Does C provide a way to declare an extern variable as 'read-only', but define it as writeable?

Question

I\'m developing a hardware abstraction library for an embedded product using GCC C. Within the library there is a variable that should be read-only to the application that l

Answer 1

Within the library there is a variable that should be read-only to the application that links the library, but can be modified from within the compilation unit that defines it.

The solution is to use object-oriented design, namely something called private encapsulation.

module.h

int module_get_x (void);

module.c

static int x;

int module_get_x (void)
{
  return x;
}

main.c

#include "module.h"

int main(void)
{
  int value = module_get_x();
}

If the variable must be read-write inside the module, then this is the only proper way to do this. All other ways are just some flavour of spaghetti programming.

Answer 2

You can use pointer to get past the issue.

module.c

static int readwrite = 0;
int const * const readonly = &readwrite;

module.h

extern int const * const readonly;

If there is no reason to expose address of the variable, you should prefer Lundins getter approach to have proper encapsulation.