Provided by: libmm-dev_1.4.2-5ubuntu3_amd64 
NAME
OSSP mm - Shared Memory Allocation
VERSION
OSSP mm 1.4.2 (15-Aug-2006)
SYNOPSIS
#include "mm.h"
Global Malloc-Replacement API
int MM_create(size_t size, const char *file);
int MM_permission(mode_t mode, uid_t owner, gid_t group);
void MM_reset(void);
void MM_destroy(void);
int MM_lock(mm_lock_mode mode);
int MM_unlock(void);
void *MM_malloc(size_t size);
void *MM_realloc(void *ptr, size_t size);
void MM_free(void *ptr);
void *MM_calloc(size_t number, size_t size);
char *MM_strdup(const char *str);
size_t MM_sizeof(void *ptr);
size_t MM_maxsize(void);
size_t MM_available(void);
char *MM_error(void);
Standard Malloc-Style API
MM *mm_create(size_t size, char *file);
int mm_permission(MM *mm, mode_t mode, uid_t owner, gid_t group);
void mm_reset(MM *mm);
void mm_destroy(MM *mm);
int mm_lock(MM *mm, mm_lock_mode mode);
int mm_unlock(MM *mm);
void *mm_malloc(MM *mm, size_t size);
void *mm_realloc(MM *mm, void *ptr, size_t size);
void mm_free(MM *mm, void *ptr);
void *mm_calloc(MM *mm, size_t number, size_t size);
char *mm_strdup(MM *mm, const char *str);
size_t mm_sizeof(MM *mm, void *ptr);
size_t mm_maxsize(void);
size_t mm_available(MM *mm);
char *mm_error(void);
void mm_display_info(MM *mm);
Low-level Shared Memory API
void *mm_core_create(size_t size, char *file);
int mm_core_permission(void *core, mode_t mode, uid_t owner, gid_t group);
void mm_core_delete(void *core);
int mm_core_lock(void *core, mm_lock_mode mode);
int mm_core_unlock(void *core);
size_t mm_core_size(void *core);
size_t mm_core_maxsegsize(void);
size_t mm_core_align2page(size_t size);
size_t mm_core_align2click(size_t size);
Internal Library API
void mm_lib_error_set(unsigned int, const char *str);
char *mm_lib_error_get(void);
int mm_lib_version(void);
DESCRIPTION
The OSSP mm library is a 2-layer abstraction library which simplifies the usage of shared memory between
forked (and this way strongly related) processes under Unix platforms. On the first (lower) layer it
hides all platform dependent implementation details (allocation and locking) when dealing with shared
memory segments and on the second (higher) layer it provides a high-level malloc(3)-style API for a
convenient and well known way to work with data-structures inside those shared memory segments.
The abbreviation OSSP mm is historically and originally comes from the phrase ``memory mapped'' as used
by the POSIX.1 mmap(2) function. Because this facility is internally used by this library on most
platforms to establish the shared memory segments.
LIBRARY STRUCTURE
This library is structured into three main APIs which are internally based on each other:
Global Malloc-Replacement API
This is the most high-level API which directly can be used as replacement API for the POSIX.1 memory
allocation API (malloc(2) and friends). This is useful when converting heap based data structures to
shared memory based data structures without the need to change the code dramatically. All which is
needed is to prefix the POSIX.1 memory allocation functions with `"MM_"', i.e. `"malloc"' becomes
`"MM_malloc"', `"strdup"' becomes `"MM_strdup"', etc. This API internally uses just a global `"MM *"'
pool for calling the corresponding functions (those with prefix `"mm_"') of the Standard Malloc-Style
API.
Standard Malloc-Style API
This is the standard high-level memory allocation API. Its interface is similar to the Global Malloc-
Replacement API but it uses an explicit `"MM *"' pool to operate on. That is why every function of
this API has an argument of type `"MM *"' as its first argument. This API provides a comfortable way
to work with small dynamically allocated shared memory chunks inside large statically allocated
shared memory segments. It is internally based on the Low-Level Shared Memory API for creating the
underlying shared memory segment.
