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.TH PROCESS_VM_READV 2 2020-06-09 "Linux" "Linux Programmer's Manual"
.SH NAME
process_vm_readv, process_vm_writev \- transfer data between process address spaces
.SH SYNOPSIS
.nf
.B #include <sys/uio.h>
.PP
.BI "ssize_t process_vm_readv(pid_t " pid ,
.BI "                         const struct iovec *" local_iov ,
.BI "                         unsigned long " liovcnt ,
.BI "                         const struct iovec *" remote_iov ,
.BI "                         unsigned long " riovcnt ,
.BI "                         unsigned long " flags ");"
.PP
.BI "ssize_t process_vm_writev(pid_t " pid ,
.BI "                          const struct iovec *" local_iov ,
.BI "                          unsigned long " liovcnt ,
.BI "                          const struct iovec *" remote_iov ,
.BI "                          unsigned long " riovcnt ,
.BI "                          unsigned long " flags ");"
.fi
.PP
.RS -4
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.RE
.PP
.BR process_vm_readv (),
.BR process_vm_writev ():
.PD 0
.ad l
.RS 4
.BR _GNU_SOURCE
.RE
.ad
.PD
.SH DESCRIPTION
These system calls transfer data between the address space
of the calling process ("the local process") and the process identified by
.IR pid
("the remote process").
The data moves directly between the address spaces of the two processes,
without passing through kernel space.
.PP
The
.BR process_vm_readv ()
system call transfers data from the remote process to the local process.
The data to be transferred is identified by
.IR remote_iov
and
.IR riovcnt :
.IR remote_iov
is a pointer to an array describing address ranges in the process
.IR pid ,
and
.IR riovcnt
specifies the number of elements in
.IR remote_iov .
The data is transferred to the locations specified by
.IR local_iov
and
.IR liovcnt :
.IR local_iov
is a pointer to an array describing address ranges in the calling process,
and
.IR liovcnt
specifies the number of elements in
.IR local_iov .
.PP
The
.BR process_vm_writev ()
system call is the converse of
.BR process_vm_readv ()\(emit
transfers data from the local process to the remote process.
Other than the direction of the transfer, the arguments
.IR liovcnt ,
.IR local_iov ,
.IR riovcnt ,
and
.IR remote_iov
have the same meaning as for
.BR process_vm_readv ().
.PP
The
.I local_iov
and
.I remote_iov
arguments point to an array of
.I iovec
structures, defined in
.IR <sys/uio.h>
as:
.PP
.in +4n
.EX
struct iovec {
    void  *iov_base;    /* Starting address */
    size_t iov_len;     /* Number of bytes to transfer */
};
.EE
.in
.PP
Buffers are processed in array order.
This means that
.BR process_vm_readv ()
completely fills
.I local_iov[0]
before proceeding to
.IR local_iov[1] ,
and so on.
Likewise,
.I remote_iov[0]
is completely read before proceeding to
.IR remote_iov[1] ,
and so on.
.PP
Similarly,
.BR process_vm_writev ()
writes out the entire contents of
.I local_iov[0]
before proceeding to
.IR local_iov[1] ,
and it completely fills
.I remote_iov[0]
before proceeding to
.IR remote_iov[1] .
.PP
The lengths of
.I remote_iov[i].iov_len
and
.I 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.
.PP
The
.I flags
argument is currently unused and must be set to 0.
.PP
The values specified in the
.I liovcnt
and
.I riovcnt
arguments must be less than or equal to
.BR IOV_MAX
(defined in
.I <limits.h>
or accessible via the call
.IR sysconf(_SC_IOV_MAX) ).
.\" In time, glibc might provide a wrapper that works around this limit,
.\" as is done for readv()/writev()
.PP
The count arguments and
.IR local_iov
are checked before doing any transfers.
If the counts are too big, or
.I 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.
.PP
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
.I 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
.I iovec
element.
(Instead, split the remote read into two
.I remote_iov
elements and have them merge back into a single write
.I local_iov
entry.
The first read entry goes up to the page boundary,
while the second starts on the next page boundary.)
.PP
Permission to read from or write to another process
is governed by a ptrace access mode
.B PTRACE_MODE_ATTACH_REALCREDS
check; see
.BR ptrace (2).
.SH RETURN VALUE
On success,
.BR process_vm_readv ()
returns the number of bytes read and
.BR 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
.I iovec
elements.
These system calls won't perform a partial transfer that splits a single
.I iovec
element.)
The caller should check the return value to determine whether
a partial read/write occurred.
.PP
On error, \-1 is returned and
.I errno
is set appropriately.
.SH ERRORS
.TP
.B EFAULT
The memory described by
.I local_iov
is outside the caller's accessible address space.
.TP
.B EFAULT
The memory described by
.I remote_iov
is outside the accessible address space of the process
.IR pid .
.TP
.B EINVAL
The sum of the
.I iov_len
values of either
.I local_iov
or
.I remote_iov
overflows a
.I ssize_t
value.
.TP
.B EINVAL
.I flags
is not 0.
.TP
.B EINVAL
.I liovcnt
or
.I riovcnt
is too large.
.TP
.B ENOMEM
Could not allocate memory for internal copies of the
.I iovec
structures.
.TP
.B EPERM
The caller does not have permission to access the address space of the process
.IR pid .
.TP
.B ESRCH
No process with ID
.I pid
exists.
.SH VERSIONS
These system calls were added in Linux 3.2.
Support is provided in glibc since version 2.15.
.SH CONFORMING TO
These system calls are nonstandard Linux extensions.
.SH NOTES
The data transfers performed by
.BR process_vm_readv ()
and
.BR process_vm_writev ()
are not guaranteed to be atomic in any way.
.PP
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).
.\" Original user is MPI, http://www.mcs.anl.gov/research/projects/mpi/
.\" See also some benchmarks at http://lwn.net/Articles/405284/
.\" and http://marc.info/?l=linux-mm&m=130105930902915&w=2
.SH EXAMPLES
The following code sample demonstrates the use of
.BR process_vm_readv ().
It reads 20 bytes at the address 0x10000 from the process with PID 10
and writes the first 10 bytes into
.I buf1
and the second 10 bytes into
.IR buf2 .
.PP
.EX
#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;
}
.EE
.SH SEE ALSO
.BR readv (2),
.BR writev (2)
.SH COLOPHON
This page is part of release 5.10 of the Linux
.I man-pages
project.
A description of the project,
information about reporting bugs,
and the latest version of this page,
can be found at
\%https://www.kernel.org/doc/man\-pages/.