What are the two types of comments in C and what's the gotcha with multi-line comments?

Single-line // and multi-line /* … */ — and the gotcha is that /* */ comments don't nest, so the first */ ends the whole comment.

// Single-line comment (C99 onwards)

/* Multi-line
   comment */

/* Nested comments DON'T work:
   /* This breaks! */
   The comment ended at the first */
*/

The nesting problem:

/* Comment out this code
   x = 5; /* initialize x */
   y = 10;
*/
// ERROR: Comment ended at first */, leaving "y = 10; */" as code!

Solutions for commenting out code:

1. Use // for each line
2. Use #if 0 ... #endif (preprocessor trick)

#if 0
   x = 5;
   y = 10;  /* This comment is fine now */
#endif