access violation in int array

[=?Utf-8?B?R2Vvcmdl?=]

Hello everyone,
There is error message when executing my program,

Unhandled exception at 0x00411a49 in test_entern.exe : 0xC0000005: Access
violation reading location 0x00000002.

It is very simple, does anyone know what is wrong with the program?

I have tested that when changing from extern int* p_int to extern int
p_int[16], my program is ok. But I think the two statements should be the
same, right?

foo.c

int p_int [16] = {2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17};
 

goo.c

extern int* p_int;
 
int main (int argc, char** argv)
{
int i;
int sum = 0;
for (i = 0; i < 16; i++)
{
sum += p_int [i]; // access violation
}
 
return 0;
}
 

thanks in advance,
George

[=?Utf-8?B?R2Vvcmdl?=]

Thanks Bo Persson!
Your description is very clear. Have a good weekend!
regards,
George

"Bo Persson" wrote:

George wrote:
:: Hi Bo Persson,
::
::
:: I think the root cause is, compiler thinks the value of extern
:: int* p_int is the 1st element of the array -- i.e. the value (real
:: value) of the 1st element of the array is treated as the the
:: address information,

Yes, because your extern declaration says that it is. :-)

:: then when using p_int [i] in goo.c, it
:: de-referencing the value of the 1st element as the address?

int* p_int;

says that there is only one element, and that it is a pointer.

::
:: For example, the array is,
::
:: int p_int [16] = {2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15,
:: 16, 17};
::
:: p_int[0] in goo.c will make 0x00000002 as the value of pointer
:: p_int, then de-referencing the address of 0x00000002 -- which will
:: cause access violation exception?
::
:: Is my understanding correct?

Yes.

An extern declaration cannot "make" something into what it is not. It
is just telling the compiler how things really are, somewhere else.

It is very important that what you tell the compiler is really true!
One way to improve on this, is to have the declaration in a .h file
and include it in both .cpp files. That way the compiler can often
tell if it is not consistent.

Bo Persson