请编写函数int fun ,a是一个M行N列的二维数组,函数的功能是求出二维数组周边元素之和,作为函数值返回.
#include <conio.h>#include <stdio.h>#define  M  4#define  N  5int  fun ( int a[M][N] ){ }NONO( ){/* 请在此函数内打开文件,输入测试数据,调用 fun 函数,    输出数据,关闭文件.*/  int i,j,y,k,aa[M][N] ;  FILE *rf,*wf ;  rf = fopen("bc4.in","r") ;  wf = fopen("bc4.out","w") ;  for(k = 0 ; k < 10 ; k++) {    for(i = 0 ; i < M ; i++)    for(j = 0 ; j < N ; j++) fscanf(rf,"%d",&aa[i][j]) ;    y = fun ( aa );    fprintf(wf,"%d\n",y) ;  }  fclose(rf) ;  fclose(wf) ;}main( ){  int  aa[M][N]={{1,3,5,7,9},                  {2,9,9,9,4},                  {6,9,9,9,8},                  {1,3,5,7,0}};   int  i,j,y;   printf ( "The original data is :\n" );   for ( i=0; i<M; i++ )   {   for ( j =0; j<N; j++ ) printf( "%6d",aa[i][j] );       printf ("\n");   }   y = fun ( aa );   printf( "\nThe  sum:  %d\n" ,y );   printf("\n");   NONO( );}