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

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);

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  speci-
       fies 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 process_vm_readv()--it transfers
       data from the local process to the remote process.  Other than the direction of the trans-
       fer,  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 pro-
       ceeding 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 inacces-
       sible 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  capa-
       bility  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, effec-
       tive  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 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
       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
       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 guaran-
       teed 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 3.55 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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