process_vm_readv(2) System Calls Manual process_vm_readv(2)
NAME
process_vm_readv, process_vm_writev - transfer data between process ad-
dress spaces
LIBRARY
Standard C library (libc, -lc)
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 call-
ing 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 describ-
ing 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 re-
mote 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 struc-
tures, described in iovec(3type).
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 be-
fore proceeding to remote_iov[1], and so on.
Similarly, process_vm_writev() writes out the entire contents of lo-
cal_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 trans-
fers. If the counts are too big, or local_iov is invalid, or the ad-
dresses refer to regions that are inaccessible to the local process,
none of the vectors will be processed and an error will be returned im-
mediately.
Note, however, that these system calls do not check the memory regions
in the remote process until just before doing the read/write. Conse-
quently, 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 avoid-
ing spanning memory pages (typically 4 KiB) in a single remote iovec el-
ement. (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 to indicate the error.
ERRORS
EFAULT The memory described by local_iov is outside the caller's acces-
sible address space.
EFAULT The memory described by remote_iov is outside the accessible ad-
dress 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 struc-
tures.
EPERM The caller does not have permission to access the address space
of the process pid.
ESRCH No process with ID pid exists.
STANDARDS
Linux.
HISTORY
Linux 3.2, glibc 2.15.
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 al-
lowing 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).
EXAMPLES
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.
#define _GNU_SOURCE
#include <stdlib.h>
#include <sys/types.h>
#include <sys/uio.h>
int
main(void)
{
char buf1[10];
char buf2[10];
pid_t pid = 10; /* PID of remote process */
ssize_t nread;
struct iovec local[2];
struct iovec remote[1];
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)
exit(EXIT_FAILURE);
exit(EXIT_SUCCESS);
}
SEE ALSO
readv(2), writev(2)
Linux man-pages 6.9.1 2024-05-02 process_vm_readv(2)
Generated by dwww version 1.16 on Tue Dec 16 04:15:08 CET 2025.