difference in implementation between operator new and new []


C

C++Liliput

Hi,
I was looking at the implementation of operator new and operator
new[] in gcc source code and found that the implementation is exactly
the same. The only difference is that the size_t argument passed to
the operators is calculated correctly during runtime and passed in.
Inside the implementation both operators (new and new[]) do a simple
malloc(). Ditto for operator delete/delete[]. I have two questions:
1) Who passes in the size_t argument at runtime?
2) If the operator implementations are virtually the same (at least
for gcc), can we assume that mismatched new[]/delete and new/delete[]
calls will not result in any problem? I ask this because I was doing
an exercise of finding memory leaks inside some C++ code. When I
executed the code with Valgrind, I got a bunch of mismatched new[]/
delete errors. However there were no reported memory leaks as such.
However I assumed that doing a new[] and then a delete would most
probably leak memory because delete would probably delete only the
first object of the array (since the implementation of operator delete
will not know about the array size). So I wrote a small C++ program
where I do a lot of new[]'s and delete's on my Linux box and then
monitored the memory footprint using the top command. However contrary
to my expectations, there were no memory leaks. That is when I started
looking into the implementation of operator new/new[] and operator
delete/delete[] in gcc source code and found out that there is no
difference in the internal implementation of the scalar and the
corresponding vector operator.
 
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T

tony_in_da_uk

Hi,
I was looking at the implementation of operator new and operator
new[] in gcc source code and found that the implementation is exactly
the same. The only difference is that the size_t argument passed to
the operators is calculated correctly during runtime and passed in.
Inside the implementation both operators (new and new[]) do a simple
malloc(). Ditto for operator delete/delete[]. I have two questions:
1) Who passes in the size_t argument at runtime?
2) If the operator implementations are virtually the same (at least
for gcc), can we assume that mismatched new[]/delete and new/delete[]
calls will not result in any problem? I ask this because I was doing
an exercise of finding memory leaks inside some C++ code. When I
executed the code with Valgrind, I got a bunch of mismatched new[]/
delete errors. However there were no reported memory leaks as such.
However I assumed that doing a new[] and then a delete would most
probably leak memory because delete would probably delete only the
first object of the array (since the implementation of operator delete
will not know about the array size). So I wrote a small C++ program
where I do a lot of new[]'s and delete's on my Linux box and then
monitored the memory footprint using the top command. However contrary
to my expectations, there were no memory leaks. That is when I started
looking into the implementation of operator new/new[] and operator
delete/delete[] in gcc source code and found out that there is no
difference in the internal implementation of the scalar and the
corresponding vector operator.

If new/delete and new[]/delete[] happen to be implemented on top of
malloc()/free() in your implementation, and given that new[] must
allocate contiguous memory in one block and that free() doesn't
require the size_t as a parameter (the memory subsystem remembers the
size anyway), this all hangs together. That's why it - observably -
"works". What doesn't hang together is the construction and
destruction of your elements: new[] and delete[] ensure each
individual object has it's destructor or destructor called
(respectively). Not using the correct operation doesn't give the
compiler the opportunity to ensure this is done correctly. Even if
you're working with POD data that ostensibly needs no construction/
destruction, what you're doing is undefined by the Standard, and even
a future revision of your compiler (let alone a different compiler/
machine/OS) might not layer over malloc()/free() so transparently.
You're asking for a crash.

Cheers, Tony
 
K

Kai-Uwe Bux

C++Liliput said:
Hi,
I was looking at the implementation of operator new and operator
new[] in gcc source code and found that the implementation is exactly
the same. The only difference is that the size_t argument passed to
the operators is calculated correctly during runtime and passed in.
Inside the implementation both operators (new and new[]) do a simple
malloc(). Ditto for operator delete/delete[]. I have two questions:
1) Who passes in the size_t argument at runtime?

It's computed magically by the implementation. See the example [5.3.4/12]:

? new T results in a call of operator new(sizeof(T)),
? ...
? new T[5] results in a call of operator new[](sizeof(T)*5+x), and
? ...

2) If the operator implementations are virtually the same (at least
for gcc), can we assume that mismatched new[]/delete and new/delete[]
calls will not result in any problem?

a) You are relying on the behavior of a particular implementation and
b) NO, not even with g++. Compile with g++ and run under valgrind:

struct X {

char * buffer;

X ( void ) {
buffer = new char;
}

~X ( void ) {
delete buffer;
}

};

int main ( void ) {
X* xp = new X [16];
delete xp; // mismatch !
}


Best

Kai-Uwe Bux
 
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K

korben dallas

C++Liliput said:
I was looking at the implementation of operator new and operator
new[] in gcc source code and found that the implementation is exactly
the same. The only difference is that the size_t argument passed to
the operators is calculated correctly during runtime and passed in.
Inside the implementation both operators (new and new[]) do a simple
malloc(). Ditto for operator delete/delete[].
OK.

I have two questions:
1) Who passes in the size_t argument at runtime?

When the compiler generates the code for new-expression ('new' or
'new[]' form), it also generates the code that will calculate the
run-time size and pass it to 'operator new'/'operator new[]' functions.

I hope you know that new-expression that you use in your code is not
even remotely the same as 'operator new' function. The call to the
'operator new' function is nothing but just a small part of what
new-expression actually does. The same applies to delete-expression and
'operator delete' function.
2) If the operator implementations are virtually the same (at least
for gcc), can we assume that mismatched new[]/delete and new/delete[]
calls will not result in any problem?

No. Why? In this question you must be referring to mismatched
new-expression and delete-expression formats. Once again, the actual
call to the appropriate 'operator new' function is just a small part of
what new-expression translates to. The same applies to
delete-expression. If you really want to know whether they are
"compatible" in some particular implementation, you need to compare the
entire code that is generated for these expressions, not just the code
for 'operator new'/'operator delete'. The code for the expression is
normally generated intrinsically, i.e. you won't find any "source code"
for it in the library. You'll have analyze the assembly code, and do it
for every possible combination of types, compiler settings, etc. This is
a useless waste of time, if you ask me.
I ask this because I was doing
an exercise of finding memory leaks inside some C++ code. When I
executed the code with Valgrind, I got a bunch of mismatched new[]/
delete errors. However there were no reported memory leaks as such.

Memory leaks? What made you to conclude that there must be memory leaks
specifically? Mismatched new/delete result in undefined behavior.
There's no way to predict how it will manifest itself. The belief in
that it should specifically cause "memory leaks" is an urban legend. I
don't know where it comes from.
 

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