Hi
I want to lock or prevent a portion of memory which I allocated. So I tried MLOCK, MPROTECT and some like this. But all these functions works only on page border. Can I know why that so.
Because that's how virtual memory works.
All those permissions take memory too, and have to be stored somewhere. Imagine how much memory would be required to remember access permissions for each individual byte of memory on your system! They had to simplify it somewhere, and pages is how they did that. Cut memory into larger, more manageable chunks.
If you need to individually protect parts, why not keep pointers to pages in your structure?
Quote:
Is that possible to protect a portion of memory which is in middle of the page.
No.
By the way: It's against the rules to spam multiple topics of the same question rephrased across different forums. If you're not answered immediately, wait -- we're not "on call".
P.S. It's possible to cause a bus error in the middle of a page by mapping in a file that's shorter than a page, but you only get errors trying to read/write past EOF, nowhere else.
Last edited by Corona688; 03-24-2011 at 11:26 AM..
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Discussion started by: dcicc
4 Replies
LEARN ABOUT NETBSD
mlock
MLOCK(2) BSD System Calls Manual MLOCK(2)NAME
mlock, munlock -- lock (unlock) physical pages in memory
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <sys/mman.h>
int
mlock(void *addr, size_t len);
int
munlock(void *addr, size_t len);
DESCRIPTION
The mlock system call locks into memory the physical pages associated with the virtual address range starting at addr for len bytes. The
munlock call unlocks pages previously locked by one or more mlock calls. The entire range of memory must be allocated.
After an mlock call, the indicated pages will cause neither a non-resident page nor address-translation fault until they are unlocked. They
may still cause protection-violation faults or TLB-miss faults on architectures with software-managed TLBs. The physical pages remain in
memory until all locked mappings for the pages are removed. Multiple processes may have the same physical pages locked via their own virtual
address mappings. A single process may likewise have pages multiply-locked via different virtual mappings of the same pages or via nested
mlock calls on the same address range. Unlocking is performed explicitly by munlock or implicitly by a call to munmap which deallocates the
unmapped address range. Locked mappings are not inherited by the child process after a fork(2).
Since physical memory is a potentially scarce resource, processes are limited in how much they can lock down. A single process can mlock the
minimum of a system-wide ``wired pages'' limit and the per-process RLIMIT_MEMLOCK resource limit.
Portable code should ensure that the addr and len parameters are aligned to a multiple of the page size, even though the NetBSD implementa-
tion will round as necessary.
RETURN VALUES
A return value of 0 indicates that the call succeeded and all pages in the range have either been locked or unlocked. A return value of -1
indicates an error occurred and the locked status of all pages in the range remains unchanged. In this case, the global location errno is
set to indicate the error.
ERRORS
mlock() will fail if:
[EAGAIN] Locking the indicated range would exceed either the system or per-process limit for locked memory.
[EINVAL] The length is negative; or the address or length given is not page aligned and the implementation does not round.
[ENOMEM] Some portion of the indicated address range is not allocated. There was an error faulting/mapping a page.
[EPERM] mlock() was called by non-root on an architecture where locked page accounting is not implemented.
munlock() will fail if:
[EINVAL] The length is negative; or the address or length given is not page aligned and the implementation does not round.
[ENOMEM] Some portion of the indicated address range is not allocated. Some portion of the indicated address range is not locked.
SEE ALSO fork(2), mincore(2), mmap(2), munmap(2), setrlimit(2), getpagesize(3)STANDARDS
The mlock() and munlock() functions conform to IEEE Std 1003.1b-1993 (``POSIX.1'').
HISTORY
The mlock() and munlock() functions first appeared in 4.4BSD.
BUGS
The per-process resource limit is a limit on the amount of virtual memory locked, while the system-wide limit is for the number of locked
physical pages. Hence a process with two distinct locked mappings of the same physical page counts as 2 pages against the per-process limit
and as only a single page in the system limit.
BSD February 28, 2011 BSD