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 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).

       +-----------+---------------+------------------------------+
       |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
       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  mal-
       loc_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  mem-
       ory.  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
", mi.arena);
	   printf("# of free chunks (ordblks):		  %d
", mi.ordblks);
	   printf("# of free fastbin blocks (smblks):	  %d
", mi.smblks);
	   printf("# of mapped regions (hblks): 	  %d
", mi.hblks);
	   printf("Bytes in mapped regions (hblkhd):	  %d
", mi.hblkhd);
	   printf("Max. total allocated space (usmblks):  %d
", mi.usmblks);
	   printf("Free bytes held in fastbins (fsmblks): %d
", mi.fsmblks);
	   printf("Total allocated space (uordblks):	  %d
", mi.uordblks);
	   printf("Total free space (fordblks): 	  %d
", mi.fordblks);
	   printf("Topmost releasable block (keepcost):   %d
", 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]]]
", 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 ==============
");
	   display_mallinfo();

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

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

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

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

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

	   exit(EXIT_SUCCESS);
       }

SEE ALSO

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

COLOPHON

       This page is part of release 4.15 of the Linux man-pages project.  A description of the project, information about reporting bugs, and the
       latest version of this page, can be found at https://www.kernel.org/doc/man-pages/.

Linux								    2017-09-15							       MALLINFO(3)