OSSP mm - Shared Memory Allocation
OSSP mm 1.4.2 (15-Aug-2006)
#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);
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).
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.
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.
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).
http://www.ossp.org/pkg/lib/mm/
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.
Ralf S. Engelschall
rse@engelschall.com
www.engelschall.com