A macro for calculating the size of a buffer for a C-style string containing a (signed/unsigned) integer:

  #define CS_INT_SIZE(int_type) ((size_t)(0.30103 * sizeof(int_type) * 8) + 2 + 1)

  int x = -1000;
  char buf[CS_INT_SIZE(x)]; // instead of char buf[100];
  snprintf(buf, sizeof(buf), "%d", x);

Another macro for calculating the length of a string literal in compile time (just like strlen would do). Note the extra check for string literal.

  #define CSLLEN(s)           (sizeof(s "") - 1)
  int len = CSLLEN("hello"); // len == 5 here

Another one for logging variable arguments or no arguments at all.

  #define Log_Trace_(...) Log(__FILE__, __LINE__, __func__, LOG_TYPE_TRACE, "" __VA_ARGS__)
  void Log(const char* file, int line, const char* func, int type, const char* fmt, ...);
  Log_Trace(); // prints just the name of the file, line and func
  Log_Trace("Error %d", error); // prints the same as above and the error number