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
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¶
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.
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 3.44 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/.