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       mallinfo, mallinfo2 - obtain memory allocation information


       #include <malloc.h>

       struct mallinfo mallinfo(void);
       struct mallinfo2 mallinfo2(void);


       These  functions  return  a  copy  of  a  structure  containing  information  about memory
       allocations performed by malloc(3) and related functions.  The structure returned by  each
       function contains the same fields.  However, the older function, mallinfo(), is deprecated
       since the type used for the fields is too small (see BUGS).

       Note that not all allocations are visible to these functions; see BUGS and consider  using
       malloc_info(3) instead.

       The mallinfo2 structure returned by mallinfo2() is defined as follows:

           struct mallinfo2 {
               size_t arena;     /* Non-mmapped space allocated (bytes) */
               size_t ordblks;   /* Number of free chunks */
               size_t smblks;    /* Number of free fastbin blocks */
               size_t hblks;     /* Number of mmapped regions */
               size_t hblkhd;    /* Space allocated in mmapped regions
                                    (bytes) */
               size_t usmblks;   /* See below */
               size_t fsmblks;   /* Space in freed fastbin blocks (bytes) */
               size_t uordblks;  /* Total allocated space (bytes) */
               size_t fordblks;  /* Total free space (bytes) */
               size_t keepcost;  /* Top-most, releasable space (bytes) */

       The mallinfo structure returned by the deprecated mallinfo() function is exactly the same,
       except that the fields are typed as int.

       The structure fields 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   This  field  is  unused,  and  is always 0.  Historically, it was the "highwater
                 mark" for allocated space—that is, the maximum amount of  space  that  was  ever
                 allocated   (in   bytes);   this  field  was  maintained  only  in  nonthreading

       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).


       The mallinfo2() function was added in glibc 2.33.


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

       │InterfaceAttributeValue                                          │
       │mallinfo(), mallinfo2() │ Thread safety │ MT-Unsafe init const:mallopt                   │
       mallinfo()/ mallinfo2() 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()/ mallinfo2() is safe enough, others modify with non-atomically maybe not.


       These  functions  are  not  specified  by POSIX or the C standards.  A mallinfo() 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 that is returned by the older mallinfo() function 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 mallinfo2() 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 mallinfo2 mi;

           mi = mallinfo2();

           printf("Total non-mmapped bytes (arena):       %zu\n", mi.arena);
           printf("# of free chunks (ordblks):            %zu\n", mi.ordblks);
           printf("# of free fastbin blocks (smblks):     %zu\n", mi.smblks);
           printf("# of mapped regions (hblks):           %zu\n", mi.hblks);
           printf("Bytes in mapped regions (hblkhd):      %zu\n", mi.hblkhd);
           printf("Max. total allocated space (usmblks):  %zu\n", mi.usmblks);
           printf("Free bytes held in fastbins (fsmblks): %zu\n", mi.fsmblks);
           printf("Total allocated space (uordblks):      %zu\n", mi.uordblks);
           printf("Total free space (fordblks):           %zu\n", mi.fordblks);
           printf("Topmost releasable block (keepcost):   %zu\n", mi.keepcost);

       main(int argc, char *argv[])
       #define MAX_ALLOCS 2000000
           char *alloc[MAX_ALLOCS];
           int numBlocks, 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 (int 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 (int 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), mallopt(3)


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