内存管理——二级指针

#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <stdlib.h>

void get_memory_fail(char *local_p)
{
    /*
     * (gdb) p local_p
     * $11 = 0x0
     * (gdb) p &p
     * $12 = (char **) 0xbffff30c
     *
     * (gdb) p local_p
     * $14 = 0x0
     * (gdb) p &local_p
     * $15 = (char **) 0xbffff2f0
     *
     * 1)可以看出，local_p = p = 0x0，也就是两个的值是一样的
     * 2)但是二者地址不一样,分配内存的时候，local_p指向新的内存,p丝毫没变
     * 这也就所用这个方法拿不到内存的根本原因
     */
    local_p = (char*)malloc(100);
}

void get_memory_ok(char **local_p)
{
    /*
     * (gdb) p p
     * $1 = 0x0
     * (gdb) p &p
     * $2 = (char **) 0xbffff30c
     * (gdb) s
     *
     * (gdb) p local_p
     * $3 = (char **) 0xbffff30c
     * (gdb) p &local_p
     * $4 = (char ***) 0xbffff2f0
     * (gdb) p *local_p
     * $5 = 0x804b070
     *
     *1)p和local_p所占用的地址不一样
     *2)local_p指向p的地址，也就是0xbffff30c
     *3)*local_p指向p的值，于是通过*local_p就可以改变p的值
     */
    *local_p = (char*)malloc(100);
}


char *get_memory_ok2(void)
{
    /*
     * local_p的值为(char*)malloc(100)所在的内存
     * p = get_memory_ok2();p的值得到local_p的值
     *
     */
    char *local_p = NULL;
    local_p = (char*)malloc(100);
    return local_p;
}


int main()
{
    char *p = NULL;
    get_memory_fail(p);


    get_memory_ok(&p);
    free(p);
    p = NULL;


    p = get_memory_ok2();
    free(p);
    p = NULL;


    return 0;
}