how to convert an 128 bit binary number to decimal number

"0xFF FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF" is a character string.

"255^16+255^15+....+255^2+255" is a character string, presumably intended to represent another character string with the "..." replaced by other symbols.

These two character strings do not represent the same number.  An expression equivalent to the first expression would be something like "(255*(256^15))+(255*(256^14))+...+(255*(256^2))+(255*256)+255".

The only variable parts of this expression are the "255"s.  You would need to plug in the character representation of each byte of the numbinary string.

The line

numbinary[i]= 0xFF;

indicates that you intend to store data in numbinary in binary form.  To learn how to convert a binary number to character representation, search this site.  This question must have been answered many times already.

I want to get the decimal result of 0xFF FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF.

Anyone could compute the "0xFF FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF"'s decimal result and save in a char array?

0xFF ---->> 255                    
8bits           char[3]

0xFFFF ---->>65535
16bits            char[5]

0xFFFFFFFF---->>4294967295
32bits                  char[10]

0xFFFFFFFFFFFFFFFF---->>18446744073709551615
64bits                                char[20]
............................................................................>>above could be computed by compiler , 32bits we could use unsigned int, 64bits we could use unsigned long

0xFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF---->>18446744073709551616^2-1
128bits                                                          maybe char[40]

In this case, how should I do?

That clarification helps.  A few more questions:

Do you want this answer just for the number 0xFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF you have specified, or do you want a more general solution?

If you just want the answer for this number, is it important to have a program that calculates it, or do you just want to know what the number is?

If you want a program, either for just this number or in general, does the program have to be limited to the basic facilities of C++, or can you use an arbitrary precision integer arithmetic library that somebody else has already created?

>>If you want a program
yes, I want a programme that implement the 128-bit number operations.
I could implement the operations between two 128-bit numbers, but I couldn't get the decimal output according to the 128-bit numbers.
What I want  is to convert the 128-bit binary number to a decimal number.

To convert binary to decimal you need at the very least a routine to subtract 128-bit numbers.

Then apply this algorithm:

( build a table of powers of ten, 128-bits wide, up to whatever power of ten is just below 2^128 )

subtract the biggest number of the table from X.

repeat that until the number goes negative.

The number of times you can do that is your first digit.

move to the next smallest power of ten

repeat all that until you're down to the 10^0 entry.

 example for 16 bits:

     table:   10000
                   1000
                    100
                      10
                        1



X = 12345

subtract 10000 from X until it goes negative.
times you can do this:  1

X is now 2345

subtract 1000 from X until it goes negative
times you can do this:  2

...  and  so on.

1. how do u know the biggest number's power of the table through programming?
2. how do u implement the subtraction, use binary number to implement, then you face how to convert the ten power to binary number?

I'd suggest you look at one or more of the libraries listed on this page:

http://en.wikipedia.org/wiki/Arbitrary-precision_arithmetic

>>Just keep a table of powers of ten
I also should have a table of power of 2 , and implement their conversions.
I've got the idea, thank you.

Thanks to all.