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3.4.3 Functions To Lock And Unlock Pages

The symbols in this section are declared in sys/mman.h. These functions are defined by POSIX.1b, but their availability depends on your kernel. If your kernel doesn't allow these functions, they exist but always fail. They are available with a Linux kernel.

Portability Note: POSIX.1b requires that when the mlock and munlock functions are available, the file unistd.h define the macro _POSIX_MEMLOCK_RANGE and the file limits.h define the macro PAGESIZE to be the size of a memory page in bytes. It requires that when the mlockall and munlockall functions are available, the unistd.h file define the macro _POSIX_MEMLOCK. The GNU C Library conforms to this requirement.

— Function: int mlock (const void *addr, size_t len)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

mlock locks a range of the calling process' virtual pages.

The range of memory starts at address addr and is len bytes long. Actually, since you must lock whole pages, it is the range of pages that include any part of the specified range.

When the function returns successfully, each of those pages is backed by (connected to) a real frame (is resident) and is marked to stay that way. This means the function may cause page-ins and have to wait for them.

When the function fails, it does not affect the lock status of any pages.

The return value is zero if the function succeeds. Otherwise, it is -1 and errno is set accordingly. errno values specific to this function are:

ENOMEM
  • At least some of the specified address range does not exist in the calling process' virtual address space.
  • The locking would cause the process to exceed its locked page limit.

EPERM
The calling process is not superuser.
EINVAL
len is not positive.
ENOSYS
The kernel does not provide mlock capability.

You can lock all a process' memory with mlockall. You unlock memory with munlock or munlockall.

To avoid all page faults in a C program, you have to use mlockall, because some of the memory a program uses is hidden from the C code, e.g. the stack and automatic variables, and you wouldn't know what address to tell mlock.

— Function: int munlock (const void *addr, size_t len)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

munlock unlocks a range of the calling process' virtual pages.

munlock is the inverse of mlock and functions completely analogously to mlock, except that there is no EPERM failure.

— Function: int mlockall (int flags)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

mlockall locks all the pages in a process' virtual memory address space, and/or any that are added to it in the future. This includes the pages of the code, data and stack segment, as well as shared libraries, user space kernel data, shared memory, and memory mapped files.

flags is a string of single bit flags represented by the following macros. They tell mlockall which of its functions you want. All other bits must be zero.

MCL_CURRENT
Lock all pages which currently exist in the calling process' virtual address space.
MCL_FUTURE
Set a mode such that any pages added to the process' virtual address space in the future will be locked from birth. This mode does not affect future address spaces owned by the same process so exec, which replaces a process' address space, wipes out MCL_FUTURE. See Executing a File.

When the function returns successfully, and you specified MCL_CURRENT, all of the process' pages are backed by (connected to) real frames (they are resident) and are marked to stay that way. This means the function may cause page-ins and have to wait for them.

When the process is in MCL_FUTURE mode because it successfully executed this function and specified MCL_CURRENT, any system call by the process that requires space be added to its virtual address space fails with errno = ENOMEM if locking the additional space would cause the process to exceed its locked page limit. In the case that the address space addition that can't be accommodated is stack expansion, the stack expansion fails and the kernel sends a SIGSEGV signal to the process.

When the function fails, it does not affect the lock status of any pages or the future locking mode.

The return value is zero if the function succeeds. Otherwise, it is -1 and errno is set accordingly. errno values specific to this function are:

ENOMEM
  • At least some of the specified address range does not exist in the calling process' virtual address space.
  • The locking would cause the process to exceed its locked page limit.

EPERM
The calling process is not superuser.
EINVAL
Undefined bits in flags are not zero.
ENOSYS
The kernel does not provide mlockall capability.

You can lock just specific pages with mlock. You unlock pages with munlockall and munlock.

— Function: int munlockall (void)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

munlockall unlocks every page in the calling process' virtual address space and turn off MCL_FUTURE future locking mode.

The return value is zero if the function succeeds. Otherwise, it is -1 and errno is set accordingly. The only way this function can fail is for generic reasons that all functions and system calls can fail, so there are no specific errno values.