what does it do

Question

what does the %ld do ?

Answer 1

%ld is a format specifier for long int type.This is implemented with printf() function for printing the long integer value stored in the variable.
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Answer 2

It is the format specifier for a long signed integer. It is often (mis)used to print the size of an object or variable pre-C99.

Use %ld when you want to print long integers that can be both positive and negative and won't fit in an int.