C语言文件读写操作

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

int main(){
	 FILE * fp1;
    FILE * fp2;
    fp1 = fopen("cal.txt","r");
    fp2 = fopen("res.txt","w");

    char * p = (char*)malloc(sizeof(char) * 20);
    while(!feof(fp1)){
        int a,b,sum;
        char op;
        memset(p, 0, 20);
        p = fgets(p,20,fp1);
        // printf("%s",p);
        sscanf(p, "%d%c%d=\n",&a,&op,&b);
        switch (op)
        {
            case '/' : sum = a/b ; break;
            case '*' : sum = a*b ; break;
            case '+' : sum = a+b ; break;
            case '-' : sum = a-b ; break; 
        }
        memset(p,0,20);
        sprintf(p,"%d%c%d=%d\n",a,op,b,sum);
        fputs(p,fp2);
    }
    free(p);
    fclose(fp1);
    fclose(fp2);
    return 0;
}

// cal.txt
1*10=
10/2=
1+3=
2-1=