C question -- init string without trailing null possible?

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theusch
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Posted by theusch: Wed. Sep 27, 2017 - 07:21 PM
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I seem to recall a discussion on this in the past, but cannot find it.

 

I want to put an array of structs into flash.  (but flash or EEPROM or SRAM shouldn't matter, I don't think)  Each has a name field of exactly 8 bytes.  I want to give initial value, with no trailing null.  CodeVision objects:

 

Error: C:\OptiCode\HY148\HY148p.c(4660): initializer "string" exceeds array dimension

Stripped-down trivial example (yeah, I know if there were no other fields I could just paste them together...)

 

struct	xproduct_struct
{
unsigned char	type;		//
unsigned char	name[8];	//
};
typedef struct xproduct_struct	xxxProduct;

flash			xxxProduct	products_fixed[] =
{
	{	1, "DICTATE "},
	{	2, "FROGHAIR"},
};

I guess I could init each element of the array individually -- that would be onerous:

{ 2, {'F','R','O','G','H','A','I','R'}},

You can put lipstick on a pig, but it is still a pig.

I've never met a pig I didn't like, as long as you have some salt and pepper.

Tags:

Tools, Compilers and General Programming
Last Edited: Wed. Sep 27, 2017 - 07:23 PM
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sternst
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Posted by sternst: Wed. Sep 27, 2017 - 07:35 PM
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Your code is correct and achieves what you want, initializing the array without the trailing null. Looks like a bug to me that the compiler doesn't accept it.

 

Edit:

 

The standard says:

An array of character type may be initialized by a character string literal or UTF−8 string literal, optionally enclosed in braces. Successive bytes of the string literal (including the terminating null character if there is room or if the array is of unknown size) initialize the elements of the array.

 

Stefan Ernst

Last Edited: Wed. Sep 27, 2017 - 07:43 PM
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ki0bk
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Posted by ki0bk: Wed. Sep 27, 2017 - 07:50 PM
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sternst wrote:

Your code is correct and achieves what you want, initializing the array without the trailing null. Looks like a bug to me that the compiler doesn't accept it.

 

Edit:

 

The standard says:

An array of character type may be initialized by a character string literal or UTF−8 string literal, optionally enclosed in braces. Successive bytes of the string literal (including the terminating null character if there is room or if the array is of unknown size) initialize the elements of the array.

 

 

Which standard is that from?

 

The problem with standards is...... there are so d*m many of them!

 

 

 

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theusch
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Posted by theusch: Wed. Sep 27, 2017 - 07:56 PM
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sternst wrote:
Your code is correct and achieves what you want, initializing the array without the trailing null. Looks like a bug to me that the compiler doesn't accept it.

Well, poo -- I think you are right.  I poked into GCC/Studio7 and got a clean build with

...
struct	xproduct_struct
{
	unsigned char	type;		//
	unsigned char	name[8];	//
};
typedef struct xproduct_struct	xxxProduct;

__flash xxxProduct	const products_fixed[] =
{
	{ 2, {'F','R','O','G','H','A','I','R'}},
	{	1, "DICTATE "},
	{	2, "FROGHAIR"},
};
...
int main(void)
{
	myPWM_init();
	while(1)
	{
		OCR1C=9999;
		PORTB = products_fixed[0].name[PINB];
	}
}

...and the generated code looks reasonable -- I see the LPM...

154:	e3 b1       	in	r30, 0x03	; 3
 156:	f0 e0       	ldi	r31, 0x00	; 0
 158:	eb 51       	subi	r30, 0x1B	; 27
 15a:	ff 4f       	sbci	r31, 0xFF	; 255
 15c:	84 91       	lpm	r24, Z
 15e:	85 b9       	out	0x05, r24	; 5

... and the table in flash...

000000e4 <__trampolines_end>:
  e4:	02 46       	sbci	r16, 0x62	; 98
  e6:	52 4f       	sbci	r21, 0xF2	; 242
  e8:	47 48       	sbci	r20, 0x87	; 135
  ea:	41 49       	sbci	r20, 0x91	; 145
  ec:	52 01       	movw	r10, r4
  ee:	44 49       	sbci	r20, 0x94	; 148
  f0:	43 54       	subi	r20, 0x43	; 67
  f2:	41 54       	subi	r20, 0x41	; 65
  f4:	45 20       	and	r4, r5
  f6:	02 46       	sbci	r16, 0x62	; 98
  f8:	52 4f       	sbci	r21, 0xF2	; 242
  fa:	47 48       	sbci	r20, 0x87	; 135
  fc:	41 49       	sbci	r20, 0x91	; 145
  fe:	52 00       	.word	0x0052	; ????

