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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | STANDARDS | HISTORY | NOTES | EXAMPLES | SEE ALSO | COLOPHON |
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process_vm_readv(2) System Calls Manual process_vm_readv(2)
process_vm_readv, process_vm_writev - transfer data between
process address spaces
Standard C library (libc, -lc)
#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
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, 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 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 4 KiB) 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.)
Permission to read from or write to another process is governed by
a ptrace access mode PTRACE_MODE_ATTACH_REALCREDS check; see
ptrace(2).
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.
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.
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
structures.
EPERM The caller does not have permission to access the address
space of the process pid.
ESRCH No process with ID pid exists.
Linux.
Linux 3.2, glibc 2.15.
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).
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);
}
readv(2), writev(2)
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Linux man-pages 6.15 2025-05-17 process_vm_readv(2)
Pages that refer to this page: process_madvise(2), ptrace(2), syscalls(2), capabilities(7)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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