NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | CONFORMING TO | NOTES | EXAMPLE | SEE ALSO | COLOPHON

PROCESS_VM_READV(2)       Linux Programmer's Manual      PROCESS_VM_READV(2)

NAME         top

       process_vm_readv,  process_vm_writev  - transfer data between process
       address spaces

SYNOPSIS         top

       #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         top

       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 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         top

       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         top

       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
              accessible 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
              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         top

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

CONFORMING TO         top

       These system calls are nonstandard Linux extensions.

NOTES         top

       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         top

       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         top

       readv(2), writev(2)

COLOPHON         top

       This page is part of release 3.65 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)