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Bit-wise AND in C language

Consider the following statement given in C book by Dennis ritchie

The bitwise AND operator & is often used to mask off some set of bits,

for example

n = n & 0177;

sets to zero all but the low-order 7 bits of n.

Regarding bit-wise AND operator, my understanding is as follows

11010010 & 01101010 = 01000010

i.e., sets to 0 if the corresponding bit in any operand is 0

But in the above quoted statement, it is told that the the bits are 0 except the last 7.

If we expand 0177 in binary, it will be 10110001 (8-bits), so we cannot say about lower order 8 bits. Where i went wrong?

Solution

  • Quoting C11, chapter §6.4.4.1, (emphasis mine)

    [...] An octal constant consists of the prefix 0 optionally followed by a sequence of the digits 0 through 7 only. [...]

    So, an integer constant like 0177 is octal, which is the same as decimal 127 or hex 7F. The binary representation gives you 01111111, which is the true in

    [...] sets to zero all but the low-order 7 bits of n.