Programming Forums > Application Development > C

iradic · Oct 22nd, 2006, 7:10 PM

I would like to correct the following so that gcc no more complains with "warning: statement with no effect" ... if possible:

(Toggle Plain Text)

static char	*spend;

static void listto(char *p1, FILE *fp)
{
	p1--;
	for (; *p1++; p1 < spend)
        ....

Thanks

Sane · Oct 22nd, 2006, 7:14 PM

p1 is a pointer to some value. You're trying to decrement the address, instead of the value, when you go: "p1--".

(Toggle Plain Text)

(*p1)--;

Try that. It derefences the pointer first before decrementing the value.

I'd also like to note that...

- Your for loop arguments aren't in the correct order. It goes (initiation ; loop condition ; iteration). You have your iteration where the loop condition should be, and vice-versa.

- You might need to bracket *p1++, to make (*p1)++. I believe the order of operations needs to derefence first, before incrementing. Try either way, see which one works.

The Dark · Oct 22nd, 2006, 9:41 PM

Sane was right in his notes - the reason you are getting the error is because the for loop needs to be reversed.
However, I dont think you want to be decrementing the value, rather than the pointer as it looks like this function should be using pointer arithmetic. This depends on what your function is supposed to be doing.
Something like:

(Toggle Plain Text)

static char	*spend;

static void listto(char *p1, FILE *fp)
{
	p1--;
	for (; p1 < spend; p1++ )
           ....
}

Sane · Oct 22nd, 2006, 10:20 PM

Oh, so it could be pointer arithmetic? All right. I guess "*p1++" kind of threw me off. In that case, you want "p1 < spend" to be "*p1 < spend", unless spend happens to be a pointer as well, in which case that would be some pretty weird programming. o_O

Then you'll also need to change "*p1++" to "p1++".

The Dark · Oct 22nd, 2006, 10:24 PM

spend is a pointer - it is right there in the code.

Sane · Oct 22nd, 2006, 10:25 PM

Oh fuck! Hahaha. I'm awesome like that. Sorry. :p

Heh. Thanks for pointing that out.

In an attempt to redeem myself, I will post what I believe to be the "correct" code... haha.

(Toggle Plain Text)

static char	*spend;

static void listto(char *p1, FILE *fp)
{
	p1--;
	for (; p1 < spend; p1++)
        ....

iradic · Oct 23rd, 2006, 9:27 AM

Ok thanks ...

For now I will just remove "p1 < spend", and see how it goes...

Thanks bye

DaWei · Oct 23rd, 2006, 9:48 AM

Maybe you didn't understand the implications of your responses, what with quite a bit of chaff being bandied about. The for loop works like this:

for (initial value, if any ; condition for repeating the loop ; action at end of loop)

because you had it out of order, thusly:

for (; *p1++; p1 < spend)

There was no initialization (intentional, I'm sure, and allowed)

The continuation condition was *p1++, which might or might not evaluate to true.

The action was no action at all, rather just an expression (p1 < spend). Evaluation of this expression had no effect on action of the code. Thus the message.

Warnings are issued when the compiler detects something that is strange enough to suggest that there is actually a mistake on the programmer's part, but there is no real violation of the rules. They are actually "thinking errors", most of the time. Such appears to be the case here. Write your for correctly, the warning will go away.

grumpy · Oct 23rd, 2006, 10:43 AM

I'll go further than Dawei. The construct "for (; *p1++; p1 < spend)" is technically valid, but is a rather atypical usage of a for loop (it is normally expected that the action at the end of the loop will have a detectable effect; your compiler is moaning because "p1 < spend" does not). If that code actually does what you intend (I sort of doubt that, but it might), you might want to restructure the code in another form (eg a do/while loop).

Just arbitrarily removing code that makes a compiler complain is not good practice. It is better to understand why the compiler is complaining -- in particular, by considering if the complete body of code is actually doing what you intend.

iradic · Oct 23rd, 2006, 1:13 PM

Code is not mine, it's from minised project...

