The canonical evil pointer trick is using prev_xor_next to construct a doubly linked list.
Could you put an example?
Ask HN: Favorite pointer tricks in C?
11–20 of 81 posts
Re: Ask HN: Favorite pointer tricks in C?
#12You can demonstrate pointer arithmetic by showing how you would work with the strstr function. It's the clearest and most understandable reason for someone to see why you'd even discuss this topic I think. I talk to some people without C experience and they hear that idea and get scared. I usually explain how strstr works and that seems to always make sense to them. Good luck!
char buf[1024]; /* should be enough */
I actually write this a lot in my code. Not code that I intend to share with others, of course. But I do find it amusing about the value I put in brackets. I find it amusing that I have an OCD-like predisposition to make it a power of two. And I find it amusing how the number between the brackets has increased over the last ten years, from a frugal 64 to an opulent 1024. This, to me, is progress.
Re: Ask HN: Favorite pointer tricks in C?
#13I like the trick of using the last few bits of aligned pointers to store something useful. It's tricky and has to be done correctly. A class wrapper around the pointer would be better.
For e.g., a constraint in an AVL tree requires that the difference in sizes of left and right subtrees be -1, 0 or 1 (just 3 values, which requires 2 bits). A 4-byte aligned pointer would be enough. =)
Re: Ask HN: Favorite pointer tricks in C?
#14It's kind of an anti-pattern, but I've found this to actually enhance clarity in a few places:
int moved = (is_reading ? read : write)(fd, buf, len);Re: Ask HN: Favorite pointer tricks in C?
#15You can demonstrate pointer arithmetic by showing how you would work with the strstr function. It's the clearest and most understandable reason for someone to see why you'd even discuss this topic I think. I talk to some people without C experience and they hear that idea and get scared. I usually explain how strstr works and that seems to always make sense to them. Good luck!
char buf[1024]; /* should be enough */ I actually write this a lot in my code. Not code that I intend to share with others, of course. But I do find it amusing about the value I put in brackets. I find it amusing that I have an OCD-like predisposition to make it a power of two. And I find it amusing how the number between the brackets has increased over the last ten years, from a frugal 64 to an opulent 1024. This, t…
In fact, a lot of code does this to avoid mallocs. Just be sure to check the length before you put anything into the buffer.
Also, making it a power of 2 is a good idea if you are concerned about alignment.
Re: Ask HN: Favorite pointer tricks in C?
#16pointers to structs for things like network protocols...
struct packet_header { uint from_addr; uint to_addr; ushort flags; ... }
packet *p;
read(socket, somebuf, sizeof(packet));
p = &somebuf;
printf("from = %u to = %u flags = %u\n",p->from_addr, p->to_addr, p->flags);
Re: Ask HN: Favorite pointer tricks in C?
#17Re: Ask HN: Favorite pointer tricks in C?
#18Earlier quoted context omitted.
char buf[1024]; /* should be enough */ I actually write this a lot in my code. Not code that I intend to share with others, of course. But I do find it amusing about the value I put in brackets. I find it amusing that I have an OCD-like predisposition to make it a power of two. And I find it amusing how the number between the brackets has increased over the last ten years, from a frugal 64 to an opulent 1024. This, t…
In fact, a lot of code does this to avoid mallocs. Just be sure to check the length before you put anything into the buffer. Also, making it a power of 2 is a good idea if you are concerned about alignment.
Just be sure to check the length before you put anything into the buffer.
No need to check. The comment reassures both me and future maintainers that I have countenanced this potential pitfall! Assign away....
Re: Ask HN: Favorite pointer tricks in C?
#19foo[5] is the same thing as * (foo + 5) (since "foo" is the address of the first element), which is equivalent to * (5 + foo) which is equivalent to 5[foo]! EDIT: hn ate up my * s.
Re: Ask HN: Favorite pointer tricks in C?
#20Copy-free contiguous subsets of arrays are fairly simple but often convenient. If you want elements 5 through 33 of big_array, you just get a pointer to element 5, and keep track separately of the length. A common case is where you split an array into two non-overlapping subparts, in which case, if you no longer need the original, you can treat each subpart as if it were a separate array. Saves the work of allocating two new arrays for the split parts, which is necessary in many other languages. Useful for efficiently implementing things like decision-tree learners.