mallinfo, mallinfo2 - obtain memory allocation information
Standard C library (
libc,
-lc)
#include <malloc.h>
struct mallinfo mallinfo(void);
struct mallinfo2 mallinfo2(void);
These functions return a copy of a structure containing information about memory
allocations performed by
malloc(3) and related functions. The structure
returned by each function contains the same fields. However, the older
function,
mallinfo(), is deprecated since the type used for the fields
is too small (see BUGS).
Note that not all allocations are visible to these functions; see BUGS and
consider using
malloc_info(3) instead.
The
mallinfo2 structure returned by
mallinfo2() is defined as
follows:
struct mallinfo2 {
size_t arena; /* Non-mmapped space allocated (bytes) */
size_t ordblks; /* Number of free chunks */
size_t smblks; /* Number of free fastbin blocks */
size_t hblks; /* Number of mmapped regions */
size_t hblkhd; /* Space allocated in mmapped regions
(bytes) */
size_t usmblks; /* See below */
size_t fsmblks; /* Space in freed fastbin blocks (bytes) */
size_t uordblks; /* Total allocated space (bytes) */
size_t fordblks; /* Total free space (bytes) */
size_t keepcost; /* Top-most, releasable space (bytes) */
};
The
mallinfo structure returned by the deprecated
mallinfo()
function is exactly the same, except that the fields are typed as
int.
The structure fields contain the following information:
- arena
- The total amount of memory allocated by means other than
mmap(2) (i.e., memory allocated on the heap). This figure includes
both in-use blocks and blocks on the free list.
- ordblks
- The number of ordinary (i.e., non-fastbin) free
blocks.
- smblks
- The number of fastbin free blocks (see
mallopt(3)).
- hblks
- The number of blocks currently allocated using
mmap(2). (See the discussion of M_MMAP_THRESHOLD in
mallopt(3).)
- hblkhd
- The number of bytes in blocks currently allocated using
mmap(2).
- usmblks
- This field is unused, and is always 0. Historically, it was
the "highwater mark" for allocated space—that is, the
maximum amount of space that was ever allocated (in bytes); this field was
maintained only in nonthreading environments.
- fsmblks
- The total number of bytes in fastbin free blocks.
- uordblks
- The total number of bytes used by in-use allocations.
- fordblks
- The total number of bytes in free blocks.
- keepcost
- The total amount of releasable free space at the top of the
heap. This is the maximum number of bytes that could ideally (i.e.,
ignoring page alignment restrictions, and so on) be released by
malloc_trim(3).
The
mallinfo2() function was added in glibc 2.33.
For an explanation of the terms used in this section, see
attributes(7).
Interface |
Attribute |
Value |
mallinfo (), mallinfo2 () |
Thread safety |
MT-Unsafe init const:mallopt |
mallinfo()/
mallinfo2() would access some global internal objects.
If modify them with non-atomically, may get inconsistent results. The
identifier
mallopt in
const:mallopt mean that
mallopt()
would modify the global internal objects with atomics, that make sure
mallinfo()/
mallinfo2() is safe enough, others modify with
non-atomically maybe not.
These functions are not specified by POSIX or the C standards. A
mallinfo() function exists on many System V derivatives, and was
specified in the SVID.
Information is returned for only the main memory allocation area.
Allocations in other arenas are excluded. See
malloc_stats(3) and
malloc_info(3) for alternatives that include information about other
arenas.
The fields of the
mallinfo structure that is returned by the older
mallinfo() function are typed as
int. However, because some
internal bookkeeping values may be of type
long, the reported values
may wrap around zero and thus be inaccurate.
The program below employs
mallinfo2() to retrieve memory allocation
statistics before and after allocating and freeing some blocks of memory. The
statistics are displayed on standard output.
The first two command-line arguments specify the number and size of blocks to be
allocated with
malloc(3).
The remaining three arguments specify which of the allocated blocks should be
freed with
free(3). These three arguments are optional, and specify (in
order): the step size to be used in the loop that frees blocks (the default is
1, meaning free all blocks in the range); the ordinal position of the first
block to be freed (default 0, meaning the first allocated block); and a number
one greater than the ordinal position of the last block to be freed (default
is one greater than the maximum block number). If these three arguments are
omitted, then the defaults cause all allocated blocks to be freed.
