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mallinfo(3) man page for review

I've written the man page page below to document glibc's mallinfo(3).
Review comments welcome!



'\" t
.\" Copyright (c) 2012 by Michael Kerrisk <>
.\" 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.\" manual page may be incorrect or out-of-date.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.
.TH MALLINFO 3  2012-04-28 "Linux" "Linux Programmer's Manual"
mallinfo \- obtain memory allocation information
.B #include <malloc.h>

.B struct mallinfo mallinfo(void);
.BR mallinfo ()
function returns a copy of a structure containing information about
memory allocations performed by
.BR malloc (3)
and related functions.
This structure is defined as follows:
.in +4n
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
.I mallinfo
structure contain the following information:

.TP 10
.I arena
The total amount of memory allocated by means other than
.BR mmap (2)
(i.e., memory allocated on the heap).
This figure includes both in-use blocks and blocks on the free list.
.I ordblks
The number of ordinary (i.e., non-fastbin) free blocks.
.I smblks
The number of fastbin free blocks (see
.BR mallopt (3)).
.I hblks
The number of blocks currently allocated using
.BR mmap (2).
.I hblkhd
The number of bytes in blocks currently allocated using
.BR mmap (2).
.I usmblks
The "highwater mark" for allocated space\(emthat is,
the maximum amount of space that was ever allocated.
This field is maintained only in nonthreading environments.
.I fsmblks
The total number of bytes in fastbin free blocks.
.I uordblks
The total number of bytes used by in-use allocations.
.I fordblks
The total number of bytes in free blocks.
.I 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
.BR malloc_trim (3).
.\" Available already in glibc 2.0, possibly earlier
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.
.\" See the 24 Aug 2011 mail by Paul Pluzhnikov:
.\"     "[patch] Fix mallinfo() to accumulate results for all arenas"
.\" on
.B Information is returned for only the main memory allocation area.
Allocations in other arenas are excluded.
.BR malloc_stats (3)
.BR malloc_info (3)
for alternatives that include information about other arenas.

The fields of the
.I mallinfo
structure are typed as
.IR int .
However, because some internal bookkeeping values may be of type
.IR long ,
the reported values may wrap around zero and thus be inaccurate.
The program below employs
.BR 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
.BR malloc (3).

The remaining three arguments specify which of the allocated blocks
should be freed with
.BR 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:
.in +4n
$ \fB./a.out 1000 100 2\fP
============== 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
.SS Program source
#include <malloc.h>
#include "tlpi_hdr.h"

static 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);

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");

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

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

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

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

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

.ad l
.BR mmap (2),
.BR malloc (3),
.BR malloc_info (3),
.BR malloc_stats (3),
.BR malloc_trim (3),
.BR mallopt (3)

Attachment: mallinfo.3
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