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Full Discussion: Mismatch free() / delete / delete [C++]
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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LEARN ABOUT DEBIAN

std::messages_byname

std::messages_byname< _CharT >(3cxx)									      std::messages_byname< _CharT >(3cxx)

NAME

       std::messages_byname< _CharT > -

SYNOPSIS

       Inherits std::messages< _CharT >.

   Public Types
       typedef int catalog
       typedef _CharT char_type
       typedef basic_string< _CharT > string_type

   Public Member Functions
       messages_byname (const char *__s, size_t __refs=0)
       void close (catalog __c) const
       string_type get (catalog __c, int __set, int __msgid, const string_type &__s) const
       catalog open (const basic_string< char > &__s, const locale &__loc) const
       catalog open (const basic_string< char > &, const locale &, const char *) const

   Static Public Attributes
       static locale::id id

   Protected Member Functions
       string_type _M_convert_from_char (char *) const
       char * _M_convert_to_char (const string_type &__msg) const
       virtual void do_close (catalog) const
       virtual string_type do_get (catalog, int, int, const string_type &__dfault) const
       template<> string do_get (catalog, int, int, const string &) const
       template<> wstring do_get (catalog, int, int, const wstring &) const
       virtual catalog do_open (const basic_string< char > &, const locale &) const

   Static Protected Member Functions
       static __c_locale _S_clone_c_locale (__c_locale &__cloc)  throw ()
       static void _S_create_c_locale (__c_locale &__cloc, const char *__s, __c_locale __old=0)
       static void _S_destroy_c_locale (__c_locale &__cloc)
       static __c_locale _S_get_c_locale ()
       static const char * _S_get_c_name ()  throw ()
       static __c_locale _S_lc_ctype_c_locale (__c_locale __cloc, const char *__s)

   Protected Attributes
       __c_locale _M_c_locale_messages
       const char * _M_name_messages

Detailed Description
   template<typename _CharT>class std::messages_byname< _CharT >
       class messages_byname [22.2.7.2].

       Definition at line 1912 of file locale_facets_nonio.h.

Member Data Documentation
   template<typename _CharT > locale::id std::messages< _CharT >::id [static],	[inherited]
       Numpunct facet id.

       Definition at line 1713 of file locale_facets_nonio.h.

Author
       Generated automatically by Doxygen for libstdc++ from the source code.

libstdc++							  Tue Nov 27 2012			      std::messages_byname< _CharT >(3cxx)
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