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
information about memory allocations performed by
malloc(3) and related
functions. This 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; /* Maximum total allocated space (bytes) */
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
- The "highwater mark" for allocated space—that is, the
maximum amount of space that was ever allocated. This field is 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
attributes(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
mallopt in
const:mallopt mean that
mallopt() would modify the global
internal objects with atomics, that make sure
mallinfo() is safe
enough, others modify with non-atomically maybe not.
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.
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 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.
EXAMPLE¶
The program below employs
mallinfo() 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
Program source¶
#include <malloc.h>
#include "tlpi_hdr.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, j, freeBegin, freeEnd, freeStep;
size_t blockSize;
if (argc < 3 || strcmp(argv[1], "--help") == 0)
usageErr("%s num-blocks block-size [free-step [start-free "
"[end-free]]]\n", argv[0]);
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 (j = 0; j < numBlocks; j++) {
if (numBlocks >= MAX_ALLOCS)
fatal("Too many allocations");
alloc[j] = malloc(blockSize);
if (alloc[j] == NULL)
errExit("malloc");
}
printf("\n============== After allocating blocks ==============\n");
display_mallinfo();
for (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 4.10 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/.