My company uses this question in pre-interview test. After I learn the answer, I find it very clever. And this is actually pretty practical: I know one major open source project uses this trick extensively.QUESTION 8: Given a pointer to member a within a struct, write a routine that returns a pointer to the struct.struct s {... int a;…}; struct s *get_s_ptr(int *a_ptr){// implement this.}

struct s* get_s_ptr(int *a_ptr){ return (struct s*)((char*)a_ptr - (int)&((struct s*)0)->a);}Hope there's no typo .

that's right XD, and that's how linux kernel implements its linked list. I handles all the pre-interview tests for my company (but i don't make decision, I just arrange and administrate the test). 2 years and hundreds of applicant, the # of ppl who got that right is a single digit.

not exactly great programming style though.

QuoteMy company uses this question in pre-interview test.What kind of company is your company? You are looking for a special skill? QuoteHope there's no typo I suppose we'll never know! undocuments code

To Cuchulainn:Our company makes firewall and related network equipment. The coding is either 1)kernel module or 2)low level daemon. I'm just a junior developer who happened to have some HR duty that was not a part of job description...To ehermo:Well, it is kind of confusion. Normally a container structure contains the element. but in this case, the element contains the container structure node. You can iterate/search the container structure node, then uses a macro to extract element. One advantage is that you can easily have an element belongs to multiple container structure. My company actually used this style for many other data structure too: hash table, trees etc.