Low-Level Shared Memory API
This is the basis of the whole OSSP mm library. It provides low-level functions for creating shared
memory segments with mutual exclusion (in short mutex) capabilities in a portable way. Internally the
shared memory and mutex facility is implemented in various platform-dependent ways. A list of
implementation variants follows under the next topic.
SHARED MEMORY IMPLEMENTATION
Internally the shared memory facility is implemented in various platform-dependent ways. Each way has its
own advantages and disadvantages (in addition to the fact that some variants aren't available at all on
some platforms). The OSSP mm library's configuration procedure tries hard to make a good decision. The
implemented variants are now given for overview and background reasons with their advantages and
disadvantages and in an ascending order, i.e. the OSSP mm configuration mechanism chooses the last
available one in the list as the preferred variant.
Classical mmap(2) on temporary file (MMFILE)
Advantage: maximum portable. Disadvantage: needs a temporary file on the filesystem.
mmap(2) via POSIX.1 shm_open(3) on temporary file (MMPOSX)
Advantage: standardized by POSIX.1 and theoretically portable. Disadvantage: needs a temporary file
on the filesystem and is is usually not available on existing Unix platform.
SVR4-style mmap(2) on "/dev/zero" device (MMZERO)
Advantage: widely available and mostly portable on SVR4 platforms. Disadvantage: needs the
"/dev/zero" device and a mmap(2) which supports memory mapping through this device.
SysV IPC shmget(2) (IPCSHM)
Advantage: does not need a temporary file or external device. Disadvantage: although available on
mostly all modern Unix platforms, it has strong restrictions like the maximum size of a single shared
memory segment (can be as small as 100KB, but depends on the platform).
4.4BSD-style mmap(2) via "MAP_ANON" facility (MMANON)
Advantage: does not need a temporary file or external device. Disadvantage: usually only available
on BSD platforms and derivatives.
LOCKING IMPLEMENTATION
As for the shared memory facility, internally the locking facility is implemented in various platform-
dependent ways. They are again listed in ascending order, i.e. the OSSP mm configuration mechanism
chooses the last available one in the list as the preferred variant. The list of implemented variants is:
4.2BSD-style flock(2) on temporary file (FLOCK)
Advantage: exists on a lot of platforms, especially on older Unix derivatives. Disadvantage: needs a
temporary file on the filesystem and has to re-open file-descriptors to it in each(!) fork(2)'ed
child process.
SysV IPC semget(2) (IPCSEM)
Advantage: exists on a lot of platforms and does not need a temporary file. Disadvantage: an unmeant
termination of the application leads to a semaphore leak because the facility does not allow a
``remove in advance'' trick (as the IPC shared memory facility does) for safe cleanups.
SVR4-style fcntl(2) on temporary file (FCNTL)
Advantage: exists on a lot of platforms and is also the most powerful variant (although not always
the fastest one). Disadvantage: needs a temporary file.
MEMORY ALLOCATION STRATEGY
The memory allocation strategy the Standard Malloc-Style API functions use internally is the following:
Allocation
If a chunk of memory has to be allocated, the internal list of free chunks is searched for a minimal-
size chunk which is larger or equal than the size of the to be allocated chunk (a best fit strategy).
If a chunk is found which matches this best-fit criteria, but is still a lot larger than the
requested size, it is split into two chunks: One with exactly the requested size (which is the
resulting chunk given back) and one with the remaining size (which is immediately re-inserted into
the list of free chunks).
If no fitting chunk is found at all in the list of free chunks, a new one is created from the spare
area of the shared memory segment until the segment is full (in which case an out of memory error
occurs).
Deallocation
If a chunk of memory has to be deallocated, it is inserted in sorted manner into the internal list of
free chunks. The insertion operation automatically merges the chunk with a previous and/or a next
free chunk if possible, i.e. if the free chunks stay physically seamless (one after another) in
memory, to automatically form larger free chunks out of smaller ones.
This way the shared memory segment is automatically defragmented when memory is deallocated.
