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Section: C Library Functions (3)
Updated: 21 May 2006
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mallopt, malloc_trim, malloc_usable_size, malloc_stats, malloc_get_state, malloc_set_state, mallinfo - dynamic allocation tuning  


#include <malloc.h>

int mallopt (int param, int value);
int malloc_trim (size_t pad);
size_t malloc_usable_size (void *ptr);
void malloc_stats (void);
void *malloc_get_state (void);
int malloc_set_state (void *ptr);
struct mallinfo mallinfo (void);  


These functions allow glibc-specific interaction with the dynamic memory allocation routines.

mallopt() allows tuning of the parameters affecting allocation. param is one of the constants listed below; value should be specified in bytes.

Free chunks not larger than the given value are not joined to adjacent free chunks; larger ones are joined. This is intended to optimize future requests for small chunks of the same size as previously freed chunks. Allowed values are in the range [0-80]; a value of 0 causes all free chunks to be joined. Default: 64.
Unused memory is returned to the OS when the size available to be returned exceeds the given value.

Note that not all unused memory is able to be returned to the OS; in particular, it is not possible to return an unused block when an in-use block lies between it and the ``top'' of the data segment. However, the free block may be used to satisfy future allocation requests.

Smaller values for this parameter cause sbrk() to be called more frequently with a negative argument, reducing memory usage, but with increased overhead of extra syscalls. A value of -1 disables trimming. Default: 128*1024.

When sbrk() is called with a positive argument to allocate additional address space, the given value specifies an additional amount to be allocated, beyond what is necessary to satisfy the request. This value also defines an amount of address space which is not released to the OS when sbrk() is called with a negative argument. Again, the intent is to minimize the number of syscalls, without needlessly using large usage of memory. Default: 0 (allocation requests are internally rounded up to the page size, and the extra allocated size is already sufficient to reduce the syscall overhead).
When an allocation request larger than the given value cannot be satisfied by an existing free chunk, the memory is guaranteed to be obtained with mmap(). Smaller requests might be allocated with either of mmap() or sbrk(). mmap()-allocated memory can be immediately returned to the OS when it is freed, but this is not true for all memory allocated with sbrk(); however, memory allocated by mmap() and later freed is neither joined nor reused, so the overhead is greater. Default: 128*1024.
The given value sets the maximum number of mmap()-allocated chunks allowed to be in use at a given time (even if the size of the allocation request exceeds the value of the M_MMAP_THRESHOLD parameter). This is useful on systems where the mmap() implementation scales poorly. A value of 0 disables the use of mmap(). Default: 65536.
The 3 low bits of this value control the behavior when heap corruption is detected.

If set to 0, errors are silently ignored (but the behavior of the program is undefined).

If the low bit is set, an error message is printed; If the third-lowest bit is unset, the message is surrounded by asterisks ("*") and includes the relevant pointer address.

If the second-lowest bit is set, the program is then terminated with abort().

The default value for glibc 2.3.3 and earlier was 1, causing only an informative message to be output. glibc-2.3.4 changes the default to 3, which also causes the program to abort.

malloc_trim() explicitly requests that any unused memory space be returned to the OS. Note that this happens automatically when free() is called with a sufficiently large chunk; see the M_TRIM_THRESHOLD and M_TOP_PAD parameters, above. pad specifies the number of bytes to be retained for use in future allocation requests; when called by free(), this is the value of M_TOP_PAD.

malloc_usable_size() returns the number of bytes available in the dynamically allocated buffer ptr, which may be greater than the requested size (but is guaranteed to be at least as large, if the request was successful). Typically, you should store the requested allocation size rather than use this function.

malloc_stats() outputs to stderr some information about the program's usage of dynamic memory. Information for each arena is displayed.

system bytes is the amount of address space obtained from the OS

in use bytes is the amount of that space which the program has requested and are in use.

max mmap regions is largest number of mmap() regions allocated at a given time.

max mmap bytes is the largest total amount of address space ever allocated by mmap() at a given time.

system bytes and in use bytes output appears twice, once excluding mmap, and later including mmap().

malloc_get_state() returns a ... malloc_set_state()

mallinfo() returns a struct mallinfo with allocation information, similar to what is printed by malloc_stats(). The return value is a structure, not a pointer; it is thread-safe. Only the information for the main arena is returned. The structure contains the following members:

int arena;
Address space used by the dynamic allocation routines, not including that from mmap()
int ordblks;
Count of independent, unused chunks
int smblks;
Count of chunks small enough to qualify for fast allocation without being joined to adjacent free chunks; this field is not meaningful in the glibc implementation.
int hblks;
Count of chunks allocated by mmap()
int hblkhd;
Address space allocated by the dynamic allocation routines using mmap()
int usmblks;
Largest amount of address space ever used by the process; this field is not meaningful in the glibc implementation.
int fsmblks;
Total unused address space in small, non-joined chunks; this field is not meaningful in the glibc implementation.
int uordblks;
Dynamically allocated address space used by the program, including book-keeping overhead per allocation of sizeof(void *) bytes.
int fordblks;
Unused address space, including that from mmap()
int keepcost;
Upper bound on the address space available for return to the OS by malloc_trim().


mallopt() returns 1 on success, and 0 on failure.

malloc_trim() returns 1 if any memory was returned to the OS, and 0 otherwise.

malloc_usable_size() returns the usable size of the allocated region beginning at ptr, or 0 if ptr is NULL.

malloc_get_state() returns a pointer to a description of the state of the allocation routines, or NULL on error.

malloc_set_state() returns 0 on success, and nonzero on error.

mallinfo() returns a structure containing information about the dynamic memory use of the program.  


The glibc malloc implementation is modified to allow use of multiple "arenas"; the malloc_stats() output is not as described in the header files and documentation, and the mallinfo() function only returns information for the main arena.

The default values listed for the mallopt() parameters may vary between installations, and should only serve as a guideline while tweaking the values; refer to the source code for your distribution's glibc package to establish the real defaults.  


Since libc 5.4.23, the mallopt() tuning parameters are accessible at runtime through the following environment variables:


Only the first byte of MALLOC_CHECK_ is considered; "10" is interpreted as 1, and "64" is interpreted as 6.  


mallopt() and mallinfo(), and M_MXFAST are defined by SVID/XPG.

That standard also defines the mallopt() parameters M_NLBLKS, M_GRAIN, and M_KEEP, but these values have no effect in the glibc implementation.

The remainder of these functions and variables are GNU extensions, and should not be used in programs intended to be portable.  


glibc uses a dynamic allocation routines heavily based on the implementation by Doug Lea <dl@cs.oswego.edu>.  


sbrk(2), mmap(2), stderr(2), malloc_hook(3), malloc(3), cfree(3), memalign(3), pvalloc(3).




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