process_vm_readv

       address spaces

SYNOPSIS
       #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);

DESCRIPTION
       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
       process to the local process.  The data to be transferred is identified
       by remote_iov and riovcnt: remote_iov is a pointer to an array describ-
       ing address ranges in the process pid, and riovcnt specifies the number
       of  elements  in  remote_iov.  The data is transferred to the locations
       specified 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
       process_vm_readv()--it  transfers  data  from  the local process to the
       remote process.  Other than the direction of the  transfer,  the  argu-
       ments 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
       process_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.

       Similarly,  process_vm_writev()  writes  out  the  entire  contents  of

       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
       immediately.

       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
       process, 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.)

RETURN VALUE
       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.

ERRORS
       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.

       ENOMEM Could not allocate memory  for  internal  copies  of  the  iovec
       These system calls are nonstandard Linux extensions.

NOTES
       The    data    transfers    performed    by    process_vm_readv()   and
       process_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).

EXAMPLE
       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
       buf2.

       #include <sys/uio.h>

       int
       main(void)
       {
           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;
           else
               return 0;
       }

SEE ALSO
       readv(2), writev(2)

COLOPHON
       This page is part of release 3.54 of the Linux  man-pages  project.   A
       description  of  the project, and information about reporting bugs, can
       be found at http://www.kernel.org/doc/man-pages/.



Linux                             2012-04-25               PROCESS_VM_READV(2)
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