using printf to print out floating values

by

printf() is not typesafe.

The arguments that you pass to printf() are treated according to what you promise the compiler.

Also, floats are promoted to doubles when passed through variadic arguments.

So when you promise the compiler %f the first time (for xf), the compiler gobbles up an entire double (usually 8 byte) from the arguments, swallowing your float in the process. Then the second %f cuts right into the zero mantissa of the second double.

Here’s a picture of your arguments:

+-0-1-2-3-+-0-1-2-3-+-0-1-2-3-4-5-6-7-+-0-1-2-3-4-5-6-7-+
|    x    |    x    |        f        |        f        |
+---------+---------+-----------------+-----------------+

%d--------|%f----------------|%f---------------|%d------|

But f looks like this (having been promoted to double):

f = 3FF8000000000000

Let’s draw it again with values, and speculating about your machine endianness:

| 05000000 | 05000000 | 00000000 0000F83F | 00000000 0000F83F |
| %d, OK   | %f, denormal...    | %f, denormal...   | %d, OK  |

Note that 1073217536 is 0x3FF80000.

More Related Contents:

  1. printf(“%p”) and casting to (void *)
  2. How to printf a memory address in C
  3. C: Which character should be used for ptrdiff_t in printf?
  4. Why doesn’t scanf need an ampersand for strings and also works fine in printf (in C)?
  5. Create a pointer to two-dimensional array
  6. Pointers in C: when to use the ampersand and the asterisk?
  7. What is an opaque pointer in C?
  8. What is the reason for using a double pointer when adding a node in a linked list?
  9. Post-increment on a dereferenced pointer?
  10. What exactly is a C pointer if not a memory address?
  11. Reversing a string in C
  12. printf format specifiers for uint32_t and size_t
  13. Why do I need to use type** to point to type*?
  14. Behaviour of printf when printing a %d without supplying variable name
  15. Why is printf with a single argument (without conversion specifiers) deprecated?
  16. Using intptr_t instead of void*?
  17. What are function pointers used for, and how would I use them?
  18. What’s a modern term for “array/pointer equivalence”?
  19. How to use “zd” specifier with `printf()`?
  20. Passing too many arguments to printf
  21. Doesn’t a 2D array decay to pointer to pointer
  22. printf with “%d” of numbers starting with 0 (ex “0102”) giving unexpected answer (ex ‘”66″)
  23. What primitive data type is time_t? [duplicate]
  24. Printf long long int in C with GCC?
  25. Array of size 0 at the end of struct [duplicate]
  26. Dereference void pointer
  27. cross-platform printing of 64-bit integers with printf
  28. What precisely does the %g printf specifier mean?
  29. Turbo C++: Why does printf print expected values, when no variables are passed to it?
  30. Is sprintf(buffer, “%s […]”, buffer, […]) safe?

Leave a Comment