The problem is that the b format is for unsigned integers only, so your -1 gets turned into an unsigned int and printing that overflows your field width, giving you lots of ones.The help for sprintf says :- 

%b    an unsigned integer, in binary

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BTW sprintf always prints the entire number even if it's bigger than the field width, so this is never going to work for 10 bit binary.

In reply to Re: Converting negative number to binary with specific width by RichardK
in thread Converting negative number to binary with specific width by pjkang7

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