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Perhaps I don't fully understand extern. With the XC8 C compiler, it seems like I can get away without using it.

In the file keypad.c I have a function signed char keypadGetPressedKeyLabel(void). In the header file keypad.h I have:

signed char keypadGetPressedKeyLabel(void);

I thought I would have to use extern, ie:

extern signed char keypadGetPressedKeyLabel(void);

The project builds without extern. And it works. Might this due to the fact that keypad.c and keypad.h are all part of the project? I'm using MPLABX.

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  • \$\begingroup\$ Are any other files using keypad.h? \$\endgroup\$ Commented Nov 11, 2014 at 20:05

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extern is not strictly necessary for function prototypes in .h files - whether or not the function is actually used in one or many different .c files in your project.
Whether you have extern int foo(void); or just int foo(void); in your .h your compiler will read is as a function prototype either way.

Its necessary for global variables though since unlike functions they don't have prototypes.
So if you have an int thing; in one .c file and you want to use it in another .c file then you'll need an extern int thing; to tell the compiler that it does exist 'somewhere' in your project.

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  • \$\begingroup\$ Correct. Also, you can include initialization only on the definition of the variable e.g. int a = 0; it is an error to repeat this again in an external declaration e.g. extern int a = 0; // error \$\endgroup\$ Commented Nov 11, 2014 at 20:37
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Functions are extern by default in C; that's why it works without the keyword.

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    \$\begingroup\$ This is really the correct answer, but I think it would also benefit from a brief explanation of what extern actually means. \$\endgroup\$ Commented Nov 11, 2014 at 21:08
  • \$\begingroup\$ Great answers; PICs XC8 compiler uses extern looking for external files, not just as a global declaration. \$\endgroup\$ Commented Jan 1, 2017 at 18:04
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extern is really a sign to the linker, that the symbol is defined in a different object file. As you only seem to have one object file, it doesn't make any difference whether it's there or not.

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extern is largely obsolete for functions in other CUs in many of the smaller simpler compilers. If a symbol isn't found locally it will be searched for in other CUs and libraries. You provide a function prototype, but not the actual function, in the CU where you want to use it.

However, extern is required for variables that are shared between CUs. You can't define a prototype for a variable, so the extern keyword in this situation has to be used to create the variable equivalent of a function prototype.

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