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Operating Systems Linux Mismatch free() / delete / delete [C++] Post 302985019 by Corona688 on Thursday 3rd of November 2016 06:03:06 PM
Old 11-03-2016
Quote:
Originally Posted by SA_Palani
You are right that send is to alter the value of Struct p1.
Changing its contents won't cause a crash.

You actually can't typecast things in the way you show in your example, which leaves me more and more curious to what you're actually doing.

That it's passed as a * & gives it the ability to reach in and mangle p1 itself - the pointer.

What I mean is this:

Code:
#include <iostream>
using namespace std;

void mangle_pointer(char * & x) {
        // Set the pointer to a blatantly wrong value
        x = (char *)123456789;
}

int main(void) {
        char *mem=new char;

        cerr << "mem is " << (unsigned long)mem << endl;

        mangle_pointer(mem);

        cerr << "mem is now "<< (unsigned long)mem <<
          " and will crash when deleted" << endl;

        delete mem; // Deleting any value that you didn't get from new is bad
}

This is a trivial example. Tracking down memory crashes is usually a lot harder, the crash is generally a side-effect of an overrun which stomped on the values of your variables or some important hidden values inside the heap itself. An overrun can leap straight through rational boundaries and stomp on whatever happens to be next to it in memory, with no regard for the line order of your program. If the overrun happened to something in a local variable, this can corrupt other local variables, or corrupt your return vector and cause a crash next time you return from any function. If it happened in the heap, it can corrupt other things you have stored in the heap, or corrupt the heap itself, causing delete to mess up in strange ways the next time you delete anything.

This is to say, "delete myvariable" crashing often has nothing to do with "myvariable". Debris can land far away from an explosion.

Last edited by Corona688; 11-03-2016 at 07:12 PM..
 

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delete(n)                                                           [incr Tcl]                                                           delete(n)

__________________________________________________________________________________________________________________________________________________

NAME
delete - delete things in the interpreter SYNOPSIS
itcl::delete option ?arg arg ...? _________________________________________________________________ DESCRIPTION
The delete command is used to delete things in the interpreter. It is implemented as an ensemble, so extensions can add their own options and extend the behavior of this command. By default, the delete command handles the destruction of namespaces. The option argument determines what action is carried out by the command. The legal options (which may be abbreviated) are: delete class name ?name...? Deletes one or more [incr Tcl] classes called name. This deletes all objects in the class, and all derived classes as well. If an error is encountered while destructing an object, it will prevent the destruction of the class and any remaining objects. To destroy the entire class without regard for errors, use the "delete namespace" command. delete object name ?name...? Deletes one or more [incr Tcl] objects called name. An object is deleted by invoking all destructors in its class hierarchy, in order from most- to least-specific. If all destructors are successful, data associated with the object is deleted and the name is removed as a command from the interpreter. If the access command for an object resides in another namespace, then its qualified name can be used: itcl::delete object foo::bar::x If an error is encountered while destructing an object, the delete command is aborted and the object remains alive. To destroy an object without regard for errors, use the "rename" command to destroy the object access command. delete namespace name ?name...? Deletes one or more namespaces called name. This deletes all commands and variables in the namespace, and deletes all child names- paces as well. When a namespace is deleted, it is automatically removed from the import lists of all other namespaces. KEYWORDS
namespace, proc, variable, ensemble itcl 3.0 delete(n)
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