I'm wondering if one needs to assign a bit-stream of 78 bits to a variable(later on to be processed), what datatype can be used? What is the max bit can be stored into a variable in C?

  • How about an array ob bytes? also 78 is not divisible by 8. no matter what type you take, you will have fractional parts.
    – Kwasmich
    Commented Nov 18, 2019 at 16:10
  • But I can still store 78 into anything multiple of 8. Its not a problem rest of the bits can be zero.
    – floppy380
    Commented Nov 18, 2019 at 16:19
  • Would in 64 can store 64 bits? How about if more than 64 and less than 128 bits are to assigned? Is it impossible?
    – floppy380
    Commented Nov 18, 2019 at 16:21
  • FWIW, sizeof(intmax_t) == 8, at least on the AVR-based boards. Commented Nov 18, 2019 at 18:00
  • Here is a template class that handles this, github.com/mikaelpatel/Cosa/blob/master/cores/cosa/Cosa/…. Commented Nov 18, 2019 at 18:52

1 Answer 1


There is no builtin data type that can store 78 bits. However, it depends a bit what you want to store. Like the following cases:

  • If the 78 bits are one 'entity' (e.g. one really big value), you can store it as an array of bytes (10 bytes of 8 bits = 80 bits, leaving 2 bits unused)
  • If the 78 bits can be split up in smaller parts, you can:
    • Create a structure or class to contain the sub elements into fields (instance variables), using for each sub type a normal type (e.g. signed/unsigned 8, 16 or 32 bits).
    • Create bit fields to save some memory if there are small items and memory space is an issue.
    • Or a combination of the above

If you have many of these items and you want to save space (thus the 2 unused bits of each 80 bits), you can create a big array that contains many items, and use bit operations to get each item. E.g. the first element will be in the first 10 bytes, where the last 2 bits are not part of the element. The second item will use the remaining 2 bits and the following 78 - 2 = 76 bits, etc. This saves only 2/80 * 100% = 2.5% so only use this when really needed.

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