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

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 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
               mmap(2).

     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).

ATTRIBUTES

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

     +-----------+---------------+------------------------------+
     |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.

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 mal-
     loc_info(3)  for alternatives that include information about other are-
     nas.

     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 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 (uord-
         blks):      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); }