If we have a const char pointer initialized by a string literal, such as const char *str = "Hi!", then we cannot use str as a compile-time constant. This makes sense, str is a variable after all. But addresses are compile time constants, so we could use &str just fine.
The following workaround compiles with GCC and clang, with -Wall, -Wextra, -Wpedantic, -Weverything (clang only), and -std=c89).
#include <stdio.h>
/* Bad:
* static const char *str = "Hi!";
*/
static const char str[4] = {'H', 'i', '!', '\0'};
static struct {
double x;
const char *string;
} my_struct = {1.0, str};
int main(void)
{
puts(my_struct.string);
return 0;
}
After some searching, I was unable to find something in the C standard that would verify that this is legal and portable. My guess is that static const char str[4] is an array, so using str in the structs initialization is the same thing as using an address, which is a compile time constant, whereas using static const char *str is a pointer to an array, so using it amounts to using a variable, which is not a compile time constant.
Any references to the relevant sections in the standard are greatly appreciated.
If we have a const char pointer initialized by a string literal, such as const char *str = "Hi!", then we cannot use str as a compile-time constant. This makes sense, str is a variable after all. But addresses are compile time constants, so we could use &str just fine.
The following workaround compiles with GCC and clang, with -Wall, -Wextra, -Wpedantic, -Weverything (clang only), and -std=c89).
#include <stdio.h>
/* Bad:
* static const char *str = "Hi!";
*/
static const char str[4] = {'H', 'i', '!', '\0'};
static struct {
double x;
const char *string;
} my_struct = {1.0, str};
int main(void)
{
puts(my_struct.string);
return 0;
}
After some searching, I was unable to find something in the C standard that would verify that this is legal and portable. My guess is that static const char str[4] is an array, so using str in the structs initialization is the same thing as using an address, which is a compile time constant, whereas using static const char *str is a pointer to an array, so using it amounts to using a variable, which is not a compile time constant.
Any references to the relevant sections in the standard are greatly appreciated.
Share Improve this question asked Feb 17 at 15:49 Ryan MaguireRyan Maguire 1078 bronze badges 4 |1 Answer
Reset to default 4You seem to be asking whether str, having been defined as a static const char str[4], can be used to initialize a member of a static structure.
C 2024 6.6 specifies constant expressions and includes:
More latitude is permitted for constant expressions in initializers. Such a constant expression shall be, or evaluate to, one of the following:
…
— an address constant, or…
and:
An address constant is a null pointer, a pointer to an lvalue designating an object of static storage duration,…
str is an array, not a pointer. However, in this use, it is automatically converted to a pointer to its first element. That first element is an object of static storage duration, and hence str evaluates to a pointer to an object of static storage duration, so it may be used as an initializer.
static const char str[] = "Hi!";to define and initialize an array of the correct size for the string literal. – Bodo Commented Feb 17 at 17:13stris potentially reserved by the implementation for any use. – Ian Abbott Commented Feb 17 at 17:18my_struct.stringwith an actual string literal"Hi!":static struct { … } my_struct = {1.0, "Hi!"};. – Ian Abbott Commented Feb 17 at 17:29