====== A small list of C brainteasers ======

Here's a small list of C brainteasers that a few friends and I came up with and/or found.  (All problems, except where specified, in C99.)

  * Can you write some C of the form ''sizeof ( ... printf("hey!\n") ... )'' which results in ''hey!'' being printed?
    * No, you may not put a close paren in the first ''...''.
    * ''sizeof'' comes in two forms in C -- ''sizeof ( TYPENAME )'', and ''sizeof EXPRESSION''.  Can you do it with both forms?
    * Can you do it without emitting any warnings?
  * Can you write some C that has the tokens ''[ * ]'' in a row?
  * How about the tokens ''restrict 5''?
  * Can you write an ''a.c'' that results in: <code>jwise@jwise-lt-linux:~$ grep '#' a.c
jwise@jwise-lt-linux:~$ grep '{' a.c
jwise@jwise-lt-linux:~$ cc -std=c99 -pedantic -Wall -o a a.c && ./a
hey!</code>
    * ... portably?
  * Can you write an ''a2.c'' that results in: <code>jwise@jwise-lt-linux:~$ grep '#' a2.c
jwise@jwise-lt-linux:~$ grep '{' a2.c
jwise@jwise-lt-linux:~$ grep '\[' a2.c
jwise@jwise-lt-linux:~$ grep '"' a2.c
jwise@jwise-lt-linux:~$ grep "'" a2.c
jwise@jwise-lt-linux:~$ gcc -std=c89 -pedantic -o a2 a2.c && ./a2
hey!</code>
    * ... portably?
  * Can you write a program ''a3.c'' that results in: <code>jwise@jwise-lt-linux:~$ grep '"' a3.c
jwise@jwise-lt-linux:~$ grep "'" a3.c
jwise@jwise-lt-linux:~$ grep ';' a3.c
jwise@jwise-lt-linux:~$ gcc -std=c99 -pedantic -Wall -o a3 a3.c && ./a3
hey!</code>
    * Portably?
  * Can you write a program ''a4.c'' that results in: <code>jwise@jwise-lt-linux:~$ grep '"' a4.c
jwise@jwise-lt-linux:~$ grep "'" a4.c
jwise@jwise-lt-linux:~$ grep ';' a4.c
jwise@jwise-lt-linux:~$ grep -E '{[^}]|{$' a4.c
jwise@jwise-lt-linux:~$ gcc -std=c99 -pedantic -Wall -o a4 a4.c && ./a4
hey!</code>
    * Without using the tricks from ''a'' or ''a2''?
    * The only implemenation I know of requires the compiler to take a specific view of the specification; you'll need ''gcc'''s view of the specification, not ''clang'''s.
  * //(taken from elsewhere on the Web)// Without running it, what is the output of the following program? \\ ''int main(void) {int n = sizeof(0)["abcdefghij"]; printf("%d\n", n); return 0; }''
    * It is not machine-specific.
  * A common idiom in C is ''assert(!"That's not gone well")''.  Unfortunately, a common idiom in C amongst careless programmers is ''assert("That's not gone well")'', or the potentially even more amusing ''assert("!That's not gone well")''.  Can you write a macro that chokes with a compile time failure on the latter two cases?
    * More concretely: it is easy to write a static assert macro that fails to compile if a given expression is either a) false, or b) not compile-time constant.  Your task is to write a static assert macro that fails to compile if a given expression is a) true **and** b) a compile-time constant!
    * Here are some test cases, if you care: <code>
ASSERT_CHECKER(time());
ASSERT_CHECKER(0);
ASSERT_CHECKER("abc"[2]);
ASSERT_CHECKER(!"biteme");
ASSERT_CHECKER("biteme");
ASSERT_CHECKER(2 + 2);</code> The last two should fail, by the C99 specification.  There is an easier way to do it with a GCC extension, but you do not need any GCC extensions to do this.
  * //(C++11-only)// Find a use for the token sequence ''......''.

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~~DISQUS~~