内存分配地址计算方法

Question

内存分配地址计算方法

发布于
2017-10-08

更新于
2017-10-08

在看UNIX/Linux系统编程手册这本书，基础比较差，看的比较慢，书中的这个程序看懂之后有个疑问。
如下代码：

/*************************************************************************\
*                  Copyright (C) Michael Kerrisk, 2015.                   *
*                                                                         *
* This program is free software. You may use, modify, and redistribute it *
* under the terms of the GNU General Public License as published by the   *
* Free Software Foundation, either version 3 or (at your option) any      *
* later version. This program is distributed without any warranty.  See   *
* the file COPYING.gpl-v3 for details.                                    *
\*************************************************************************/

/* Listing 7-1 */

/* free_and_sbrk.c

   Test if free(3) actually lowers the program break.

   Usage: free_and_sbrk num-allocs block-size [step [min [max]]]

   Try: free_and_sbrk 1000 10240 2 1 1000
        free_and_sbrk 1000 10240 1 1 999
        free_and_sbrk 1000 10240 1 500 1000

        (Only the last of these should see the program break lowered.)
*/
#define _BSD_SOURCE
#include "tlpi_hdr.h"

#define MAX_ALLOCS 1000000

int
main(int argc, char *argv[])
{
    char *ptr[MAX_ALLOCS];
    int freeStep, freeMin, freeMax, blockSize, numAllocs, j;

    printf("\n");

    if (argc < 3 || strcmp(argv[1], "--help") == 0)
        usageErr("%s num-allocs block-size [step [min [max]]]\n", argv[0]);

    numAllocs = getInt(argv[1], GN_GT_0, "num-allocs");
    if (numAllocs > MAX_ALLOCS)
        cmdLineErr("num-allocs > %d\n", MAX_ALLOCS);

    blockSize = getInt(argv[2], GN_GT_0 | GN_ANY_BASE, "block-size");

    freeStep = (argc > 3) ? getInt(argv[3], GN_GT_0, "step") : 1;
    freeMin =  (argc > 4) ? getInt(argv[4], GN_GT_0, "min") : 1;
    freeMax =  (argc > 5) ? getInt(argv[5], GN_GT_0, "max") : numAllocs;

    if (freeMax > numAllocs)
        cmdLineErr("free-max > num-allocs\n");

    printf("Initial program break:          %10p\n", sbrk(0));

    printf("Allocating %d*%d bytes\n", numAllocs, blockSize);
    for (j = 0; j < numAllocs; j++) {
        ptr[j] = malloc(blockSize);
        if (ptr[j] == NULL)
            errExit("malloc");
    }

    printf("Program break is now:           %10p\n", sbrk(0));

    printf("Freeing blocks from %d to %d in steps of %d\n",
                freeMin, freeMax, freeStep);
    for (j = freeMin - 1; j < freeMax; j += freeStep)
        free(ptr[j]);

    printf("After free(), program break is: %10p\n", sbrk(0));

    exit(EXIT_SUCCESS);
}

执行./free_and_sbrk 1000 10240 2
这个会分配1000个内存块，每一块大小是10240bytes,最后一个参数是释放时的step，每隔一个内存块释放一个内存块。
下面时我的运行截图
图片描述