In the following example run of the program, 1000 allocations of 100 bytes are
performed, and then every second allocated block is freed:
$ ./a.out 1000 100 2
============== Before allocating blocks ==============
Total non-mmapped bytes (arena): 0
# of free chunks (ordblks): 1
# of free fastbin blocks (smblks): 0
# of mapped regions (hblks): 0
Bytes in mapped regions (hblkhd): 0
Max. total allocated space (usmblks): 0
Free bytes held in fastbins (fsmblks): 0
Total allocated space (uordblks): 0
Total free space (fordblks): 0
Topmost releasable block (keepcost): 0
============== After allocating blocks ==============
Total non-mmapped bytes (arena): 135168
# of free chunks (ordblks): 1
# of free fastbin blocks (smblks): 0
# of mapped regions (hblks): 0
Bytes in mapped regions (hblkhd): 0
Max. total allocated space (usmblks): 0
Free bytes held in fastbins (fsmblks): 0
Total allocated space (uordblks): 104000
Total free space (fordblks): 31168
Topmost releasable block (keepcost): 31168
============== After freeing blocks ==============
Total non-mmapped bytes (arena): 135168
# of free chunks (ordblks): 501
# of free fastbin blocks (smblks): 0
# of mapped regions (hblks): 0
Bytes in mapped regions (hblkhd): 0
Max. total allocated space (usmblks): 0
Free bytes held in fastbins (fsmblks): 0
Total allocated space (uordblks): 52000
Total free space (fordblks): 83168
Topmost releasable block (keepcost): 31168
#include <malloc.h>
#include <stdlib.h>
#include <string.h>
static void
display_mallinfo2(void)
{
struct mallinfo2 mi;
mi = mallinfo2();
printf("Total non-mmapped bytes (arena): %zu\n", mi.arena);
printf("# of free chunks (ordblks): %zu\n", mi.ordblks);
printf("# of free fastbin blocks (smblks): %zu\n", mi.smblks);
printf("# of mapped regions (hblks): %zu\n", mi.hblks);
printf("Bytes in mapped regions (hblkhd): %zu\n", mi.hblkhd);
printf("Max. total allocated space (usmblks): %zu\n", mi.usmblks);
printf("Free bytes held in fastbins (fsmblks): %zu\n", mi.fsmblks);
printf("Total allocated space (uordblks): %zu\n", mi.uordblks);
printf("Total free space (fordblks): %zu\n", mi.fordblks);
printf("Topmost releasable block (keepcost): %zu\n", mi.keepcost);
}
int
main(int argc, char *argv[])
{
#define MAX_ALLOCS 2000000
char *alloc[MAX_ALLOCS];
size_t blockSize, numBlocks, freeBegin, freeEnd, freeStep;
if (argc < 3 || strcmp(argv[1], "--help") == 0) {
fprintf(stderr, "%s num-blocks block-size [free-step "
"[start-free [end-free]]]\n", argv[0]);
exit(EXIT_FAILURE);
}
numBlocks = atoi(argv[1]);
blockSize = atoi(argv[2]);
freeStep = (argc > 3) ? atoi(argv[3]) : 1;
freeBegin = (argc > 4) ? atoi(argv[4]) : 0;
freeEnd = (argc > 5) ? atoi(argv[5]) : numBlocks;
printf("============== Before allocating blocks ==============\n");
display_mallinfo2();
for (size_t j = 0; j < numBlocks; j++) {
if (numBlocks >= MAX_ALLOCS) {
fprintf(stderr, "Too many allocations\n");
exit(EXIT_FAILURE);
}
alloc[j] = malloc(blockSize);
if (alloc[j] == NULL) {
perror("malloc");
exit(EXIT_FAILURE);
}
}
printf("\n============== After allocating blocks ==============\n");
display_mallinfo2();
for (size_t j = freeBegin; j < freeEnd; j += freeStep)
free(alloc[j]);
printf("\n============== After freeing blocks ==============\n");
display_mallinfo2();
exit(EXIT_SUCCESS);
}
mmap(2),
malloc(3),
malloc_info(3),
malloc_stats(3),
malloc_trim(3),
mallopt(3)