MALLINFO(3) Linux Programmer's Manual MALLINFO(3)
NAME
mallinfo - obtain memory allocation information
SYNOPSIS
#include <malloc.h>
struct mallinfo mallinfo(void);
DESCRIPTION
The mallinfo() function returns a copy of a structure containing infor-
mation about memory allocations performed by malloc(3) and related
functions.
Note that not all allocations are visible to mallinfo(); see BUGS and
consider using malloc_info(3) instead.
The returned structure is defined as follows:
struct mallinfo {
int arena; /* Non-mmapped space allocated (bytes) */
int ordblks; /* Number of free chunks */
int smblks; /* Number of free fastbin blocks */
int hblks; /* Number of mmapped regions */
int hblkhd; /* Space allocated in mmapped regions (bytes) */
int usmblks; /* See below */
int fsmblks; /* Space in freed fastbin blocks (bytes) */
int uordblks; /* Total allocated space (bytes) */
int fordblks; /* Total free space (bytes) */
int keepcost; /* Top-most, releasable space (bytes) */
};
The fields of the mallinfo structure 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).
ATTRIBUTES
For an explanation of the terms used in this section, see at-
tributes(7).
┌───────────┬───────────────┬──────────────────────────────┐
│Interface │ Attribute │ Value │
├───────────┼───────────────┼──────────────────────────────┤
│mallinfo() │ Thread safety │ MT-Unsafe init const:mallopt │
└───────────┴───────────────┴──────────────────────────────┘
mallinfo() would access some global internal objects. If modify them
with non-atomically, may get inconsistent results. The identifier mal-
lopt in const:mallopt mean that mallopt() would modify the global in-
ternal objects with atomics, that make sure mallinfo() is safe enough,
others modify with non-atomically maybe not.
CONFORMING TO
This function is not specified by POSIX or the C standards. A similar
function exists on many System V derivatives, and was specified in the
SVID.
BUGS
Information is returned for only the main memory allocation area. Al-
locations in other arenas are excluded. See malloc_stats(3) and mal-
loc_info(3) for alternatives that include information about other are-
nas.
The fields of the mallinfo structure are typed as int. However, be-
cause some internal bookkeeping values may be of type long, the re-
ported values may wrap around zero and thus be inaccurate.
EXAMPLES
The program below employs mallinfo() to retrieve memory allocation sta-
tistics 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 posi-
tion 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
Program source
#include <malloc.h>
#include <stdlib.h>
#include <string.h>
static void
display_mallinfo(void)
{
struct mallinfo mi;
mi = mallinfo();
printf("Total non-mmapped bytes (arena): %d\n", mi.arena);
printf("# of free chunks (ordblks): %d\n", mi.ordblks);
printf("# of free fastbin blocks (smblks): %d\n", mi.smblks);
printf("# of mapped regions (hblks): %d\n", mi.hblks);
printf("Bytes in mapped regions (hblkhd): %d\n", mi.hblkhd);
printf("Max. total allocated space (usmblks): %d\n", mi.usmblks);
printf("Free bytes held in fastbins (fsmblks): %d\n", mi.fsmblks);
printf("Total allocated space (uordblks): %d\n", mi.uordblks);
printf("Total free space (fordblks): %d\n", mi.fordblks);
printf("Topmost releasable block (keepcost): %d\n", mi.keepcost);
}
int
main(int argc, char *argv[])
{
#define MAX_ALLOCS 2000000
char *alloc[MAX_ALLOCS];
int numBlocks, freeBegin, freeEnd, freeStep;
size_t blockSize;
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_mallinfo();
for (int 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_mallinfo();
for (int j = freeBegin; j < freeEnd; j += freeStep)
free(alloc[j]);
printf("\n============== After freeing blocks ==============\n");
display_mallinfo();
exit(EXIT_SUCCESS);
}
SEE ALSO
mmap(2), malloc(3), malloc_info(3), malloc_stats(3), malloc_trim(3),
mallopt(3)
COLOPHON
This page is part of release 5.10 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
Linux 2020-11-01 MALLINFO(3)
Generated by dwww version 1.14 on Thu Jan 23 01:07:49 CET 2025.