Why is `extern` used in this MPLAB C example?

Question

In the MPLAB XC8 Compiler User Guide , an example on page 162 (reproduced below) uses the extern keyword in conjunction with the @ specifier. Given that we are specifying the address ourselves, why is this needed? It's not going to be allocating any memory per se.

The only reason I can think of is maybe extern variables aren't zeroed at startup. But then, C variables generally contain garbage anyway until you explicitly assign to them. So...I dunno.

Maybe it has something to do with it being in a header file? To avoid multiple #include statements causing a "variable already declared" error of some sort?

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If the pointer has to access objects in data memory, you need to define a different object to act as a dummy target. For example, if the checksum was to be calculated over 10 bytes starting at address 0x90 in data memory, the following code could be used.

 const char * cp; extern char inputData[10] @ 0x90; cp = &inputData; // cp is incremented over inputData and used to read values there

No memory is consumed by the extern declaration, and this can be mapped over the top of existing objects.

Answer 1

It makes no difference. It's mainly a choice of being explicit vs. being implicit.