functions returning pointers

Question

I came across a program in c:

int bags[5]={20,5,20,3,20};
int main()
{
    int pos=5,*next();
    *next()=pos;   //problem with this line(should give error :lvalue required)
    printf("%d %d %d",pos,*next(),bags[0]);
    return 0; 
}
int *next()
{
    int i;
    for(i=0;i<5;i++)
        if(bags[i]==20)
            return (bags+i); 
    printf("error");
    exit(0);
}

the output to the program is 5 20 5 though i expected it to be lvalue required. can anyone tell the reason???

Doesn't compile in codepad (codepad.org/98m9rEjj). The problematic line indeed gives an error. What compiler did you use? — ugoren
– ugoren, Commented Apr 21, 2012 at 18:52
@ugoren remove two * from each sid eof the sentence, this is a formmating issue. — MByD
– MByD, Commented Apr 21, 2012 at 18:53
This code isn't legal C code. Lines 4 and 5 seem to have syntax errors. — unexpectedvalue
– unexpectedvalue, Commented Apr 21, 2012 at 18:53
@ssteinberg - line four is a valid function declaration, line 5 - see my previous comment. — MByD
– MByD, Commented Apr 21, 2012 at 18:57
this code is No problem. *(int pointer) = 5; //normal syntax — BLUEPIXY
– BLUEPIXY, Commented Apr 22, 2012 at 7:50

MByD · Accepted Answer · 2012-04-21 18:56:14Z

2

While next() is not lvalue, *next() is, this is an int, addressed by the return value if *next()(which is an int in thebags` array).

In other words, while you can't assign to the return value of a function, you can assign to the value which is addressed by the return value.

edited Apr 21, 2012 at 18:56

answered Apr 21, 2012 at 18:51

MByD

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3 Comments

Raj Over a year ago

,@ugoren -how can *next() be an lvalue?? *next() is a function call which returns the address(a constant)..and how can a constant be an lvalue

Raj Over a year ago

*next() will not be an int. it will be an address pointing towards a integer value.

MByD Over a year ago

Not exactly, *next() is an int variable pointed by the return value of next.

ugoren · Accepted Answer · 2012-04-21 18:58:55Z

2

next() is not an lvalue, because function return values are not.
*next() is an lvalue, because its a value contained at some known memory address. Whether this address was obtained as a function's return value or otherwise (e.g. it was stored in a variable), doesn't matter.

answered Apr 21, 2012 at 18:58

ugoren

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Comments

bames53 · Accepted Answer · 2012-04-21 19:03:30Z

0

The result of the dereference operator * is an lvalue.

"The unary * operator denotes indirection. [...] if it points to an object, the result is an lvalue designating the object." C99 6.5.3.2/4

answered Apr 21, 2012 at 19:03

bames53

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