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NAME

       process_vm_readv, process_vm_writev - transfer data between process 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);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       process_vm_readv(), process_vm_writev():
           _GNU_SOURCE

DESCRIPTION

       These  system  calls  transfer data between the address space of the calling 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 describing  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 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 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 local_iov[0] before
       proceeding to local_iov[1], and it completely fills  remote_iov[0]  before  proceeding  to
       remote_iov[1].

       The  lengths of remote_iov[i].iov_len and local_iov[i].iov_len do not have to be the same.
       Thus, it is possible to split a single local buffer into multiple remote buffers, or  vice
       versa.

       The flags argument is currently unused and must be set to 0.

       The  values  specified  in the liovcnt and riovcnt arguments must be less than or equal to
       IOV_MAX (defined in <limits.h> or accessible via the call sysconf(_SC_IOV_MAX)).

       The count arguments and local_iov are checked before doing any transfers.  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.  Consequently, 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  4 KiB) 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.)

       Permission to read from or write to another process is governed by a  ptrace  access  mode
       PTRACE_MODE_ATTACH_REALCREDS check; see ptrace(2).

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 partial 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

       EFAULT The memory described by local_iov is outside the caller's accessible address space.

       EFAULT The memory described by remote_iov is outside the accessible address space  of  the
              process pid.

       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.

       ENOMEM Could not allocate memory for internal copies of the iovec structures.

       EPERM  The caller does not have permission to access the address space of the process pid.

       ESRCH  No process with ID pid exists.

VERSIONS

       These system calls were added in Linux 3.2.  Support is provided in  glibc  since  version
       2.15.

CONFORMING TO

       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[0].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

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