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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)

> `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)

> `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)

> `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)

> `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.