#include <stdio.h>

int main(int argc, char const *argv[])
{
    FILE *fp = fopen("cal.txt", "r");
    char buf[100];
    int a, b;
    char sign;
    if (fp == NULL)
    {
        perror("Open file:");
        return -1;
    }

    while ((fgets(buf, sizeof(buf), fp)) != NULL)
    {
        sscanf(buf, "%d%c%d", &a, &sign, &b);
        //printf("%c ,", sign);
        switch (sign)
        {
        case '+':
            printf("%d+%d=%d\n", a, b, a + b);
            break;
        case '-':
            printf("%d-%d=%d\n", a, b, a - b);
            break;
        case '*':
            printf("%d*%d=%d\n", a, b, a * b);
            break;
        case '/':
            if (b != 0)
            {
                int remain = a % b;
                printf("%d/%d=%d,余数为:%d\n", a, b, a / b, remain);
            }
            else
            {
                printf("错误除数不能为零，请修改除数的值");
            }

            break;

        default:
            break;
        }
    }

    return 0;
}

cal.txt文件内容：
34567+45678=
1234-886=
123*234=
656/122=