How to test memory allocation with new ?

[jjleto]

Hello,

How do I handle the following :

------>
#include <iostream>
using namespace std;

int main()
{
int n = 0x7FFFFFFF;
char *pp = new char(n);
if ( pp != NULL ) {
pp[0] = 0;
pp[n-1] = 0;
cout << "OK" << endl;
} else {
cout << "FAILED" << endl;
}
}
------>


It compiles OK, but when I run it, I get a segmentation fault. I thought
that testing a NULL value was enough for testing memory allocation (or
perhaps it is a bug ? I use gcc (GCC) 3.3.4)

Regards,
jjleto

 

[Sharad Kala]

Hello,

How do I handle the following :

------>
#include <iostream>
using namespace std;

int main()
{
int n = 0x7FFFFFFF;
char *pp = new char(n);

This does not do what you are thinking. It allocates memory for one char and
tries to initialize it with 0x7FFFFFFF. To allocate an array use -
char *pp = new char[...];

if ( pp != NULL ) {

This is a big misconception that new returns NULL on failure. The fact is
that it throws std::bad_alloc exception on failure (unless you specify
nothrow)

pp[0] = 0;
pp[n-1] = 0;

This is the cause of your segmentation fault. You are trying to write to an
illegal memory location.

cout << "OK" << endl;
} else {
cout << "FAILED" << endl;
}
}

Sharad

 

[jjleto]

char *pp = new char[...];

Thanks, the segmentation fault was actually for the reason you gave.

Nevertheless, I still have an error when running this program ("give up"
but not a segfault)

#include <iostream>
using namespace std;

int main()
{
int n = 0x7FFFFFFF;
char *pp = new char[n];
if ( pp != NULL ) {
pp[0] = 0;
pp[n-1] = 0;
cout << "OK" << endl;
} else {
cout << "FAILED" << endl;
}
}

How do I test memory allocation with new ?

 

[Sharad Kala]

char *pp = new char[...];

Thanks, the segmentation fault was actually for the reason you gave.

Nevertheless, I still have an error when running this program ("give up"
but not a segfault)

How do I test memory allocation with new ?

I told you in my earlier reply too that new throws std::bad_alloc on
exception. You need to wrap the new in a try-catch block (catching
std::bad_alloc).

Sharad

 

[Sharad Kala]

char *pp = new char[...];

Thanks, the segmentation fault was actually for the reason you gave.

Nevertheless, I still have an error when running this program ("give up"
but not a segfault)

How do I test memory allocation with new ?

I told you in my earlier reply too that new throws std::bad_alloc on
exception. You need to wrap the new in a try-catch block (catching
std::bad_alloc).

Something like -
try{
char *pp = new char[n];
//...
}
catch(std::bad_alloc)
{
cout << "New failed";
}

Sharad

 

[Shailesh Humbad]

char *pp = new char[...];

Thanks, the segmentation fault was actually for the reason you gave.

Nevertheless, I still have an error when running this program ("give up"
but not a segfault)

How do I test memory allocation with new ?

I told you in my earlier reply too that new throws std::bad_alloc on
exception. You need to wrap the new in a try-catch block (catching
std::bad_alloc).

Something like -
try{
char *pp = new char[n];
//...
}
catch(std::bad_alloc)
{
cout << "New failed";
}

Sharad

Note for those using MS VC++ (5.0 or higher), it has a bug in the
implementation of new where it doesn't throw the std::bad_alloc
exception on failure. I just found this out when I tried your example.

http://support.microsoft.com/?kbid=167733

 

[Sharad Kala]

Something like -
try{
char *pp = new char[n];
//...
}
catch(std::bad_alloc)
{
cout << "New failed";
}

Sharad

Note for those using MS VC++ (5.0 or higher), it has a bug in the
implementation of new where it doesn't throw the std::bad_alloc
exception on failure. I just found this out when I tried your example.

Well there was no Standard formally in place in the time of MS VC 6.0
(that's 1997) and its ancestors. So one can't actually blame the compilers
for being non-conformant.

