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

       mallinfo - obtain memory allocation information

       #include <malloc.h>

       struct mallinfo mallinfo(void);

       The mallinfo() function returns a copy of a structure containing infor-
       mation 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

       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 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

       |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
       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

       Information  is  returned  for  only  the  main memory allocation area.
       Allocations in other arenas are excluded.  See malloc_stats(3) and mal-
       loc_info(3)  for alternatives that include information about other are-

       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 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
           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");


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

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Linux                             2017-09-15                       MALLINFO(3)
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