1 malloc()

声明:

void* malloc (size_t size);

其中size_t代表unsigned int。
malloc:分配一块size Byte大小的内存，返回一个指向该块内存开始的指针，指针类型是void。

void * memset ( void * ptr, int value, size_t num );

示例:

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

/* Micro */
#define MEMASERT(p) if(!p){sprintf(stderr,"Alloc memory failed!"); exit(-1);}
#define NUMBER_OF_STUDENT 2

/* Prototypes */
typedef struct _Student{
    char * name;
    char * major;
    unsigned int age;
    unsigned int id;
}Student;


void show_student_info(Student *pstudents, int n)
{
    int i;
    Student * ps;

    printf("+------------+------------+----------+----------+\n");
    printf("| Name       | Major      | Age      | ID       |\n");
    printf("+------------+------------+----------+----------+\n");
    for (i = 0, ps = pstudents; i < n; i++, ps++)
    {
        printf("| %-11s| %-11s| %-9s| %-9s|\n", ps->name, 
            ps->major, ps->age, ps->id);
    }
    printf("+------------+------------+----------+----------+\n");
}


int main(void)
{
    Student *pstudents = NULL;
    pstudents = (Student *)malloc(NUMBER_OF_STUDENT*sizeof(Student));
    MEMASERT(pstudents);
    memset(pstudents, 0, NUMBER_OF_STUDENT*sizeof(Student));

    pstudents->name = "wwchaonj";
    pstudents->major = "science";
    pstudents->age = "24";
    pstudents->id = "007";

    printf("Student Info: \n");
    show_student_info(pstudents, 2);

    if (!pstudents){
        free(pstudents);
        pstudents = NULL;
    }
    system("pause");

    return 0;
}

2 calloc()

声明:

void* calloc (size_t num, size_t size);

为一个大小为num的数组分配内存，每个元素的大小是size，把每个元素初始化为0。
示例:

int main(void)
{
    ...
    Student *pstudents = NULL;
    pstudents = (Student *)calloc(NUMBER_OF_STUDENT, sizeof(Student));
    MEMASERT(pstudents);
    ...
}

3 realloc()

声明:

void* realloc (void* ptr, size_t size);

将ptr所指向的内存空间的大小改为size个字节.
如果新分配的内存比原内存大, 那么原内存的内容保持不变, 增加的空间不进行初始化.
如果新分配的内存比原内存小, 那么新内存保持原内存的内容, 增加的空间不进行初始化.
返回指向新分配空间的指针; 若内存不够,则返回NULL, 原ptr指向的内存区不变.
示例:

int main（void）
{
    ...
    printf("Student Info: \n");
    show_student_info(pstudents, 1);

    printf("Realloc memory:\n");
    pstudents = (Student *)realloc(pstudents, NUMBER_OF_STUDENT*sizeof(Student));
    MEMASERT(pstudents);
    ps = pstudents + 1;
    memset(ps, 0, sizeof(Student));
    ps->name = "JackMa";
    ps->major = "mathmatics";
    ps->age = "23";
    ps->id = "008";
    show_student_info(pstudents, 2);
    ...
}

4 free()

声明:

void free (void* ptr);

回收由malloc/calloc/realloc分配的内存空间。

5 malloc/calloc/realloc区别与联系

差异：
1) malloc/realloc不对新分配的内存进行初始化，当前值不确定；而calloc分配的内存全部初始化为0；

相同：
1) malloc/calloc/realloc均返回void*指针，使用前需要进行强制类型转换；
2) malloc/calloc/realloc分配的内存不再使用时需调用free进行内存回收。

6 参考链接：

[1] http://www.cplusplus.com/reference/cstdlib/
[2] malloc calloc realloc 作用、用法、区别、实现原理
[3] C语言动态内存管理malloc、calloc、realloc、free的用法和注意事项

C语言malloc/calloc/realloc/free堆内存管理