Difference between scanf and scanf_s

It is a function that belongs specifically to the Microsoft compiler.

scanf originally just reads whatever console input you type and assign it to a type of variable.

If you have an array called first_name[5] and you use scanf for "Alex", there is no problem. If you have the same array and assign "Alexander", you can see it exceeds the 5 slots that the array contains, so C will still write it on memory that doesn't belong to the array and it might or might not crash the program, depending if something tries to access and write on that memory slot that doesn't belongs to first_name. This is where scanf_s comes in.

scanf_s has an argument(parameter) where you can specify the buffer size and actually control the limit of the input so you don't crash the whole building.

scanf_s() is not described by the C99 Standard (or previous ones).

If you want to use a compiler that targets C99 (or previous) use scanf().

For C11 Standard (and eventually later ones) scanf_s() is much harder to use than scanf() for improved security against buffer overflows.

C11 fscanf_s(): http://port70.net/~nsz/c/c11/n1570.html#K.3.5.3.2

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If you have a C99 compiler with extras that provides scanf_s() as an extension and don't mind losing portability, check your compiler documentation.