mallinfo(3) — Linux manual page


mallinfo(3)             Library Functions Manual             mallinfo(3)

NAME         top

       mallinfo, mallinfo2 - obtain memory allocation information

LIBRARY         top

       Standard C library (libc, -lc)

SYNOPSIS         top

       #include <malloc.h>

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

DESCRIPTION         top

       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

           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

       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.

              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

              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

              The total number of bytes in fastbin free blocks.

              The total number of bytes used by in-use allocations.

              The total number of bytes in free blocks.

              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         top

       For an explanation of the terms used in this section, see
       │ 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.

STANDARDS         top


HISTORY         top

              glibc 2.0.  SVID.

              glibc 2.33.

BUGS         top

       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.

EXAMPLES         top

       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

       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

           $ ./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];
           size_t blockSize, numBlocks, freeBegin, freeEnd, freeStep;

           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 (size_t 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 (size_t j = freeBegin; j < freeEnd; j += freeStep)

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


SEE ALSO         top

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

COLOPHON         top

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Linux man-pages 6.9.1          2024-06-15                    mallinfo(3)

Pages that refer to this page: malloc_hook(3)malloc_info(3)malloc_stats(3)mallopt(3)