How # flag in printf works?

Question

#include 
int main()
{
  float x;
  x=(int)(float)(double)(5.5);
  printf(\"%#u\",x);
  return 0;
}

How the # flag in the p

Answer 1

Using this flag with any other than the listed conversions is undefined behaviour. Don't use it with other conversions.

The value should be converted to an "alternate form".

For o conversions, the first character of the output string is made zero (by prefixing a 0 if it was not zero already).

For x and X conversions, a nonzero result has the string "0x" (or "0X" for X conversions) prepended to it.

For a, A, e, E, f, F, g, and G conversions, the result will always contain a decimal point, even if no digits follow it (normally, a decimal point appears in the results of those conversions only if a digit follows).

For g and G conversions, trailing zeros are not removed from the result as they would otherwise be.

For other conversions, the result is undefined.

(taken from the printf(3)-manpage. Wording is essentially the same as in the standard. Emphasis mine)