checking pointer is non-null causes segmentation fault in c++

markhod · 01-11-2005, 07:28 AM

Hi,

In some c+ code I do;

if (A[0])

where A is some vector of pointers and simply doing this check causes a segmenation fault. I have no idea how to fix this (I checked the vector is within array bounds which is the only thing I could think of). Any ideas on some tests I could try?

Thankss,

Mark

bigearsbilly · 01-11-2005, 07:59 AM

How about

Code:

if ( *A  )

bigearsbilly · 01-11-2005, 08:01 AM

then again, silly me
what's the definition of A?

Is it static, are you initialising it?

need more info....

kees-jan · 01-11-2005, 08:30 AM

Basically, if a simple line like

Code:

if(A[0]))

is causing a segmentation fault, then A equals null. Try

Code:

if(A && A[0])

Groetjes,

Kees-Jan

itsme86 · 01-11-2005, 09:43 AM

A doesn't have to necessarily be NULL to crash the program. It could be pointing to any invalid memory address.

Code:

{
  int *a;

  if(a[0])
    puts("foo");
}

That could also crash, but a is just random garbage, not necessarily NULL, and your check to see if A is NULL doesn't quite cover all the bases.

bigearsbilly · 01-11-2005, 09:46 AM

exactly,

the definition at least may help.

markhod · 01-11-2005, 10:53 AM

Well the definition turns out to be not so simple.

I have a bunch of objects O and put them in a compnay-in-house container of O's (lets call it Cont).

In-House Cont;

for (int i = 0; i< some_value; i++){
Cont[i] = new O();
}

or (int i = 0; i< some_value; i++){
if (Cont[i]) //crash here
}

If I do Cont[i] I get returned a pointer to the ith O. It seems to me I know that new object exists so when I do Cont[i] I should get returned a valid pointer? Obviously I dont and since I see no reason for this I am stuck...

Cheers,

Mark

markhod · 01-11-2005, 10:57 AM

I am confused by some responses. A null pointer should not cause a crash just because I ask if it is null, surely? If it is null and I try to use the pointer to access a member function then I expect a crash, but not just from saying if(ptr). and theen only using it if it is non-null.

bigearsbilly · 01-11-2005, 11:08 AM

yeh but
Cont[i] is referring to something called Cont
namely *(Cont +i)
Which if Cont is invalid, i.e. the container itself
then it will sigsev.
Or you are going over the end of the Cont container.

If in_house is just an array of pointers why use it?
why not use just an array of type O?
does the container have an initial size or is it smart enough (or not)
to resize?

markhod · 01-11-2005, 11:13 AM

>yeh but
>Cont[i] is referring to something called Cont
>namely *(Cont +i)
>Which if Cont is invalid, i.e. the container itself
>then it will sigsev.
>Or you are going over the end of the Cont container.

No, I checked the size of the container is 1 and the crash occurs when accessing the oth element...

>If in_house is just an array of pointers why use it?

because of rules governing how we persist data to the database (which has to have standard things for people to find when they analyse the data at a later date)

>why not use just an array of type O?
>does the container have an initial size or is it smart enough (or not) to resize?

it has no initial size. Its like a std::vector in that it resizes itself

Hko · 01-11-2005, 11:28 AM

Quote:

Originally posted by markhod
I am confused by some responses. A null pointer should not cause a crash just because I ask if it is null, surely?

True, but when you do "if (A[0]) ..." or "if (*A) ...", then you're already dereferencing the pointer A. In other words, such an if-statement is not checking if the pointer is null, but it's checking if the memory pointed to by A is null/0. If A itself is null, or if it points to an invalid address the program crashes because it reads from and memory address it may not have access to.

To test if pointer A is null, you could do:

Code:

if (A == NULL) ...
/* or: */
if (A) ....

Still, like itsme86 said, if it's not null, it may still contain an invalid address resulting in a segmentation fault, e.g. when a pointer is not initialized. This is probably what happens in your case:

Code:

for (int i = 0; i< some_value; i++){
    Cont[i] = new O();
}

for (int i = 0; i< some_value; i++){
    // if "some_value" is higher than "some_value" in
    // the loop above,...
    if (Cont[i]) // ...it will probably crash here when i
              //  exceeds the "some_value" of the first loop.
}