which is the biggest datatype in c language to store large values?

Question

what is the largest datatype in c in which I can store a large values like 10^17 etc? and also tell how to take user input,store,print them in this particular case.

Answer 1

A particular implemention might optionally support implementation-defined extended integer types (N1570 6.2.5 Types):

4) There may also be implementation-defined extended signed integer types.³⁸⁾ The standard and extended signed integer types are collectively called signed integer types.³⁹)

6) The unsigned integer types that correspond to the extended signed integer types are the extended unsigned integer types. The standard and extended unsigned integer types are collectively called unsigned integer types^.40)

The motivitation is likely to allow 128-bit wide integers, but it seems to be no interest of it. It's more likely that particular implementation supports such integers via extension such as __int128 in gcc.

However, if your requirement is up to 10¹⁷, then you will be fine with the standard unsigned integer type such as unsigned long long int. It guaranteed to be at least 64-bits wide, thus possible range is at least [0, 2⁶⁴-1], that is slightly more than 18 * 10¹⁸.

Note that long long types were introduced in C99, but it's likely that even older C89 compiler supports them through extension. Check you compiler's documentation for any details.

Both printf() and scanf() expects prefix ll in format specifier. For instance:

unsigned long long n = ULLONG_MAX;
printf("lld", n);