C-内存和类型

2020-09-29

C内存和类型

测试源码

void *vp = calloc(4,4);

int *int_arr = vp;
for (int i = 0; i < 4; ++i){
    printf("int array[%d] => %d\n", i, int_arr[i]);
    int_arr[i] = i;
}
for (int i = 0; i < 4; ++i){
    printf("int array[%d] => %d\n", i, int_arr[i]);
}

char *char_arr = (char *)vp;
for (int i = 0; i < 16; ++i) {
    char_arr[i] = 'a' + i;
}
char_arr[15] = '\0';
printf("char_arr = %s\n", char_arr);

memset(vp,0,16);

memset(vp, 12, 1);
strcpy(vp+4,"Tom Jerry");

struct person {
    int32_t age;
    char name[12];
} *person_ptr;

person_ptr = vp;
printf("person age:%d, name : %s\n",person_ptr->age, person_ptr->name);

memset(vp+4,0,12);
strcpy(vp+4, "man");
strcpy(vp + 4 + 6,"fat");

struct annimal {
    int32_t type;
    char type_name[6];
    char description[6];
} *annimal_ptr;
annimal_ptr = (struct annimal *)person_ptr;
printf("annimal type: %d, type_name: %s, description: %s\n",annimal_ptr->type, annimal_ptr->type_name, annimal_ptr->description);
free(vp);

输出

int array[0] => 0
int array[1] => 0
int array[2] => 0
int array[3] => 0
int array[0] => 0
int array[1] => 1
int array[2] => 2
int array[3] => 3
char_arr = abcdefghijklmno
person age:12, name : Tom Jerry
annimal type: 12, type_name: man, description: fat

分析

关于变量类型和内存。申请了4×4字节的内存，内存是与变量无关的，初始化时的内存值为。

1	00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 │ ················ │

16字节内存作为整型数组使用

执行到下边的代码之后的内存。

int *int_arr = vp;
    for (int i = 0; i < 4; ++i){
        printf("int array[%d] => %d\n", i, int_arr[i]);
        int_arr[i] = i;
    }

memory:

1	00 00 00 00 01 00 00 00 02 00 00 00 03 00 00 00 │ ················ │

一个int类型大小为4个字节，所以16字节可以转成大小为4的整型数组。

16字节内存作为字符串数组使用

执行到下面的代码，内存变化为

char *char_arr = (char *)vp;
for (int i = 0; i < 16; ++i) {
    char_arr[i] = 'a' + i;
}
char_arr[15] = '\0';

memory:

1	65 66 67 68 69 6a 6b 6c 6d 6e 6f 00 │ abcdefghijklmno· │

一个char类型占用1个字节,16个字节可以放16个字符。

16字节作为结构体变量使用

执行到下面的代码，

1
2
3

memset(vp,0,16);
memset(vp, 12, 1);
strcpy(vp+4,"Tom Jerry");

memory:

1	0c 00 00 00 54 6f 6d 20 4a 65 72 72 79 00 00 00 │ ····Tom Jerry··· │

执行下面的代码

struct person {
    int32_t age;
    char name[12];
} *person_ptr;

person_ptr = vp;
printf("person age:%d, name : %s\n",person_ptr->age, person_ptr->name);

输出

1	person age:12, name : Tom Jerry

16字节的内存被person结构体使用了。

结构体间强转

执行到以下代码

memset(vp+4,0,12);
strcpy(vp+4, "man");
strcpy(vp + 4 + 6,"fat");

struct annimal {
    int32_t type;
    char type_name[6];
    char description[6];
} *annimal_ptr;

annimal_ptr = (struct annimal *)person_ptr;
printf("annimal type: %d, type_name: %s, description: %s\n",annimal_ptr->type, annimal_ptr->type_name, annimal_ptr->description);

memory:

1	0c 00 00 00 6d 61 6e 00 00 00 66 61 74 00 00 00 │ ····man···fat··· │

输出

1	annimal type: 12, type_name: man, description: fat

总结

对于申请的内存得到的内存是无类型的，谁都可以使用。变量类型的作用是用来确定内存的边界。
int类型大小为4个字节,所以会以4个字节为单位进行切割。
char 类型大小为1个字节,以1个字节为单位对16字节的数据进行切割。
person结构体，切割成两个部分。前4个字节作为整型age,后12字节作为char数组name。
annimal结构体，将16字节切割给 type、type_name, description。

