C Language Declarations Using Global Constants
Example
Headers may be used to declare globally used read-only resources, like string tables for example.
Declare those in a separate header which gets included by any file ("Translation Unit") which wants to make use of them. It's handy to use the same header to declare a related enumeration to identify all string-resources:
resources.h:
#ifndef RESOURCES_H
#define RESOURCES_H
typedef enum { /* Define a type describing the possible valid resource IDs. */
RESOURCE_UNDEFINED = -1, /* To be used to initialise any EnumResourceID typed variable to be
marked as "not in use", "not in list", "undefined", wtf.
Will say un-initialised on application level, not on language level. Initialised uninitialised, so to say ;-)
Its like NULL for pointers ;-)*/
RESOURCE_UNKNOWN = 0, /* To be used if the application uses some resource ID,
for which we do not have a table entry defined, a fall back in
case we _need_ to display something, but do not find anything
appropriate. */
/* The following identify the resources we have defined: */
RESOURCE_OK,
RESOURCE_CANCEL,
RESOURCE_ABORT,
/* Insert more here. */
RESOURCE_MAX /* The maximum number of resources defined. */
} EnumResourceID;
extern const char * const resources[RESOURCE_MAX]; /* Declare, promise to anybody who includes
this, that at linkage-time this symbol will be around.
The 1st const guarantees the strings will not change,
the 2nd const guarantees the string-table entries
will never suddenly point somewhere else as set during
initialisation. */
#endif
To actually define the resources created a related .c-file, that is another translation unit holding the actual instances of the what had been declared in the related header (.h) file:
resources.c:
#include "resources.h" /* To make sure clashes between declaration and definition are
recognised by the compiler include the declaring header into
the implementing, defining translation unit (.c file).
/* Define the resources. Keep the promise made in resources.h. */
const char * const resources[RESOURCE_MAX] = {
"<unknown>",
"OK",
"Cancel",
"Abort"
};
A program using this could look like this:
main.c:
#include <stdlib.h> /* for EXIT_SUCCESS */
#include <stdio.h>
#include "resources.h"
int main(void)
{
EnumResourceID resource_id = RESOURCE_UNDEFINED;
while ((++resource_id) < RESOURCE_MAX)
{
printf("resource ID: %d, resource: '%s'\n", resource_id, resources[resource_id]);
}
return EXIT_SUCCESS;
}
Compile the three file above using GCC, and link them to become the program file main for example using this:
gcc -Wall -Wextra -pedantic -Wconversion -g main.c resources.c -o main
(use these -Wall -Wextra -pedantic -Wconversion to make the compiler really picky, so you don't miss anything before posting the code to SO, will say the world, or even worth deploying it into production)
Run the program created:
$ ./main
And get:
resource ID: 0, resource: '<unknown>'
resource ID: 1, resource: 'OK'
resource ID: 2, resource: 'Cancel'
resource ID: 3, resource: 'Abort'
