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Re: extern array as a pointer

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Frank Tetzel <s1445051@mail.zih.tu-dresden.de> writes:

>> > why does the following program crash at line# 20.
>> > Is it even legal to define array as an integer
>> > pointer while infact it's original definition is
>> > single dimensional array or five elements.
>> >
>> >  1 #include "stdio.h"
>> >   2
>> >   3 int foo();
>> >   4
>> >   5 int array[5];
>> >   6
>> >   7 int main()
>> >   8 {
>> >   9    foo();
>> >  10    printf("array[0] is %u", array[0]);
>> >  11    return(0);
>> >  12 }
>> >  13
>> >  14 foo.c:
>> >  15
>> >  16 extern int *array;
>> >  17
>> >  18 int foo()
>> >  19 {
>> >  20   arr[0]= 10;  
>> 
>> Did you mean array[0] here?
>> 
>> >  21   return(0);
>> >  22 }
>> >
>> >  
>> 
>> You have a normal array in one module which is defined as a pointer
>> to array in another, assuming arr was meant to be array. Try writing:
>> 
>> extern int array[];
>> 
>> in foo.c
>
> But why? Why is "extern int *array" containing the first element which
> is 0? Why doesn't the compiler or rather the linker warn about this? Is
> the type information lost for the linker?

Yes, it's sadly not common for this sort of thing to be checked at link
time.

> In the past i used *array and array[] interchangeable. I might have to
> reconsider that.

They are interchangeable only when they appear as formal parameters in a
function declaration or definition and that's only because C can't pass
"naked" array values so the [] form is redundant.  It was (probably
unfortunately) appropriated to mean the same thing in this context.

In other contexts, a pointer declaration and an array declaration are
very different.  Think of what code the compiler must generate for

  extern int *a;
  ...
  a[0]  // a names a pointer-sized object whose content must loaded
        // and used in a indirect memory access.

and

  extern int a[10];
  ...
  a[0]  // a names a 10-int sized object.  It is the address of that
        // object that must be used.

But there are all sorts of other ways to see why this distinction
matters.  For example, consider what &a means in each case.


-- 
Ben.

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extern array as a pointer sinbad <sinbad.sinbad@gmail.com> - 2016-07-12 03:06 -0700
  Re: extern array as a pointer BartC <bc@freeuk.com> - 2016-07-12 11:45 +0100
    Re: extern array as a pointer Frank Tetzel <s1445051@mail.zih.tu-dresden.de> - 2016-07-12 13:58 +0200
      Re: extern array as a pointer Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-07-12 13:33 +0100
      Re: extern array as a pointer Keith Thompson <kst-u@mib.org> - 2016-07-12 09:30 -0700
        Re: extern array as a pointer James Kuyper <jameskuyper@verizon.net> - 2016-07-12 12:52 -0400
          Re: extern array as a pointer Keith Thompson <kst-u@mib.org> - 2016-07-12 11:30 -0700
            Re: extern array as a pointer supercat@casperkitty.com - 2016-07-12 11:51 -0700
        Re: extern array as a pointer supercat@casperkitty.com - 2016-07-12 10:55 -0700
    Re: extern array as a pointer Keith Thompson <kst-u@mib.org> - 2016-07-12 09:28 -0700
  Re: extern array as a pointer Nobody <nobody@nowhere.invalid> - 2016-07-12 13:26 +0100
    Re: extern array as a pointer Keith Thompson <kst-u@mib.org> - 2016-07-12 09:26 -0700
  Re: extern array as a pointer Richard Heathfield <rjh@cpax.org.uk> - 2016-07-12 13:46 +0100
  Re: extern array as a pointer James Kuyper <jameskuyper@verizon.net> - 2016-07-12 12:33 -0400

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