How can I initialize a flexible array in rodata and create a pointer to it?

Question

In C, the code

char *c = "Hello world!";

stores Hello world!\0 in rodata and initializes c with a pointer to it. How can I do this with something other than a string?

Specifically, I am trying to define my own string type

typedef struct {
   size_t Length;
   char Data[];
} PascalString;

And then want some sort of macro so that I can say

const PascalString *c2 = PASCAL_STRING_CONSTANT("Hello world!");

And have it behave the same, in that \x0c\0\0\0Hello world! is stored in rodata and c2 is initialized with a pointer to it.

I tried using

#define PASCAL_STRING_CONSTANT(c_string_constant) \
    &((const PascalString) { \
        .Length=sizeof(c_string_constant)-1, \
        .Data=(c_string_constant), \
    })

as suggested in these questions, but it doesn't work because Data is a flexible array: I get the error error: non-static initialization of a flexible array member (with gcc, clang gives a similar error).

Is this possible in C? And if so, what would the PASCAL_STRING_CONSTANT macro look like?

To clarify

With a C string, the following code-block never stores the string on the stack:

#include <inttypes.h>
#include <stdio.h>

int main(void) {
    const char *c = "Hello world!";

    printf("test %s", c);

    return 0;
}

As we can see by looking at the assembly, line 5 compiles to just loading a pointer into a register.

I want to be able to get that same behavior with pascal strings, and using GNU extensions it is possible to. The following code also never stores the pascal-string on the stack:

#include <inttypes.h>
#include <stdio.h>

typedef struct {
   size_t Length;
   char Data[];
} PascalString;

#define PASCAL_STRING_CONSTANT(c_string_constant) ({\
        static const PascalString _tmpstr = { \
            .Length=sizeof(c_string_constant)-1, \
            .Data=c_string_constant, \
        }; \
        &_tmpstr; \
    })

int main(void) {
    const PascalString *c2 = PASCAL_STRING_CONSTANT("Hello world!");

    printf("test %.*s", c2->Length, c2->Data);

    return 0;
}

Looking at its generated assembly, line 18 is also just loading a pointer.

However, the best code I've found to do this in ANSI C produces code to copy the entire string onto the stack:

#include <inttypes.h>
#include <stdio.h>

typedef struct {
   size_t Length;
   char Data[];
} PascalString;

#define PASCAL_STRING_CONSTANT(initial_value) \
    (const PascalString *)&(const struct { \
        uint32_t Length; \
        char Data[sizeof(initial_value)]; \
    }){ \
        .Length = sizeof(initial_value)-1, \
        .Data = initial_value, \
    }

int main(void) {
    const PascalString *c2 = PASCAL_STRING_CONSTANT("Hello world!");

    printf("test %.*s", c2->Length, c2->Data);

    return 0;
}

In the generated assembly for this code, line 19 copies the entire struct onto the stack then produces a pointer to it.

I'm looking for either ANSI C code that produces the same assembly as my second example, or an explanation of why that's not possible with ANSI C.

Answer 1

This can be done with the statment-expressions GNU extension, although it is nonstandard.

#define PASCAL_STRING_CONSTANT(c_string_constant) ({\
        static const PascalString _tmpstr = { \
            .Length=sizeof(c_string_constant)-1, \
            .Data=c_string_constant, \
        }; \
        &_tmpstr; \
    })

The extension allows you to have multiple statements in a block as an expression which evaluates to the value of the last statement by enclosing the block in ({ ... }). Thus, we can declare our PascalString as a static const value, and then return a pointer to it.

For completeness, we can also make a stack buffer if we want to modify it:

#define PASCAL_STRING_STACKBUF(initial_value, capacity) \
    (PascalString *)&(struct { \
        uint32_t Length; \
        char Data[capacity]; \
    }){ \
        .Length = sizeof(initial_value)-1, \
        .Data = initial_value, \
    }

Answer 2

You can use this macro, which names the name of the variable on its contents:

#define PASCAL_STRING(name, str) \
    struct { \
        unsigned char len; \
        char content[sizeof(str) - 1]; \
    } name = { sizeof(str) - 1, str }

To create such a string. Use it like this:

const PASCAL_STRING(c2, "Hello world!");

Answer 3

I am not sure why you would want to do it, but you could do it this way. This method will store your string in the data segment and gives you a way to access it as a structure. Note that I create a packed structure to ensure that the mapping into the structure always works since I have essentially hard coded the data fields in the const expression below.

#include <stdio.h>

#pragma packed(1)
typedef struct {
   unsigned char Length;
   char Data[];
} PascalString;
#pragma pack()

const unsigned char HELLO[7] = { 
0x06,
'H','E','L','L','O','\0'
};


int main(void) {
        PascalString *  myString = (PascalString *)HELLO;
        printf("I say: %s \n", myString->Data);
}