how can I point to the start of a function in C?

I am writing a program for my class that in a VERY simplified way simulates the way an 开发者_StackOverflow中文版operating system would handle an interrupt.

In C, I have an array INTERRUPT_TABLE[] that I have declared with:

typedef void (*FN_TYPE)();

extern FN_TYPE INTERRUPT_TABLE[];

I want to set it so that each position in the array points to the beginning of a different function that is contained elsewhere in my program - for example, INTERRUPT_TABLE[0] should point to the beginning of the function handle_trap().

I thought that I could simply say: INTERRUPT_TABLE[0] = handle_trap; but that is not working. I get a compiler error that says "kernel.c:134: error: subscripted value is neither array nor pointer". Does anyone know what I am doing wrong?

Thanks for the help.

edit: figured out! I had my INTERRUPT_TABLE above the functions I was attempting to call, so they were being automatically declared as ints

Define "not working". That should work. I suspect we're missing some context. Are you tryingto initialize INTERRUPT_TABLE by saying INTERRUPT_TABLE[0] = handle_trap; somewhere at the top level? That particular syntax won't work there, but will work in a function body. Alternatively you can use initializer syntax:

  FN_TYPE INTERRUPT_TABLE[] = { handle_trap, ... };

To elaborate on Logan's answer:

Only (function and object) declarations can appear in the "external" or file context.

While FN_TYPE INTERRUPT_TABLE[] = { handle_trap, ... }; is a definition with initialization, the following has one definition and one assignment operation:

FN_TYPE INTERRUPT_TABLE[];
INTERRUPT_TABLE[0] = handle_trap;

This assignment can't appear at the top level and is interpreted as a redefinition of INTERRUPT_TABLE.

However, this doesn't match up with the error message you got back. For what it's worth, this simple program compiles successfully:

typedef void (*FN_TYPE) ();

extern FN_TYPE INTERRUPT_TABLE[];

void handle_trap() {
}

int main() {
    INTERRUPT_TABLE[0] = handle_trap;
}

When you say elsewhere, I assume it is not in the same file. So you either need a header file that declares handler_trap() or you must declare it manually in your C file before using it:

extern void handler_trap();

Otherwise the compiler has no idea that handler_trap() is a function, which value it returns or which parameters it expects.