Here is the whole function:

(Toggle Plain Text)

/* write a hex dump expansion of *p1... to fp
   p1: the source
   fp: output stream to write to */
static void listto(char *p1, FILE *fp)
{
	p1--;
	for (; *p1++; /* p1<spend */ )		/* no effect ? */
		if (isprint(*p1))
			putc(*p1, fp);		/* pass it through */
		else
		{
			putc('\\', fp);		/* emit a backslash */
			switch(*p1)
			{
			case '\b':	putc('b', fp); break;	/* BS */
			case '\t':	putc('t', fp); break;	/* TAB */
			case '\n':	putc('n', fp); break;	/* NL */
			case '\r':	putc('r', fp); break;	/* CR */
			case '\033':	putc('e', fp); break;	/* ESC */
			default:	fprintf(fp, "%02x", *p1 & 0xFF);
			}
		}
	putc('\n', fp);
}

This will give you a better idea of what's going on...

Bye

Oct 22nd, 2006, 7:10 PM	#1
iradic Programmer Join Date: Feb 2006 Posts: 36 Rep Power: 0	statement with no effect? I would like to correct the following so that gcc no more complains with "warning: statement with no effect" ... if possible: (Toggle Plain Text) static char spend; static void listto(char p1, FILE fp) { p1--; for (; p1++; p1 < spend) .... static char spend; static void listto(char p1, FILE fp) { p1--; for (; p1++; p1 < spend) .... Thanks

Oct 22nd, 2006, 7:14 PM	#2
Sane Programming Guru Join Date: Apr 2005 Posts: 1,827 Rep Power: 5	p1 is a pointer to some value. You're trying to decrement the address, instead of the value, when you go: "p1--". (Toggle Plain Text) (p1)--; (p1)--; Try that. It derefences the pointer first before decrementing the value. I'd also like to note that... - Your for loop arguments aren't in the correct order. It goes (initiation ; loop condition ; iteration). You have your iteration where the loop condition should be, and vice-versa. - You might need to bracket p1++, to make (p1)++. I believe the order of operations needs to derefence first, before incrementing. Try either way, see which one works.

Oct 22nd, 2006, 9:41 PM	#3
The Dark Expert Programmer Join Date: Jun 2005 Posts: 824 Rep Power: 4	Sane was right in his notes - the reason you are getting the error is because the for loop needs to be reversed. However, I dont think you want to be decrementing the value, rather than the pointer as it looks like this function should be using pointer arithmetic. This depends on what your function is supposed to be doing. Something like: (Toggle Plain Text) static char spend; static void listto(char p1, FILE fp) { p1--; for (; p1 < spend; p1++ ) .... } static char spend; static void listto(char p1, FILE fp) { p1--; for (; p1 < spend; p1++ ) .... }

Oct 22nd, 2006, 10:25 PM	#6
Sane Programming Guru Join Date: Apr 2005 Posts: 1,827 Rep Power: 5	Oh fuck! Hahaha. I'm awesome like that. Sorry. :p Heh. Thanks for pointing that out. In an attempt to redeem myself, I will post what I believe to be the "correct" code... haha. (Toggle Plain Text) static char spend; static void listto(char p1, FILE fp) { p1--; for (; p1 < spend; p1++) .... static char spend; static void listto(char p1, FILE fp) { p1--; for (; p1 < spend; p1++) .... Last edited by Sane; Oct 22nd, 2006 at 10:38 PM.

Oct 23rd, 2006, 9:48 AM	#8
DaWei Resident Grouch Join Date: Jun 2005 Posts: 6,453 Rep Power: 10	Maybe you didn't understand the implications of your responses, what with quite a bit of chaff being bandied about. The for loop works like this: for (initial value, if any ; condition for repeating the loop ; action at end of loop) because you had it out of order, thusly: for (; p1++; p1 < spend) There was no initialization (intentional, I'm sure, and allowed) The continuation condition was p1++, which might or might not evaluate to true. The action was no action at all, rather just an expression (p1 < spend). Evaluation of this expression had no effect on action of the code. Thus the message. Warnings are issued when the compiler detects something that is strange enough to suggest that there is actually a mistake on the programmer's part, but there is no real violation of the rules. They are actually "thinking errors", most of the time. Such appears to be the case here. Write your for correctly, the warning will go away. __________________ Abstraction doesn't make it impossible to write bad code; it makes it possible to write superior code. Contributor's Corner: Grumpy on C++ Exceptions DaWei on Pointers

