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

       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 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 at-

       |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.

       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.   Al-
       locations  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, be-
       cause some internal bookkeeping values may be of  type  long,  the  re-
       ported 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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       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at

Linux                             2019-03-06                       MALLINFO(3)
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