Sharad

 

[Cosmin Calinescu]

If you have less then 2G of memory you're scrued. I don't think you do
what you intend to do.

 

[Ulrich Lauther]

: > I told you in my earlier reply too that new throws std::bad_alloc on
: > exception. You need to wrap the new in a try-catch block (catching
: > std::bad_alloc).

: Something like -
: try{
: char *pp = new char[n];
: //...
: }
: catch(std::bad_alloc)
: {
: cout << "New failed";
: }

: Sharad


or else:

char* pp = new(nothrow) char[n];
if (! pp) { // new failed
// do something
}

 

[Larry I Smith]

Hello,

How do I handle the following :

------>
#include <iostream>
using namespace std;

int main()
{
int n = 0x7FFFFFFF;
char *pp = new char(n);
if ( pp != NULL ) {
pp[0] = 0;
pp[n-1] = 0;
cout << "OK" << endl;
} else {
cout << "FAILED" << endl;
}
}
------>


It compiles OK, but when I run it, I get a segmentation fault. I thought
that testing a NULL value was enough for testing memory allocation (or
perhaps it is a bug ? I use gcc (GCC) 3.3.4)

Regards,
jjleto

#include <iostream>
using namespace std;

int main()
{
char * pp;

// given this value, 'new' will fail if less than 2GB of free RAM
// is available
int n = 0x7FFFFFFF;

try
{
cout << "Allocate an array of " << n << " bytes in RAM" << endl;
pp = new char[n];
}
catch (bad_alloc)
{
cout << "Allocation Attempt Failed" << endl;
return 1;
}

pp[0] = 0;
pp[n - 1] = 0;
cout << "OK" << endl;

delete[] pp; // free the allocated array

return 0;
}

Regards,
Larry

 

[David Lindauer]

what is the syntax for specifying nothrow?

Thanks,

David

Hello,

How do I handle the following :

------>
#include <iostream>
using namespace std;

int main()
{
int n = 0x7FFFFFFF;
char *pp = new char(n);

This does not do what you are thinking. It allocates memory for one char and
tries to initialize it with 0x7FFFFFFF. To allocate an array use -
char *pp = new char[...];

if ( pp != NULL ) {

This is a big misconception that new returns NULL on failure. The fact is
that it throws std::bad_alloc exception on failure (unless you specify
nothrow)

pp[0] = 0;
pp[n-1] = 0;

This is the cause of your segmentation fault. You are trying to write to an
illegal memory location.

cout << "OK" << endl;
} else {
cout << "FAILED" << endl;
}
}

Sharad

 

[Old Wolf]

If you have less then 2G of memory you're scrued. I don't think you do
what you intend to do.

int n = 0x7FFFFFFF;
char *pp = new char[n];

Many operating systems have "lazy allocation", they will honour
this request but then swap maniacally or kill the application
when it tries to actually use that memory.

 

[Method Man]

Hello,

How do I handle the following :

------>
#include <iostream>
using namespace std;

int main()
{
int n = 0x7FFFFFFF;
char *pp = new char(n);

This does not do what you are thinking. It allocates memory for one char and
tries to initialize it with 0x7FFFFFFF. To allocate an array use -
char *pp = new char[...];

if ( pp != NULL ) {

This is a big misconception that new returns NULL on failure. The fact is
that it throws std::bad_alloc exception on failure (unless you specify
nothrow)

pp[0] = 0;
pp[n-1] = 0;

This is the cause of your segmentation fault. You are trying to write to an
illegal memory location.

Do you mean both statements are problematic? Shouldn't 'pp[0] = 0' be OK
since it is equivalent to *pp = 0 (assigning the allocated char the null
value)?

 

[Sharad Kala]

Do you mean both statements are problematic? Shouldn't 'pp[0] = 0' be OK
since it is equivalent to *pp = 0 (assigning the allocated char the null
value)?

No, only pp[1] is problematic. This gets evaluated to *(pp + 1), pp [0] as
you point out is same as *pp.

Sharad