Re: Can array[]=malloc()ed?

From: Eric Sosman (Eric.Sosman_at_sun.com)
Date: 10/10/03 

Date: Fri, 10 Oct 2003 12:32:52 -0400

dam_fool_2003@yahoo.com wrote:
>
> Friends,
> cannot we malloc a array?
> So, I tried the following code:
>
> int main(void)
> {
> unsigned int y[3]={1,3,6},i,j;
> for(i=0;i<3;i++)
> printf("before =%d\n",y[i]);
> *y = 7; /* 1*/
> for(j=0;j<3;j++)
> printf("after =%d\n",y[j]);
> return 0;
> }
>
> In the above I derefered *y in the line indicated by1 (correct me if I
> have used a wrong term since pointers are derefered in this way. But
> in the above I have used array but derefered as a pointer since
> an array always decays into pointers.) the array's first element value
> changes. Considering this as a kind of proof that *y pointes to the
> first element in a array I wanted to malloc the array y. I tried
> various syntax but I only get warnings.
> *y = malloc(sizeof y);

    If `y' is still declared as shown above, `*y' is an `int'.
malloc() does not return an `int' value; malloc() returns a
pointer. You cannot store a pointer value in an `int' variable.

> In spite of the warnings I have compiled the code and the code I not
> have any run time error.

    After giving you the diagnostic, the compiler apparently
tried to help you by altering your code to a correct form,
perhaps something like `*y = (int)malloc(sizeof y)'. If this
is what happened, the result was to forcibly convert the pointer
returned by malloc() into a corresponding `int' value, and store
that converted value in `*y'. The conversion might or might not
make sense; it might not even produce a legitimate `int' value;
your program might perfectly well explode at this point -- or it
might appear to do something sensible. The code as it stands is
incorrect, and there's really no telling what it might do.

> Can somebody tell me the correct syntax?

    No, because you haven't explained what you are trying to
do. The compiler (apparently) chose a correction in an attempt
to get some sense out of your code, but I am just as ignorant
as the compiler about your true intention, and my "correction"
is no more likely to be the right one than the compiler's was.
Neither of us can read your mind.

> By the malloc of above the array's first element's mem get
> allocated, Am I correct?

    I'm sorry: I cannot understand this question at all.

> What happens I we use a multi dimensional array?

    Everything depends on how you use it.

    Your understanding of arrays and of memory allocation seems
to be incomplete. I recommend that you study Sections 6 and 7
in the comp.lang.c Frequently Asked Questions (FAQ) list

        http://www.eskimo.com/~scs/C-faq/top.html

Sections 4 and 5 might also be helpful. Good luck! 

-- 
Eric.Sosman@sun.com

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