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

       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

       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 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).

       The mallinfo2() function was added in glibc 2.33.

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

       │Interface   Attribute     Value                                     │
       │mallinfo(), │ Thread safety │ MT-Unsafe init const:mallopt              │
       │mallinfo2() │               │                                           │
       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

       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),

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Linux                              2021-03-22                        MALLINFO(3)