C语言中二维数组如何申请动态分配内存

#include <stdio.h> 
#include <malloc.h> 

int main() { 

    int **a; 
    int i, j; 
    a = (int**)malloc(sizeof(int*)*3);//为二维数组分配3行 
    for (i = 0; i < 3; ++i){//为每列分配4个大小空间 
        a[i] = (int*)malloc(sizeof(int)*4); 
    } 
    //初始化 
    for (i = 0; i < 3; ++i){ 
        for (j = 0; j < 4; ++j){ 
            a[i][j] = i+j; 
        } 
    } 
    //输出测试 
    for (i = 0; i < 3; ++i){ 
        for (j = 0; j < 4; ++j){ 
            printf ("%d ", a[i][j]); 
        } 
        printf ("\n"); 
    } 
    //释放动态开辟的空间 
    for (i = 0; i < 3; ++i){ 
        free(a[i]); 
    } 
    free(a); 
    return 0; 
}