This strategy reduces memory waste and fragmentation caused by small and frequent allocations and
deallocations to a minimum.
The internal implementation of the list of free chunks is not specially optimized (for instance by using
binary search trees or even splay trees, etc), because it is assumed that the total amount of entries in
the list of free chunks is always small (caused both by the fact that shared memory segments are usually
a lot smaller than heaps and the fact that we always defragment by merging the free chunks if possible).
API FUNCTIONS
In the following, all API functions are described in detail. The order directly follows the one in the
SYNOPSIS section above.
Global Malloc-Replacement API
int MM_create(size_t size, const char *file);
This initializes the global shared memory pool with size and file and has to be called before any
fork(2) operations are performed by the application.
int MM_permission(mode_t mode, uid_t owner, gid_t group);
This sets the filesystem mode, owner and group for the global shared memory pool (has effects only if
the underlying shared memory segment implementation is actually based on external auxiliary files).
The arguments are directly passed through to chmod(2) and chown(2).
void MM_reset(void);
This resets the global shared memory pool: all chunks that have been allocated in the pool are marked
as free and are eligible for reuse. The global memory pool itself is not destroyed.
void MM_destroy(void);
This destroys the global shared memory pool and should be called after all child processes were
killed.
int MM_lock(mm_lock_mode mode);
This locks the global shared memory pool for the current process in order to perform either
shared/read-only (mode is "MM_LOCK_RD") or exclusive/read-write (mode is "MM_LOCK_RW") critical
operations inside the global shared memory pool.
int MM_unlock(void);
This unlocks the global shared memory pool for the current process after the critical operations were
performed inside the global shared memory pool.
void *MM_malloc(size_t size);
Identical to the POSIX.1 malloc(3) function but instead of allocating memory from the heap it
allocates it from the global shared memory pool.
void MM_free(void *ptr);
Identical to the POSIX.1 free(3) function but instead of deallocating memory in the heap it
deallocates it in the global shared memory pool.
void *MM_realloc(void *ptr, size_t size);
Identical to the POSIX.1 realloc(3) function but instead of reallocating memory in the heap it
reallocates it inside the global shared memory pool.
void *MM_calloc(size_t number, size_t size);
Identical to the POSIX.1 calloc(3) function but instead of allocating and initializing memory from
the heap it allocates and initializes it from the global shared memory pool.
char *MM_strdup(const char *str);
Identical to the POSIX.1 strdup(3) function but instead of creating the string copy in the heap it
creates it in the global shared memory pool.
size_t MM_sizeof(const void *ptr);
This function returns the size in bytes of the chunk starting at ptr when ptr was previously
allocated with MM_malloc(3). The result is undefined if ptr was not previously allocated with
MM_malloc(3).
size_t MM_maxsize(void);
This function returns the maximum size which is allowed as the first argument to the MM_create(3)
function.
size_t MM_available(void);
Returns the amount in bytes of still available (free) memory in the global shared memory pool.
char *MM_error(void);
Returns the last error message which occurred inside the OSSP mm library.
Standard Malloc-Style API
MM *mm_create(size_t size, const char *file);
This creates a shared memory pool which has space for approximately a total of size bytes with the
help of file. Here file is a filesystem path to a file which need not to exist (and perhaps is never
created because this depends on the platform and chosen shared memory and mutex implementation). The
return value is a pointer to a "MM" structure which should be treated as opaque by the application.
It describes the internals of the created shared memory pool. In case of an error "NULL" is returned.
A size of 0 means to allocate the maximum allowed size which is platform dependent and is between a
few KB and the soft limit of 64MB.
int mm_permission(MM *mm, mode_t mode, uid_t owner, gid_t group);
This sets the filesystem mode, owner and group for the shared memory pool mm (has effects only when
the underlying shared memory segment implementation is actually based on external auxiliary files).
