mallinfo man page

mallinfo — obtain memory allocation information


#include <malloc.h>

struct mallinfo mallinfo(void);


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:

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.
The number of ordinary (i.e., non-fastbin) free blocks.
The number of fastbin free blocks (see mallopt(3)).
The number of blocks currently allocated using mmap(2). (See the discussion of M_MMAP_THRESHOLD in mallopt(3).)
The number of bytes in blocks currently allocated using mmap(2).
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.
The total number of bytes in fastbin free blocks.
The total number of bytes used by in-use allocations.
The total number of bytes in free blocks.
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).


For an explanation of the terms used in this section, see attributes(7).

mallinfo()Thread safetyMT-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.

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.


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.


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
    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);

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");

    for (j = 0; j < numBlocks; j++) {
        if (numBlocks >= MAX_ALLOCS)
            fatal("Too many allocations");

        alloc[j] = malloc(blockSize);
        if (alloc[j] == NULL)

    printf("\n============== After allocating blocks ==============\n");

    for (j = freeBegin; j < freeEnd; j += freeStep)

    printf("\n============== After freeing blocks ==============\n");


See Also

mmap(2), malloc(3), malloc_info(3), malloc_stats(3), malloc_trim(3), mallopt(3)

Referenced By

malloc_hook(3), malloc_info(3), malloc_stats(3), mallopt(3).

Explore man page connections for mallinfo(3).