How to initialize global constant variable in main() function using pure C? - c

Question

How can I do global constant initialization like this? Is it possible at all? Or there is another way to do what I want? I mean I need global parameters gained from main() and they must be constants.
#include <stdio.h>
#include <stdlib.h>
const int var;
int main(int argc, char *argv[]) {
var = atoi(argv[1]);
/* ... */
return 0;
}

Answer 1

I need global parameters gained from main() and they must be constants.
No portable way to do directly as OP wants.
Code needs different access for reading and writing. Effectively hiding the access to the true data.
A close solution it to set and get data via functions defined in another file. Then no way to change data once set and only settable once.
main_var.h
int main_var_get(void);
void main_var_set(int v);
main_var.c
#include <stdlib.h>
#include "main_var.h"
static int var; // This could instead be a struct of many members.
// Or a pointer to a struct with many members.
static int var_init;
int main_var_get(void) {
if (!var_init) {
// Handle call of get before set, perhaps exit or return default value
exit(EXIT_FAILURE);
}
return var;
}
void main_var_set(int v) {
if (var_init) {
// Handle 2nd set, perhaps exit or ignore
exit(EXIT_FAILURE);
}
var = v;
var_init = 1;
}
main.c
#include <stdio.h>
#include "main_var.h"
int main(void) {
main_var_set(42);
...
printf("%d\n", main_var_get());
}
Another is to use a const int *. Access before setting is the same no-no as dereferencing NULL. Attempting to write *main_var_addr is UB like writing any const object.
main_var.h
extern const int *main_var_addr;
void main_var_set(int v);
main_var.c
#include <stdlib.h>
#include "main_var.h"
const int *main_var_addr = NULL;
static int var;
void main_var_set(int v) {
if (main_var_addr) {
// Handle 2nd set attempt, perhaps exit or ignore
exit(EXIT_FAILURE);
}
var = v;
main_var_addr = &var
}
main.c
#include <stdio.h>
#include "main_var.h"
int main(void) {
main_var_set(42);
...
printf("%d\n", *main_var_addr);
}

Answer 2

I don't think C allows you to initialize a constant variable elsewhere. However, you can just change var from const to static.

Answer 3

This is not possible, actually const doesn't mean the variable is constant in C, it's only mean that you are not allowed to change the value of the variable, but someone else could.
Global value are initialize before the main run, that mean you can't initialize at runtime, how this initialization is done is implementation behavior so there is no "pure C" way to do what you ask.
However, I don't see why you could not encapsulate your global:
my_var.h:
int init_my_var(int argc, char **argv);
int get_my_var(void);
my_var.c
#include <stdlib.h>
#include <limits.h>
#include <errno.h>
static int var;
int init_my_var(int argc, char **argv) {
#ifndef NDEBUG
#include <stdbool.h>
#include <assert.h>
{
static bool first = true;
assert(first);
first = false;
}
#endif
if (argc < 2) {
return 1;
}
errno = 0;
long ret = strtol(argv[1], NULL, 10);
if (errno || (ret < INT_MIN || ret > INT_MAX)) {
return 2;
}
var = (int)ret;
return 0;
}
int get_my_var(void) {
return var;
}
main.c:
#include <stdio.h>
int main(void) {
printf("%d\n", get_my_var());
if (init_my_var(2, (char *[]){"", "42"})) {
return EXIT_FAILURE;
}
printf("%d\n", get_my_var());
if (init_my_var(2, (char *[]){"", "0"})) {
return EXIT_FAILURE;
}
}