The arguments are directly passed through to chmod(2) and chown(2).
void mm_reset(MM *mm);
This resets the shared memory pool mm: all chunks that have been allocated in the pool are marked as
free and are eligible for reuse. The memory pool itself is not destroyed.
void mm_destroy(MM *mm);
This destroys the complete shared memory pool mm and with it all chunks which were allocated in this
pool. Additionally any created files on the filesystem corresponding to the shared memory pool are
unlinked.
int mm_lock(MM *mm, mm_lock_mode mode);
This locks the shared memory pool mm for the current process in order to perform either
shared/read-only (mode is "MM_LOCK_RD") or exclusive/read-write (mode is "MM_LOCK_RW") critical
operations inside the global shared memory pool.
int mm_unlock(MM *mm);
This unlocks the shared memory pool mm for the current process after critical operations were
performed inside the global shared memory pool.
void *mm_malloc(MM *mm, size_t size);
This function allocates size bytes from the shared memory pool mm and returns either a (virtual
memory word aligned) pointer to it or "NULL" in case of an error (out of memory). It behaves like the
POSIX.1 malloc(3) function but instead of allocating memory from the heap it allocates it from the
shared memory segment underlying mm.
void mm_free(MM *mm, void *ptr);
This deallocates the chunk starting at ptr in the shared memory pool mm. It behaves like the POSIX.1
free(3) function but instead of deallocating memory from the heap it deallocates it from the shared
memory segment underlying mm.
void *mm_realloc(MM *mm, void *ptr, size_t size);
This function reallocates the chunk starting at ptr inside the shared memory pool mm with the new
size of size bytes. It behaves like the POSIX.1 realloc(3) function but instead of reallocating
memory in the heap it reallocates it in the shared memory segment underlying mm.
void *mm_calloc(MM *mm, size_t number, size_t size);
This is similar to mm_malloc(3), but additionally clears the chunk. It behaves like the POSIX.1
calloc(3) function. It allocates space for number objects, each size bytes in length from the shared
memory pool mm. The result is identical to calling mm_malloc(3) with an argument of ``number *
size'', with the exception that the allocated memory is initialized to nul bytes.
char *mm_strdup(MM *mm, const char *str);
This function behaves like the POSIX.1 strdup(3) function. It allocates sufficient memory inside the
shared memory pool mm for a copy of the string str, does the copy, and returns a pointer to it. The
pointer may subsequently be used as an argument to the function mm_free(3). If insufficient shared
memory is available, "NULL" is returned.
size_t mm_sizeof(MM *mm, const void *ptr);
This function returns the size in bytes of the chunk starting at ptr when ptr was previously
allocated with mm_malloc(3) inside the shared memory pool mm. The result is undefined when ptr was
not previously allocated with mm_malloc(3).
size_t mm_maxsize(void);
This function returns the maximum size which is allowed as the first argument to the mm_create(3)
function.
size_t mm_available(MM *mm);
Returns the amount in bytes of still available (free) memory in the shared memory pool mm.
char *mm_error(void);
Returns the last error message which occurred inside the OSSP mm library.
void mm_display_info(MM *mm);
This is debugging function which displays a summary page for the shared memory pool mm describing
various internal sizes and counters.
Low-Level Shared Memory API
void *mm_core_create(size_t size, const char *file);
This creates a shared memory area which is at least size bytes in size with the help of file. The
value size has to be greater than 0 and less or equal the value returned by mm_core_maxsegsize(3).
Here file is a filesystem path to a file which need not to exist (and perhaps is never created
because this depends on the platform and chosen shared memory and mutex implementation). The return
value is either a (virtual memory word aligned) pointer to the shared memory segment or "NULL" in
case of an error. The application is guaranteed to be able to access the shared memory segment from
byte 0 to byte size-1 starting at the returned address.
int mm_core_permission(void *core, mode_t mode, uid_t owner, gid_t group);
This sets the filesystem mode, owner and group for the shared memory segment code (has effects only
when the underlying shared memory segment implementation is actually based on external auxiliary
files). The arguments are directly passed through to chmod(2) and chown(2).
void mm_core_delete(void *core);
This deletes a shared memory segment core (as previously returned by a mm_core_create(3) call). After
this operation, accessing the segment starting at core is no longer allowed and will usually lead to
a segmentation fault.
