HP-UX Reference (11i v3 07/02) - 3 Library Functions A-M (vol 6)

m
malloc(3C) malloc(3C)
the thread to borrow blocks from a global pool if available. This may be more efficient than going back to
the arena. max_cache_misses
is a hint to the caching algorithm indicating a particular number of
cache misses after which it will search the global pool for appropriate sized blocks. The algorithm also
releases unused cache blocks to the global pool. If
max_cache_misses
is 0 or not set, cache exchange is
turned off.
num_global_slots
is an indication of the size of the global cache pool. This parameter is valid only if
max_cache_misses
is turned on. The default value for num_global_slots
is 8.
The maximum number of blocks that will be cached for each thread is
bucket_size * buckets.
Here are examples of how to use
_M_CACHE_OPTS,
$ export _M_CACHE_OPTS = 1024:32:20:4:8
This means that bucket_size is 1024, buckets is 32,
retirement_age is 20 minutes,
max_cache_misses
is 4, and num_global_slots
is 8.
$ export _M_CACHE_OPTS = 1024:32:20
This is a valid configuration where the global pool is not activated.
_M_ARENA_OPTS and _M_CACHE_OPTS
have no effect on non-threaded applications, while
_M_SBA_OPTS has.
NOTE: Modifying these variables increases the chances of surfacing existing user memory defects such as
buffer overrun.
WARNINGS
malloc() functions use brk() and sbrk() (see brk(2)) to increase the address space of a process.
Therefore, an application program that uses brk() or sbrk() must not use them to decrease the address
space, because this confuses the malloc() functions.
free() and realloc() do not check their pointer argument for validity.
The following actions are considered bad programming practices and should not be done. The results are
unpredictable, probably undesirable and not supported. Examples of undesirable results are loss of data,
memory fault, bus error or infinite loop.
• Attempting to free() or realloc() a pointer not generated as the result of a call to
mal-
loc()
, realloc() , calloc(),orvalloc().
• Reading or writing data outside the boundaries of an allocated block.
• Attempting to realloc() an aligned block such as the result of valloc().
The following actions are strongly discouraged and may be unsupported in a future version of malloc():
• Attempting to
free() the same block twice.
• Depending on unmodified contents of a block after it has been freed.
• Attempting to realloc() a block after it is freed.
Undocumented features of earlier memory allocators have not been duplicated. Applications which used
any of the above bad programming practices or discouraged practices are not guaranteed to continue func-
tioning at future releases.
Compatibility
The only external difference between the old malloc() allocator and the malloc() allocator is that the
old allocator would return a NULL pointer for a request of zero bytes. The malloc() allocator returns a
valid memory address. This is not a concern for most applications.
By default, SBA (Small Block Allocation) is turned on for Itanium-based systems and is turned off for PA-
RISC systems. This was due to performance concerns. Please refer to the EXTERNAL INFLUENCES sec-
tion of this manpage for details.
SEE ALSO
brk(2), errno(2), thread_safety(5).
HP-UX 11i Version 3: February 2007 − 5 − Hewlett-Packard Company 779