declaring constant data

declaring constant data

[Rafał Pietrak]

Hi everybody,

I'm writing code for embedded ARM microcontroler using arm-none-eabi-gcc
version 8.3.1 20190703. I'd like to put my static data structure into
FLASH memory to save precious RAM. My code is like this:
---------------------------------
static const char *test_a[] /* __attribute__((section (".rodata"))) */ = {
	"test1",
	"test2",
};
static const char *test_b[] /* __attribute__((section (".rodata"))) */ = {
	"test3",
	"test4",
};
static const char *test_c[] /* __attribute__((section (".rodata"))) */ = {
	"test5",
	"test6",
};
static const struct dict_ports_s {
	const char **alias;
	const char len;
	const char *name;
} ports[] = {
	{ .name = "test0", .len = sizeof(test_a), .alias = test_a },
	{ .name = "test1", .len = sizeof(test_b), .alias = test_b },
	{ .name = "test2", .len = sizeof(test_c), .alias = test_c },
};

int main(int ac, char *av[]) {
	int i;
	for (i = 0; i < 3; i++) {
		printf("%d %s\n", ports[i].len , ports[i].alias[0]);
	}
	return 0;
}
------------------------------------------------

but neither the "const" words in declarations, nor the explicit
selection of ".RODATA" section make those constant data structure go
into read-only data section. Assembler, that gcc produces with "-S"
option looks like this:
-----------------------------------------
	.section	.data.test_a,"aw"
	.align	2
	.set	.LANCHOR0,. + 0
	.type	test_a, %object
	.size	test_a, 8
test_a:
	.word	.LC5
	.word	.LC6
	.section	.data.test_b,"aw"
	.align	2
	.set	.LANCHOR1,. + 0
	.type	test_b, %object
	.size	test_b, 8
test_b:
	.word	.LC3
	.word	.LC4
	.section	.data.test_c,"aw"
	.align	2
	.set	.LANCHOR2,. + 0
	.type	test_c, %object
	.size	test_c, 8
test_c:
	.word	.LC1
	.word	.LC2
----------------------------------------------------

so the structures are emitted into relevant ".data.test*" sections which
are writable. And that's not what I need.

How do I get them emitted into "a" section (with no "w" attribute)?

BR.

Rafał
PS: I'm not a subscriber of the list, so pls Cc: responses to me directly.

Re: declaring constant data

[Martin Sebor]

On 10/8/21 1:55 PM, Rafał Pietrak via Gcc-help wrote:
> Hi everybody,
> 
> I'm writing code for embedded ARM microcontroler using arm-none-eabi-gcc
> version 8.3.1 20190703. I'd like to put my static data structure into
> FLASH memory to save precious RAM. My code is like this:
> ---------------------------------
> static const char *test_a[] /* __attribute__((section (".rodata"))) */ = {
> 	"test1",
> 	"test2",
> };
> static const char *test_b[] /* __attribute__((section (".rodata"))) */ = {
> 	"test3",
> 	"test4",
> };
> static const char *test_c[] /* __attribute__((section (".rodata"))) */ = {
> 	"test5",
> 	"test6",
> };
> static const struct dict_ports_s {
> 	const char **alias;
> 	const char len;
> 	const char *name;
> } ports[] = {
> 	{ .name = "test0", .len = sizeof(test_a), .alias = test_a },
> 	{ .name = "test1", .len = sizeof(test_b), .alias = test_b },
> 	{ .name = "test2", .len = sizeof(test_c), .alias = test_c },
> };
> 
> int main(int ac, char *av[]) {
> 	int i;
> 	for (i = 0; i < 3; i++) {
> 		printf("%d %s\n", ports[i].len , ports[i].alias[0]);
> 	}
> 	return 0;
> }
> ------------------------------------------------
> 
> but neither the "const" words in declarations, nor the explicit
> selection of ".RODATA" section make those constant data structure go
> into read-only data section. Assembler, that gcc produces with "-S"
> option looks like this:
> -----------------------------------------
> 	.section	.data.test_a,"aw"
> 	.align	2
> 	.set	.LANCHOR0,. + 0
> 	.type	test_a, %object
> 	.size	test_a, 8
> test_a:
> 	.word	.LC5
> 	.word	.LC6
> 	.section	.data.test_b,"aw"
> 	.align	2
> 	.set	.LANCHOR1,. + 0
> 	.type	test_b, %object
> 	.size	test_b, 8
> test_b:
> 	.word	.LC3
> 	.word	.LC4
> 	.section	.data.test_c,"aw"
> 	.align	2
> 	.set	.LANCHOR2,. + 0
> 	.type	test_c, %object
> 	.size	test_c, 8
> test_c:
> 	.word	.LC1
> 	.word	.LC2
> ----------------------------------------------------
> 
> so the structures are emitted into relevant ".data.test*" sections which
> are writable. And that's not what I need.
> 
> How do I get them emitted into "a" section (with no "w" attribute)?

Try throwing a few more consts at it ;-)  This:

   static const char *test_a[] = { ... };

defines test_a[] to be a modifiable array of pointers to const char.
If you want the array itself to be read-only you need to declare it
const as well:

   static const char* const test_a[] = { ... };

Martin

> 
> BR.
> 
> Rafał
> PS: I'm not a subscriber of the list, so pls Cc: responses to me directly.
>