 

 

You can put lipstick on a pig, but it is still a pig.

I've never met a pig I didn't like, as long as you have some salt and pepper.

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theusch
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Posted by theusch: Wed. Sep 27, 2017 - 08:00 PM
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ki0bk wrote:
Which standard is that from?

You can put lipstick on a pig, but it is still a pig.

I've never met a pig I didn't like, as long as you have some salt and pepper.

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david.prentice
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Posted by david.prentice: Wed. Sep 27, 2017 - 08:01 PM
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It is perfectly legal to initialise an 8 char array with a quoted string.   It should just drop the NUL if it does not fit.

Most Compilers will whinge but it should be Warning not Error.

 

Note that a 9-char string would be an Error.

 

char name[5] = "David";  //will omit the NUL

char name[] = "David";   //will allocate 6 chars to accommodate the NUL

 

David.

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theusch
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Posted by theusch: Wed. Sep 27, 2017 - 08:08 PM
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Thanks for the eyes and the responses, people.  I'll work around it. Take care.

You can put lipstick on a pig, but it is still a pig.

I've never met a pig I didn't like, as long as you have some salt and pepper.

Last Edited: Wed. Sep 27, 2017 - 08:09 PM
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ki0bk
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Posted by ki0bk: Wed. Sep 27, 2017 - 08:14 PM
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Your example compiles ok in imagecraft as well. 

No null char's stored.

Jim

 

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JohanEkdahl
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Posted by JohanEkdahl: Wed. Sep 27, 2017 - 08:26 PM
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david.prentice wrote:
It is perfectly legal to initialise an 8 char array with a quoted string.   It should just drop the NUL if it does not fit.

Learn someting every day: Check.

 

(I might actually have known this at some time, but iif so it sure is swapped out thoroughly..)

 

 

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Posted by joeymorin: Thu. Sep 28, 2017 - 06:40 AM
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(I might actually have known this at some time, but iif so it sure is swapped out thoroughly..)

$ echo 3 > /proc/sys/vm/drop_caches

 

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Posted by skeeve: Thu. Sep 28, 2017 - 04:10 PM
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BTW dropping the null is NOT valid in C++.

I note the file name ends in .c .

gcc can be told to compile a .c file as C++.

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westfw
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Posted by westfw: Fri. Sep 29, 2017 - 06:32 AM
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I am disappointed that I can apparently not index quoted strings in initialization:

   char c = "mystring"[3];

(I mean, the compiler will optimize away references like that

static char* mystring = "mystring";
  :
  c = mystring[3];

will end up producing something like "ldi r24,'t'"...)

 

that would have made for some not-too-ugly macros for initializing size-limited arrays from arbitrary strings...   Ah well.

 

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MarekJ71
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Posted by MarekJ71: Fri. Sep 29, 2017 - 07:08 AM
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westfw wrote:

I am disappointed that I can apparently not index quoted strings in initialization:

   char c = "mystring"[3];

Are you sure? I tried to compile this and there were no errors. It works for both C and C++.

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westfw
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Posted by westfw: Fri. Sep 29, 2017 - 09:03 AM
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Are you sure? I tried to compile this and there were no errors. It works for both C and C++.

Failed for me with avr-gcc 5.4, 4.3.x, and Apple llvm 6.00

 

BillW-MacOSX-2<5027> avr-gcc -c -Os -mmcu=atmega8 gross.c
gross.c:5:10: error: initializer element is not constant
 char c = "1234"[2];
          ^

 

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MarekJ71
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Posted by MarekJ71: Fri. Sep 29, 2017 - 09:54 AM
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westfw wrote:

Are you sure? I tried to compile this and there were no errors. It works for both C and C++.

Failed for me with avr-gcc 5.4, 4.3.x, and Apple llvm 6.00

 

BillW-MacOSX-2<5027> avr-gcc -c -Os -mmcu=atmega8 gross.c
gross.c:5:10: error: initializer element is not constant
 char c = "1234"[2];
          ^

 

I know what makes the difference. My c variable was an automatic variable in main() and apparently your c was a global variable. When I moved the variable outside of main(), I also got compile error.

In early C string constants like "1234" in your example could be modified, but current standard doesn't allow this. However, string literals are still of type char[] in C, not const char[] and I think this is the reason why "1234"[2] isn't a constant expression.

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westfw
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Posted by westfw: Sat. Sep 30, 2017 - 12:51 AM
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I thought it might be that, but it also doesn't like casts to "const" either.

I guess the automatic variable initialization doesn't even need to be constant (but does optimized to an LDI...)

 

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