Pointer to constant string in struct - c

Question

I'm writing in C.
In header1.h I declare a pointer to a constant string (a constant array of char), a new type with a struct and a variable of this type, and another variable:
const char *str_ptr = "Hello world!";
struct entry_t {
const char *text;
int var;
};
typedef entry_t entry;
extern entry start_entry;
int a;
In the file func.c I define the new variable:
#include "header1.h"
entry start_entry = {str_ptr, a};
void func(void)
{
//here i use start_entry
}
But the compiler report me the error
constant expression required
referring to the initialization of start_entry, and in particular of the const char* member.
Why? Where is the error?
Naturally if I define the string as
const char str[] = "Hello world!";
and then the entry variable as
entry start_entry = {&str, a};
everything is ok.
EDIT_1
I've made two errors in reporting my code:
var is const too
struct entry_t {
const char *text;
const int var;
};
in func.c entry is const too
const entry start_entry;
EDIT_2
I don't care of var, my interest is on const char * member (even if maybe the problem is the same: i write wrong code before, also with int var i had an error...).
I'm going to redefine all:
//header1.h
const char *str_ptr = "Hello world!";
struct entry_t {
const char *text;
};
typedef entry_t entry;
extern const entry start_entry;
//func.c
#include header1.h
const entry start_entry = {str_ptr};
void func(void)
{
//here i use start_entry
}
I can't understand why start_entry has a const char* member, i define it with a const char* but there is that error.

Answer 1

You can only use compile-time constants in initializers. The variable str_ptr is not even a run-time constant. What the const qualifier does is to prevent the string pointed to by str_ptr from being modified. The pointer itself can be reassigned. Therefor you need to initialize your start entry in an initialization function instead:
#include "header1.h"
entry start_entry;
void func(void)
{
//here i use start_entry
}
void init(void)
{
start_entry.text = str_ptr;
start_entry.var = a;
}

Answer 2

The problem is that with a C compiler you cannot initialize such statements in the global scope, it is considered as code outside a function:
(the same way that you cannot call a function to get tis return value and initialize a global outside a function)
entry start_entry = {str_ptr, a};
str_ptr is not really considered as a constant in C since it is seen as a pointer. The const qualifier does nothing here.
(the problem also occurs for me with a BTW, it looks that you are using some strange C++ compiler or a special C99 option see pmg comment)
When simply calling g++, your example works.
Note about your "naturally" statement:
entry start_entry = {&str, a};
compiles (with your compiler, gcc still rejects it because of a) but since you pass the address of the pointer, not the pointer itself => unspecified behaviour: not good.
If you want to do that, I suggest that you use a C++ compiler.
What works with simple gcc call:
char str_ptr[] = "Hello world!";
entry start_entry = {str_ptr, 12}; // considered as an array, value decided by the compiler, even if const keyword is missing
What doesn't
const char *str_ptr = "Hello world!";
entry start_entry = {str_ptr, 12}; // str_ptr is considered as a variable pointer even if const
entry start_entry = {"Hello world!", a}; // a is a variable: nocantdo
Where you are right is that char [] tells the compiler that the pointer is constant, even without the const keyword BTW, whereas the const keyword is not taken into account.
Notes:
I had to fix your code so it compiles with my gcc compiler:
typedef struct {
const char *text;
int var;
} entry;
extern entry start_entry;
Another point is: avoid declaring globals in header files as you'll get multiple inclusions or undefined symbols if you have more than one code file including it.