With floating point numbers

Question

I noticed that this code compiles, but I have no idea why:

int main() { double z = 0.000000000000001E-383DD; } 

But I'm not sure what DD means at the end of the number. I looked at the standard, but this was not mentioned.

I got this number from the following command:

 $ gcc -dM -E - < /dev/null #define __DBL_MIN_EXP__ (-1021) #define __FLT_MIN__ 1.17549435e-38F #define __DEC64_DEN__ 0.000000000000001E-383DD ... 

Perhaps this is a GCC extension?

Answer 1

Correctly this is aa GCC extension to indicate a 64-bit decimal floating point .

Other extension suffixes:

• Complex literals , for example. 1.0i , 1.0j , 1.0fi , etc. This is a pure GCC extension and the standard way for C99 is to use macro I ( 1.0*I , 1.0f*I , etc.)

• Additional floating point types that are purely a GCC extension:

  • 1.0w __float80 (80-bit binary floating point)
  • 1.0q __float128 (128-bit binary floating-point)

• Decimal types with floating point , which is based on the sentence N1312: extension for the C programming language to support decimal floating point arithmetic :

  • 1.0df _Decimal32 (32-bit decimal floating-point)
  • 1.0dd _Decimal64 (64-bit decimal floating-point)
  • 1.0dl → _Decimal128 (128-bit decimal floating point)

• Fixed Point Types Based on N1169 Proposal: Extensions to Support Embedded Processors :

  • 0.5hr , 0.5r , 0.5ulr , etc. → _Fract types (fixed-point types with magnitude ≤ 1)
  • 5.0hk , 5.0k , 5.0ulk etc. → _Accum types (fixed point types)

Answer 2

Yes, this is a GCC extension to support decimal float . A literal is of type _Decimal64 , but is converted when a double variable is assigned.

Answer 3

Maybe it means a tightly packed Decimal number of 64 bits?

This wikipedia section of the article looks as if it matches the name define, no?