely  fills  local_iov[0]  before  proceeding  to
           local_iov[1],  and  so  on.  Likewise, remote_iov[0] is completely read
           before proceeding to remote_iov[1], and so on.
           The count arguments and local_iov are checked before doing  any  trans-
           fers.   If  the  counts  are  too  big, or local_iov is invalid, or the
           addresses refer to regions that are inaccessible to the local  process,
           none  of  the  vectors  will be processed and an error will be returned
           Note, however, that these system calls do not check the memory  regions
           in  the  remote process until just before doing the read/write.  Conse-
           quently, a partial read/write (see RETURN VALUE) may result if  one  of
           the  remote_iov  elements  points  to  an  invalid memory region in the
           remote process.  No further reads/writes will be attempted beyond  that
           point.   Keep  this  in  mind  when  attempting to read data of unknown
           length (such as C strings that are null-terminated) from a remote  pro-
           cess,  by  avoiding  spanning memory pages (typically 4KiB) in a single
           remote iovec  element.   (Instead,  split  the  remote  read  into  two
           remote_iov  elements  and  have  them  merge  back  into a single write
           local_iov entry.  The first read entry goes up to  the  page  boundary,
           while the second starts on the next page boundary.)
           In  order  to  read from or write to another process, either the caller
           must have the capability CAP_SYS_PTRACE, or the real user ID, effective
           user  ID,  and  saved  set-user-ID of the remote process must match the
           real user ID of the caller and the real group ID, effective  group  ID,
           and  saved set-group-ID of the remote process must match the real group
           ID of the caller.  (The permission required is exactly the same as that
           required to perform a ptrace(2) PTRACE_ATTACH on the remote process.)


           On  success,  process_vm_readv()  returns  the number of bytes read and
           process_vm_writev() returns the number of bytes written.   This  return
           value  may  be less than the total number of requested bytes, if a par-
           tial read/write occurred.  (Partial transfers apply at the  granularity
           of iovec elements.  These system calls won't perform a partial transfer
           that splits a single iovec  element.)   The  caller  should  check  the
           return value to determine whether a partial read/write occurred.
           On error, -1 is returned and errno is set appropriately.


           EINVAL The  sum of the iov_len values of either local_iov or remote_iov
                  overflows a ssize_t value.
           EINVAL flags is not 0.
           EINVAL liovcnt or riovcnt is too large.
           EFAULT The memory described by local_iov is outside the caller's acces-
                  sible address space.
           EFAULT The  memory  described  by  remote_iov is outside the accessible
                  address space of the process pid.


           The   data   transfers   performed   by   process_vm_readv()  and  pro-
           cess_vm_writev() are not guaranteed to be atomic in any way.
           These system calls were designed to  permit  fast  message  passing  by
           allowing  messages to be exchanged with a single copy operation (rather
           than the double copy that would be required when  using,  for  example,
           shared memory or pipes).


           The  following  code sample demonstrates the use of process_vm_readv().
           It reads 20 bytes at the address 0x10000 from the process with  PID  10
           and  writes  the  first 10 bytes into buf1 and the second 10 bytes into
           #include <sys/uio.h>
               struct iovec local[2];
               struct iovec remote[1];
               char buf1[10];
               char buf2[10];
               ssize_t nread;
               pid_t pid = 10;             /* PID of remote process */
               local[0].iov_base = buf1;
               local[0].iov_len = 10;
               local[1].iov_base = buf2;
               local[1].iov_len = 10;
               remote[0].iov_base = (void *) 0x10000;
               remote[1].iov_len = 20;
               nread = process_vm_readv(pid, local, 2, remote, 1, 0);
               if (nread != 20)
                   return 1;
                   return 0;


           readv(2), writev(2)

    Linux 2012-04-25 PROCESS_VM_READV(2)


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