扩展-数组

多维静态数组

静态数组int map[2][3] = {{1,2,3},{4,5,6}};在内存的布局如下,前4×6 = 24个字节分别对应map中的值。根据列的长度得出行的个数。列的大小为4×3 = 12,可以将24分成2个。

1 2	01 00 00 00 02 00 00 00 03 00 00 00 04 00 00 00 │ ················ │ 05 00 00 00 06 00 00 00 00 b5 e9 21 50 1e 21 4e │ ···········!P·!N │

静态数组：int map2[3][2] = {{1,2},{3,4},{5,6}};内存布局如下。和上面的map变量内存布局一致。不同的是解析方法。

1 2	01 00 00 00 02 00 00 00 03 00 00 00 04 00 00 00 │ ················ │ 05 00 00 00 06 00 00 00 00 2a 5b 24 65 cb 71 c8 │ ·········*[$e·q· │

静态数组: int map3[3][2][2] = {{{1,2},{3,4}},{{5,6},{7,8}},{{9,10},{11,12}}}; 内存布局如下,切割方法,最小单位为2×4字节。map3[0][0][0] => 1。

1
2
3

01 00 00 00   02 00 00 00   03 00 00 00   04 00 00 00   │ ················ │
05 00 00 00   06 00 00 00   07 00 00 00   08 00 00 00   │ ················ │
09 00 00 00   0a 00 00 00   0b 00 00 00   0c 00 00 00   │ ················ │

字符串数组：char names[3][16] = {"AAA","BBBB","CCCCCCCCCCCCCCC"}; 。

1
2
3

41 41 41 00   00 00 00 00   00 00 00 00   00 00 00 00   │ AAA············· │
42 42 42 42   00 00 00 00   00 00 00 00   00 00 00 00   │ BBBB············ │
43 43 43 43   43 43 43 43   43 43 43 43   43 43 43 00   │ CCCCCCCCCCCCCCC· │

多维动态数组

二维字符串数组创建方法一：
代码如下，4个字符串指针，决定了字符串的个数，指针所指向的地址由自己定义。

void *vp = malloc(16);
memset(vp, 0, 16);
memcpy(vp,"AAA",4);
memcpy(vp + 4, "BBB", 4);
memcpy(vp + 8, "CCC", 4);
memcpy(vp + 12, "DDD", 4);

char *str_arr_ptr[4];
str_arr_ptr[0] = vp;
str_arr_ptr[1] = vp + 4;
str_arr_ptr[2] = vp + 8;
str_arr_ptr[3] = vp + 12;
for (int i = 0; i < 4;i ++){
    printf("str_arr_ptr_%d => %s\n",i,str_arr_ptr[i]);
}

内存:

1	41 41 41 00 42 42 42 00 43 43 43 00 44 44 44 00 │ AAA·BBB·CCC·DDD· │

输出:

str_arr_ptr_0 => AAA
str_arr_ptr_1 => BBB
str_arr_ptr_2 => CCC
str_arr_ptr_3 => DDD

二维字符串数组创建方法二：

void *vp = malloc(16);
memset(vp, 0, 16);
memcpy(vp,"AAA",4);
memcpy(vp + 4, "BBB", 4);
memcpy(vp + 8, "CCC", 4);
memcpy(vp + 12, "DDD", 4);

char **str_arr_ptr2 = NULL;
// 申请了4个指针
str_arr_ptr2 = calloc(4,sizeof(char *));

str_arr_ptr2[0] = vp;

str_arr_ptr2[1] = vp + 4;

str_arr_ptr2[2] = vp + 8;

str_arr_ptr2[3] = vp + 12;
for (int i = 0; i < 4;i ++){
    printf("str_arr_ptr_%d => %s\n",i,str_arr_ptr2[i]);
}

说明
与第一种不同的是，每个指向字符串数组的指针都要自己去申请。但是，好处是动态的，大小可以自己决定。C指针编程实际上是对内存操作的编程。

栈大小

每个操作系统都会限制栈大小,总栈大小和每个函数栈大小都会有不同的限制。
8M的函数栈,其实可以通过操作系统变更的。