In a C file (say sourcefile1.c), an array is defined as follows. Here, we don’t need to mention arrary arr size explicitly in  because the size would be determined by the number of elements used in the initialization.
In another C file (say sourcefile2.c), the same array is declared for usage as follows:
In sourcefile2.c, we can use sizeof() on arr to find out the actual size of arr.
Explanation: First thing first, sizeof() operator works at compile time. So usage of sizeof on arr in sourcefile2.c won’t work because arr in sourcefile2.c is an incomplete type. Please note that arr in sourcefile1.c is a complete type because size of array got determined at compile time due to initialization.
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