#include <stdio.h>
struct SYOHIN
{
 int kosu;
 int tanka;
 int uriage;
};

void keisan(struct SYOHIN *x);

int main(void)
{
 struct SYOHIN a, b, c;
//<商品別の個数と単価の代入>
 kaisan(&a); printf("商品Ａの売り上げは%d円　\n",a.uriage);
 kaisan(&b); printf("商品Ｂの売り上げは%d円　\n",b.uriage);
 kaisan(&c); printf("商品Ｃの売り上げは%d円　\n",c.uriage);
}

void keisan(struct SYOHIN *x)
{
//<商品xの売り上げの計算>
}

In function `main':
undefined reference to `kaisan'
undefined reference to `kaisan'
undefined reference to `kaisan'