process_vm_readv(2) - Linux manual page (original) (raw)
processvmreadv(2) System Calls Manual processvmreadv(2)
NAME top
process_vm_readv, process_vm_writev - transfer data between
process address spacesLIBRARY top
Standard C library (_libc_, _-lc_)SYNOPSIS top
**#include <sys/uio.h>**
**ssize_t process_vm_readv(pid_t** _pid_**,**
**const struct iovec ***_localiov_**,**
**unsigned long** _liovcnt_**,**
**const struct iovec ***_remoteiov_**,**
**unsigned long** _riovcnt_**,**
**unsigned long** _flags_**);**
**ssize_t process_vm_writev(pid_t** _pid_**,**
**const struct iovec ***_localiov_**,**
**unsigned long** _liovcnt_**,**
**const struct iovec ***_remoteiov_**,**
**unsigned long** _riovcnt_**,**
**unsigned long** _flags_**);**Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
**process_vm_readv**(), **process_vm_writev**():
_GNU_SOURCEDESCRIPTION top
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 _remoteiov_ and _riovcnt_: _remoteiov_ is a pointer to
an array describing address ranges in the process _pid_, and _riovcnt_
specifies the number of elements in _remoteiov_. The data is
transferred to the locations specified by _localiov_ and _liovcnt_:
_localiov_ is a pointer to an array describing address ranges in
the calling process, and _liovcnt_ specifies the number of elements
in _localiov_.
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_, _localiov_, _riovcnt_, and _remoteiov_ have the
same meaning as for **process_vm_readv**().
The _localiov_ and _remoteiov_ arguments point to an array of _iovec_
structures, described in [iovec(3type)](../man3/iovec.3type.html).
Buffers are processed in array order. This means that
**process_vm_readv**() completely fills _localiov[0]_ before proceeding
to _localiov[1]_, and so on. Likewise, _remoteiov[0]_ is completely
read before proceeding to _remoteiov[1]_, and so on.
Similarly, **process_vm_writev**() writes out the entire contents of
_localiov[0]_ before proceeding to _localiov[1]_, and it completely
fills _remoteiov[0]_ before proceeding to _remoteiov[1]_.
The lengths of _remoteiov[i].iovlen_ and _localiov[i].iovlen_ 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(SCIOVMAX)_).
The count arguments and _localiov_ are checked before doing any
transfers. If the counts are too big, or _localiov_ 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 _remoteiov_ 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 _remoteiov_ elements and
have them merge back into a single write _localiov_ 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)](../man2/ptrace.2.html).RETURN VALUE top
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](../man3/errno.3.html)_ is set to indicate the error.ERRORS top
**EFAULT** The memory described by _localiov_ is outside the caller's
accessible address space.
**EFAULT** The memory described by _remoteiov_ is outside the
accessible address space of the process _pid_.
**EINVAL** The sum of the _iovlen_ values of either _localiov_ or
_remoteiov_ overflows a _ssizet_ 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.STANDARDS top
Linux.HISTORY top
Linux 3.2, glibc 2.15.NOTES top
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).EXAMPLES top
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 top
[readv(2)](../man2/readv.2.html), [writev(2)](../man2/writev.2.html)COLOPHON top
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man-pages@man7.orgLinux man-pages 6.10 2024-07-23 processvmreadv(2)
Pages that refer to this page:process_madvise(2), ptrace(2), syscalls(2), capabilities(7)