A dirty macro that convert the scala type to string type in binary representation

Handy function that convert the scala type to string type in C. This is a not-portable GCC macro to produce binary-represented string in C or C++.

Sometimes, it is useful to have a way to print a scala value in a binary representation. Unfortunately, there is no such format specifier for printf(3). Luckily, glibc has a customization feature for printf(3)-like functions. If you're interested in, read the code here. For example, you could write:

int i = 0xdeadbeef;
printf("%b\n", i);      /* print 11011110101011011011111011101111 */

If you're not using glibc, you could create some utility functions to do this. Here're some candidate function prototypes:

/* return the binary represented string of VALUE.  The width
 * of the string is CHAR_BIT * size.  The return value should
 * be free(3)ed after use. */
char *binstr(unsigned long long value, size_t size);

/* store the binary representation of VALUE in string pointed by BUF
 * with the size SIZE.  */
char *binstr(unsigned long long value, char *buf, size_t size);

I'm not satisfied with these. The first one, which returns a value that is dynamically allocated, looks heavy. Plus, the caller should supply the number of binary digit, since it always accept the value as unsigned long long. The second one, which works only when the user provided the buffer as in the second and the third parameters. It does not look heavy, but probably someone may be not happy with it, since the user always need to prepare the buffer.

I couldn't come up with a everybody-satisified solution with this.

Since most of my works are done in Linux and MacOS; both provides GCC, so I decided to drop the portability. :)

#define binstr(x)       ({ char *p = alloca(sizeof(x) * CHAR_BIT + 1);  \
      char *q = p + sizeof(x) * CHAR_BIT;                               \
      *q-- = '\0';                                                      \
      int i;                                                            \
      typeof(x) tmp = (x);                                              \
      for (i = 0; i < sizeof(x) * CHAR_BIT; i++) {                      \
	*q-- = (tmp & 1) ? '1' : '0';                                   \
	tmp >>= 1;                                                      \
      }                                                                 \
      q + 1;                                                            \
  • I'm using alloca(3), instead of malloc(3), so that the memory buffer should be freed automatically. The caller no longer need to call free(3).
  • To decide the right number of digits of the value type, binstr is a macro function.
  • I used statement expression, ({…}) (GCC extension), which allows me to declare local variables.
  • I used typeof operator (GCC extension), which allows to declare a local variable, that has the same type as the parameter.


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