想编写一个从char*检查特定字元, 但无从入手, 请各大神指教一下

例: 检查字元是 'is'

int countResult(char source[])
{

}

int main()
{
char* source = "This is an apple";
int count = 0;
count = countResult(source);

printf("Result: %d", count)//Output: Result: 2
}

是用来练习的, 因为初学C语言, 从书中看到这些, 但不太懂