Linux manual pages Section 3: Subroutines  

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).

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 "tlpi_hdr.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)
        usageErr("%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");
    display_mallinfo();

    for (j = 0; j < numBlocks; j++) {
        if (numBlocks >= MAX_ALLOCS)
            fatal("Too many allocations");

        alloc[j] = malloc(blockSize);
        if (alloc[j] == NULL)
            errExit("malloc");
    }

    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

This page is part of release 3.52 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man−pages/.


  t
Copyright (c) 2012 by Michael Kerrisk <mtk.manpagesgmail.com>

%%%LICENSE_START(VERBATIM)
Permission is granted to make and distribute verbatim copies of this
manual provided the copyright notice and this permission notice are
preserved on all copies.

Permission is granted to copy and distribute modified versions of this
manual under the conditions for verbatim copying, provided that the
entire resulting derived work is distributed under the terms of a
permission notice identical to this one.

Since the Linux kernel and libraries are constantly changing, this
manual page may be incorrect or out-of-date.  The author(s) assume no
responsibility for errors or omissions, or for damages resulting from
the use of the information contained herein.  The author(s) may not
have taken the same level of care in the production of this manual,
which is licensed free of charge, as they might when working
professionally.

Formatted or processed versions of this manual, if unaccompanied by
the source, must acknowledge the copyright and authors of this work.
%%%LICENSE_END