HP-UX Reference (11i v2 07/12) - 3 Library Functions A-M (vol 6)

m
malloc(3C) malloc(3C)
_M_SBA_OPTS is used to turn on the small block allocator, and to set up parameters for the small block
allocator, namely, maxfast, grain, and numlblks. Applications with small block allocator turned on usually
run faster than with it turned off. Small block allocator can be turned on through
mallopt(); however, it
is not early enough for C++/Java applications. The environment variable turns it on before the application
starts. The
mallopt() call can still be used the same way. If the environment variable is set, and no
small block allocator has been used, the subsequent
mallopt() calls can still overwrite whatever is set
through
_M_SBA_OPTS . If the environment variable is set, and small block allocator has been used, then
mallopt() will have no effect.
To use this environment variable,
$ export _M_SBA_OPTS = 512:100:16
This means that the maxfast size is 512, the number of small blocks is 100, and the grain size is 16. You
have to supply all 3 values, and in that order. If not, the default values will be used instead.
_M_ARENA_OPTS
has no effects on non-threaded applications, while
_M_SBA_OPTS has.
Three new global variables,
__hp_malloc_maxfast
, __hp_malloc_num_smallblocks
and
__hp_malloc_grain
are introduced for Itanium-based systems to override the
_M_SBA_OPTS
environment option. When these three variables are initialized within an application, _M_SBA_OPTS has
no effect. This way, a finely tuned application can lock in performance across different user environments.
However, as _M_ARENA_OPTS, a subsequent call to
mallopt() before any block of memory was allo-
cated will change the
malloc() behavior. By default, these three variables will be initialized to zero at
start up. It is the same as setting them to
extern int __hp_malloc_maxfast=512;
extern int __hp_malloc_num_smallblocks=100;
extern int __hp_malloc_grain=16;
By default, SBA (Small Block Allocation) is turned on for Itanium-based systems. This may contribute to
better application performance. A user can set
extern int __hp_malloc_maxfast=-1;
This will turn off SBA.
For all other possible values, please refer to mallopt().
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.
HP-UX 11i Version 2: December 2007 Update − 4 − Hewlett-Packard Company 723