Hidden features of C

Answer 1

Multi-character constants:

int x = 'ABCD';

This sets x to 0x41424344 (or 0x44434241, depending on architecture).

EDIT: This technique is not portable, especially if you serialize the int. However, it can be extremely useful to create self-documenting enums. e.g.

enum state {
    stopped = 'STOP',
    running = 'RUN!',
    waiting = 'WAIT',
};

This makes it much simpler if you're looking at a raw memory dump and need to determine the value of an enum without having to look it up.

Answer 2

C99 has some awesome any-order structure initialization.

struct foo{
  int x;
  int y;
  char* name;
};

void main(){
  struct foo f = { .y = 23, .name = "awesome", .x = -38 };
}

Answer 3

When using sscanf you can use %n to find out where you should continue to read:

sscanf ( string, "%d%n", &number, &length );
string += length;

Apparently, you can't add another answer, so I'll include a second one here, you can use "&&" and "||" as conditionals:

#include <stdio.h>
#include <stdlib.h>

int main()
{
   1 || puts("Hello\n");
   0 || puts("Hi\n");
   1 && puts("ROFL\n");
   0 && puts("LOL\n");

   exit( 0 );
}

This code will output:

Hi
ROFL

Answer 4

Function pointers. You can use a table of function pointers to implement, e.g., fast indirect-threaded code interpreters (FORTH) or byte-code dispatchers, or to simulate OO-like virtual methods.

Then there are hidden gems in the standard library, such as qsort(),bsearch(), strpbrk(), strcspn() [the latter two being useful for implementing a strtok() replacement].

A misfeature of C is that signed arithmetic overflow is undefined behavior (UB). So whenever you see an expression such as x+y, both being signed ints, it might potentially overflow and cause UB.

Answer 5

Well... I think that one of the strong points of C language is its portability and standardness, so whenever I find some "hidden trick" in the implementation I am currently using, I try not to use it because I try to keep my C code as standard and portable as possible.

Answer 6

I discoverd recently 0 bitfields.

struct {
  int    a:3;
  int    b:2;
  int     :0;
  int    c:4;
  int    d:3;
};

which will give a layout of

000aaabb 0ccccffffd

instead of without the :0;

0000aaab bccccffffd

The 0 width field tells that the following bitfields should be set on the next atomic entity (char)