CODEWITHSUNDEEP
ccompilationlinker
dbkk
dbkk

Reputation: 12842

How to find an address of a C global variable at compile time

I'm doing some code generation. Is it possible to find the future address of a global C variable at compile time?

Suppose there is a global variable in a C file:

const char[] bytecode = "generated bytecode goes here";

After that, I need to add code pointing to bytecode at compile time:

const char* ptrInsideBytecode = 0x1234 + offset; 
// 0x1234 should be the address of bytecode

It is not an option to just refer to bytecode field, the pointer must be fixed.

I know that myVar will be at some fixed position after compilation and linking, so this should be possible. How do I find the exact address of bytecode before compiling?

Upvotes: 1

Views: 1253

Answers (3)

wildplasser
wildplasser

Reputation: 44240

const char bytecode[] = "generated bytecode goes here";

// After that, I need to add code pointing to bytecode at compile time:
#define offset 6

const char *ptrInsideBytecode = bytecode + offset;

// the address you want >>>-----^^^^^^^^

Upvotes: 0

Zebra North
Zebra North

Reputation: 11492

There isn't a way to do this, because the compiler must perform the summation in order to know what the value is, however the address won't be known until the linker has run.

Depending on your platform, you may be able to place your bytecode at a fixed location in memory by using a linker script. This is only likely to work if you're on a microcontroller that doesn't have an operating system.

Upvotes: 2

Scott Hunter
Scott Hunter

Reputation: 49803

It doesn't have an address at compile time; to see why, you could compile it on a different machine than the one (or more!) that it gets executed on.

Upvotes: 0

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