int mm_core_lock(const void *core, mm_lock_mode mode);
This function acquires an advisory lock for the current process on the shared memory segment core for
either shared/read-only (mode is "MM_LOCK_RD") or exclusive/read-write (mode is "MM_LOCK_RW")
critical operations between fork(2)'ed child processes.
int mm_core_unlock(const void *core);
This function releases a previously acquired advisory lock for the current process on the shared
memory segment core.
size_t mm_core_size(const void *core);
This returns the size in bytes of core. This size is exactly the size which was used for creating the
shared memory area via mm_core_create(3). The function is provided just for convenience reasons to
not require the application to remember the memory size behind core itself.
size_t mm_core_maxsegsize(void);
This returns the number of bytes of a maximum-size shared memory segment which is allowed to allocate
via the MM library. It is between a few KB and the soft limit of 64MB.
size_t mm_core_align2page(size_t size);
This is just a utility function which can be used to align the number size to the next virtual memory
page boundary used by the underlying platform. The memory page boundary under Unix platforms is
usually somewhere between 2048 and 16384 bytes. You do not have to align the size arguments of other
OSSP mm library functions yourself, because this is already done internally. This function is
exported by the OSSP mm library just for convenience reasons in case an application wants to perform
similar calculations for other purposes.
size_t mm_core_align2word(size_t size);
This is another utility function which can be used to align the number size to the next virtual
memory word boundary used by the underlying platform. The memory word boundary under Unix platforms
is usually somewhere between 4 and 16 bytes. You do not have to align the size arguments of other
OSSP mm library functions yourself, because this is already done internally. This function is
exported by the OSSP mm library just for convenience reasons in case an application wants to perform
similar calculations for other purposes.
Low-Level Shared Memory API
void mm_lib_error_set(unsigned int, const char *str);
This is a function which is used internally by the various MM function to set an error string. It's
usually not called directly from applications.
char *mm_lib_error_get(void);
This is a function which is used internally by MM_error(3) and mm_error(3) functions to get the
current error string. It is usually not called directly from applications.
int mm_lib_version(void);
This function returns a hex-value ``0xVRRTLL'' which describes the current OSSP mm library version. V
is the version, RR the revisions, LL the level and T the type of the level (alphalevel=0,
betalevel=1, patchlevel=2, etc). For instance OSSP mm version 1.0.4 is encoded as 0x100204. The
reason for this unusual mapping is that this way the version number is steadily increasing.
RESTRICTIONS
The maximum size of a continuous shared memory segment one can allocate depends on the underlying
platform. This cannot be changed, of course. But currently the high-level malloc(3)-style API just uses
a single shared memory segment as the underlying data structure for an "MM" object which means that the
maximum amount of memory an "MM" object represents also depends on the platform.
This could be changed in later versions by allowing at least the high-level malloc(3)-style API to
internally use multiple shared memory segments to form the "MM" object. This way "MM" objects could have
arbitrary sizes, although the maximum size of an allocatable continuous chunk still is bounded by the
maximum size of a shared memory segment.
SEE ALSO
mm-config(1).
malloc(3), calloc(3), realloc(3), strdup(3), free(3), mmap(2), shmget(2), shmctl(2), flock(2), fcntl(2),
semget(2), semctl(2), semop(2).
HOME
http://www.ossp.org/pkg/lib/mm/
HISTORY
This library was originally written in January 1999 by Ralf S. Engelschall <rse@engelschall.com> for use
in the Extended API (EAPI) of the Apache HTTP server project (see http://www.apache.org/), which was
originally invented for mod_ssl (see http://www.modssl.org/).
Its base idea (a malloc-style API for handling shared memory) was originally derived from the non-
publically available mm_malloc library written in October 1997 by Charles Randall
<crandall@matchlogic.com> for MatchLogic, Inc.
In 2000 this library joined the OSSP project where all other software development projects of Ralf S.
Engelschall are located.
AUTHOR
Ralf S. Engelschall
rse@engelschall.com
www.engelschall.com