ian bell

I am passing the address of an int into a function. I want to increment the
int. Code snippet

void foo(int * pc)
{
*pc++;
}

pc does not get incremented. However if I do this:

foor(int * pc)
{
int temp;

temp = *pc;
temp++
*pc = temp;
}

then pc is incremented.

Is this a funny precedence thing with *?

Ian
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jack klein

On 25 Nov 2003 04:37:48 GMT, Ian Bell

C doesn't actually have precedence, but it does have a parse sequence
defined by the grammar, and yes this is the reason.

The expression *p++, where p is a pointer to come complete type,
evaluates the value of what p is currently point to, then increments
the pointer p, not the pointed to object.

Replace this with (*p)++.

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dave neary

On 25 Nov 2003 04:37:48 GMT, Ian Bell said:

This increments the pointer pc, but evaluates to the value pointer to by
the old value (++ binds tighter than *).
(*pc)++;
will work.

Apart from the fact that it's not that funny, yes

Cheers,
Dave.

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andy sinclair

It is. '++' has a very high precedence.
In fact ++ and * (indirection) have the same precedence but right to
left associativity. Therefore the post increment gets applied to the
pointer rather than the location and the statement has no effect.

See K&R2 p53 or http://www.difranco.net/cop2220/op-prec.htm for an
operator precendence table.

Changing to one of the following gives the expected results.
(*pc)++;
*pc+=1;

Regards
Andy
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paolo montegriffo

void foo(int *pc)
{
(*pc)++ ;
}

Unary operators like * and ++ are evalued from right to left, i.e. postfix
operator ++ has the precedence over the prefix unary operator *.

see K&R Sec 5.1 or look at C F.A.Q. - Question 4.3

Cheers
Paolo
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hans-bernhard broeker

Yes. I'll leave it as an exercise to you to find out how to fix up the
precedence. ;->

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francis glassborow

In article <clcm-20031124-0014@plethora.net>, Ian Bell
<ian@ruffrecordsDOTworldonline.co.uk> writes


Depends on your definition of 'funny'. Did you try:

void foo(int *pc){
(*pc) ++;
}


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lndresnick

unary * and ++ have the same precedence, however unary operators
associate
from right to left. You are thus incrementing the pointer in foo.
You want (*pc)++, or ++*pc, or *pc += 1.

-David
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mark gordon

On 25 Nov 2003 04:37:48 GMT

It is a standard precedence and is generally useful since it is more
common for you to want to increment a pointer than the value it points
to.

To achieve what you want without a temporary use parenthesis. E.g.
(*pc)++
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dan.pop

In <clcm-20031124-0014@plethora.net> Ian Bell <ian@ruffrecordsDOTworldonline.co.uk> writes:

This is not the same thing as passing by reference (which is not supported
by C at all).

(*pc)++;

would be enough.

What does your C book have to say on the topic?

Dan
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