man Pages(2): System Calls

memcntl(2)

NAME

memcntl- memory management control

SYNOPSIS

#include <sys/types.h>
#include <sys/mman.h>

addr

len

cmd

arg

attr

mask

DESCRIPTION

The memcntl() function allows the calling process to apply a variety of control operations over the address space identified by the mappings established for the address range [addr, addr + len).

The addr argument must be a multiple of the pagesize as returned by sysconf(3C). The scope of the control operations can be further defined with additional selection criteria (in the form of attributes) according to the bit pattern contained in attr.

The following attributes specify page mapping selection criteria:

SHARED: Page is mapped shared.
PRIVATE: Page is mapped private.

The following attributes specify page protection selection criteria. The selection criteria are constructed by a bitwise OR operation on the attribute bits and must match exactly.

PROT_READ: Page can be read.
PROT_WRITE: Page can be written.
PROT_EXEC: Page can be executed.

The following criteria may also be specified:

PROC_TEXT: Process text.
PROC_DATA: Process data.

The PROC_TEXT attribute specifies all privately mapped segments with read and execute permission, and the PROC_DATA attribute specifies all privately mapped segments with write permission.

Selection criteria can be used to describe various abstract memory objects within the address space on which to operate. If an operation shall not be constrained by the selection criteria, attr must have the value 0.

The operation to be performed is identified by the argument cmd. The symbolic names for the operations are defined in <sys/mman.h> as follows:

MC_LOCK

Lock in memory all pages in the range with attributes attr. A given page may be locked multiple times through different mappings; however, within a given mapping, page locks do not nest. Multiple lock operations on the same address in the same process will all be removed with a single unlock operation. A page locked in one process and mapped in another (or visible through a different mapping in the locking process) is locked in memory as long as the locking process does neither an implicit nor explicit unlock operation. If a locked mapping is removed, or a page is deleted through file removal or truncation, an unlock operation is implicitly performed. If a writable MAP_PRIVATE page in the address range is changed, the lock will be transferred to the private page.

At present arg is unused, but must be 0 to ensure compatibility with potential future enhancements.

MC_LOCKAS

Lock in memory all pages mapped by the address space with attributes attr. At present addr and len are unused, but must be NULL and 0 respectively, to ensure compatibility with potential future enhancements. The arg argument is a bit pattern built from the flags:

MCL_CURRENT: Lock current mappings
MCL_FUTURE: Lock future mappings

The value of arg determines whether the pages to be locked are those currently mapped by the address space, those that will be mapped in the future, or both. If MCL_FUTURE is specified, then all mappings subsequently added to the address space will be locked, provided sufficient memory is available.

MC_SYNC

Write to their backing storage locations all modified pages in the range with attributes attr. Optionally, invalidate cache copies. The backing storage for a modified MAP_SHARED mapping is the file the page is mapped to; the backing storage for a modified MAP_PRIVATE mapping is its swap area. The arg argument is a bit pattern built from the flags used to control the behavior of the operation:

MS_ASYNC: perform asynchronous writes
MS_SYNC: perform synchronous writes
MS_INVALIDATE: invalidate mappings

MS_ASYNC returns immediately once all write operations are scheduled; with MS_SYNC the function will not return until all write operations are completed.

MS_INVALIDATE invalidates all cached copies of data in memory, so that further references to the pages will be obtained by the system from their backing storage locations. This operation should be used by applications that require a memory object to be in a known state.

MC_UNLOCK

Unlock all pages in the range with attributes attr. At present arg is unused, but must be 0 to ensure compatibility with potential future enhancements.

MC_UNLOCKAS

Remove address space memory locks, and locks on all pages in the address space with attributes attr. At present addr, len, and arg are unused, but must be NULL, 0 and 0 respectively, to ensure compatibility with potential future enhancements.

The mask argument must be 0; it is reserved for future use.

Locks established with the lock operations are not inherited by a child process after fork(2). The memcntl() function fails if it attempts to lock more memory than a system-specific limit.

Due to the potential impact on system resources, all operations, with the exception of MC_SYNC, are restricted to processes with super-user effective user ID.

USAGE

The memcntl() function subsumes the operations of plock(3C) and mctl(3B).

RETURN VALUES

Upon successful completion, memcntl() returns 0; otherwise, it returns -1 and sets errno to indicate an error.

ERRORS

The memcntl() function will fail if:

EAGAIN: Some or all of the memory identified by the operation could not be locked when MC_LOCK or MC_LOCKAS was specified.
EBUSY: Some or all of the addresses in the range [addr, addr + len) are locked and MC_SYNC with the MS_INVALIDATE option was specified.
EINVAL: The addr argument specifies invalid selection criteria or is not a multiple of the page size as returned by sysconf(3C); the addr and/or len argument does not have the value 0 when MC_LOCKAS or MC_UNLOCKAS is specified; or the arg argument is not valid for the function specified.
ENOMEM: Some or all of the addresses in the range [addr, addr + len) are invalid for the address space of a process or specify one or more pages which are not mapped.
EPERM: The process's effective user ID is not super-user and MC_LOCK, MC_LOCKAS, MC_UNLOCK, or MC_UNLOCKAS was specified.

ATTRIBUTES

See attributes(5) for descriptions of the following attributes:

ATTRIBUTE TYPE	ATTRIBUTE VALUE
MT-Level	MT-Safe