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NAME

       mallinfo - obtain memory allocation information

SYNOPSIS

       #include <malloc.h>

       struct mallinfo mallinfo(void);

DESCRIPTION

       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:

       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   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.

       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).

ATTRIBUTES

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

       ┌───────────┬───────────────┬──────────────────────────────┐
       │InterfaceAttributeValue                        │
       ├───────────┼───────────────┼──────────────────────────────┤
       │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

       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

       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.

EXAMPLE

       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
       display_mallinfo(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);
       }

       int
       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]);
               exit(EXIT_FAILURE);
           }

           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");
           display_mallinfo();

           for (j = 0; j < numBlocks; j++) {
               if (numBlocks >= MAX_ALLOCS) {
                   fprintf(stderr, "Too many allocations\n");
                   exit(EXIT_FAILURE);
               }

               alloc[j] = malloc(blockSize);
               if (alloc[j] == NULL) {
                   perror("malloc");
                   exit(EXIT_FAILURE);
               }
           }

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

           for (j = freeBegin; j < freeEnd; j += freeStep)
               free(alloc[j]);

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

           exit(EXIT_SUCCESS);
       }

SEE ALSO

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

COLOPHON

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