Oct 22nd, 2006, 10:20 PM	#4
Sane Programming Guru Join Date: Apr 2005 Posts: 1,827 Rep Power: 5	Oh, so it could be pointer arithmetic? All right. I guess "p1++" kind of threw me off. In that case, you want "p1 < spend" to be "p1 < spend", unless spend happens to be a pointer as well, in which case that would be some pretty weird programming. o_O Then you'll also need to change "*p1++" to "p1++".

Oct 22nd, 2006, 10:24 PM	#5
The Dark Expert Programmer Join Date: Jun 2005 Posts: 824 Rep Power: 4	spend is a pointer - it is right there in the code.

Oct 23rd, 2006, 9:27 AM	#7
iradic Programmer Join Date: Feb 2006 Posts: 36 Rep Power: 0	Ok thanks ... For now I will just remove "p1 < spend", and see how it goes... Thanks bye

Oct 23rd, 2006, 10:43 AM	#9
grumpy Programming Guru Join Date: Jun 2005 Location: Adelaide, South Australia Posts: 1,206 Rep Power: 5	I'll go further than Dawei. The construct "for (; *p1++; p1 < spend)" is technically valid, but is a rather atypical usage of a for loop (it is normally expected that the action at the end of the loop will have a detectable effect; your compiler is moaning because "p1 < spend" does not). If that code actually does what you intend (I sort of doubt that, but it might), you might want to restructure the code in another form (eg a do/while loop). Just arbitrarily removing code that makes a compiler complain is not good practice. It is better to understand why the compiler is complaining -- in particular, by considering if the complete body of code is actually doing what you intend.

Oct 23rd, 2006, 1:13 PM	#10
iradic Programmer Join Date: Feb 2006 Posts: 36 Rep Power: 0	Code is not mine, it's from minised project... Here is the whole function: (Toggle Plain Text) /* write a hex dump expansion of p1... to fp p1: the source fp: output stream to write to / static void listto(char p1, FILE fp) { p1--; for (; p1++; / p1<spend / ) / no effect ? / if (isprint(p1)) putc(p1, fp); / pass it through / else { putc('\\', fp); / emit a backslash / switch(p1) { case '\b': putc('b', fp); break; /* BS / case '\t': putc('t', fp); break; / TAB / case '\n': putc('n', fp); break; / NL / case '\r': putc('r', fp); break; / CR / case '\033': putc('e', fp); break; / ESC / default: fprintf(fp, "%02x", p1 & 0xFF); } } putc('\n', fp); } /* write a hex dump expansion of p1... to fp p1: the source fp: output stream to write to / static void listto(char p1, FILE fp) { p1--; for (; p1++; / p1<spend / ) / no effect ? / if (isprint(p1)) putc(p1, fp); / pass it through / else { putc('\\', fp); / emit a backslash / switch(p1) { case '\b': putc('b', fp); break; /* BS / case '\t': putc('t', fp); break; / TAB / case '\n': putc('n', fp); break; / NL / case '\r': putc('r', fp); break; / CR / case '\033': putc('e', fp); break; / ESC / default: fprintf(fp, "%02x", p1 & 0xFF); } } putc('\n', fp); } This will give you a better idea of what's going on... Bye

Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)

Thread Tools
Show Printable Version Email this Page
Display Modes
Linear Mode Switch to Hybrid Mode Switch to Threaded Mode

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
SQL Select Statement Performance	King	Other Scripting Languages	6	Dec 1st, 2006 5:12 PM
Why did %S break my if statement?	somehollis	Python	3	Jun 22nd, 2006 8:49 PM
The Genie Effect - "Windows"	Kilo	Coder's Corner Lounge	3	Apr 4th, 2006 9:29 AM
Trouble with a swith statement	cjaime	C	10	Nov 8th, 2005 11:09 AM
Image distortion effect	djm_2k1	Java	0	Oct 11th, 2005 1:21 PM

Programming Forums > Application Development > C

statement with no effect?