mallinfo(3) — Linux manual page


MALLINFO(3)               Linux Programmer's Manual              MALLINFO(3)

NAME         top

       mallinfo - obtain memory allocation information

SYNOPSIS         top

       #include <malloc.h>

       struct mallinfo mallinfo(void);

DESCRIPTION         top

       The mallinfo() function returns a copy of a structure containing
       information 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;   /* 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 informa‐

       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

       usmblks   The "highwater mark" for allocated space—that is, the maxi‐
                 mum 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 ide‐
                 ally (i.e., ignoring page alignment restrictions, and so
                 on) be released by malloc_trim(3).

ATTRIBUTES         top

       For an explanation of the terms used in this section, see

       │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.

CONFORMING TO         top

       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         top

       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

       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.

EXAMPLES         top

       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 <stdlib.h>
       #include <string.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) {
               fprintf(stderr, "%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) {
                   fprintf(stderr, "Too many allocations\n");

               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         top

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

COLOPHON         top

       This page is part of release 5.08 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

Linux                            2020-06-09                      MALLINFO(3)

Pages that refer to this page: __after_morecore_hook(3)__free_hook(3)malloc_hook(3)__malloc_hook(3)malloc_info(3)__malloc_initialize_hook(3)malloc_stats(3)mallopt(3)__memalign_hook(3)__realloc_hook(3)