省略参数名称，C++ vs C

In C++, I tend to omit the parameter's name under some circumstances. But in C, I got an error when I omitted the parameter's name.

代码如下:

void foo(int);  //forward-decl, it's OK to omit the parameter's name, in both C++ and C

int main()
{
    foo(0);
    return 0;
}

void foo(int)  //definition in C, it cannot compile with gcc
{
    printf("in foo
");
}

void foo(int)  //definition in C++, it can compile with g++
{
    cout << "in foo" << endl;
}

这是为什么?我不能在 C 函数定义中省略参数名称吗?