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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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SHELL-QUOTE(1)						User Contributed Perl Documentation					    SHELL-QUOTE(1)

NAME
shell-quote - quote arguments for safe use, unmodified in a shell command SYNOPSIS
shell-quote [switch]... arg... DESCRIPTION
shell-quote lets you pass arbitrary strings through the shell so that they won't be changed by the shell. This lets you process commands or files with embedded white space or shell globbing characters safely. Here are a few examples. EXAMPLES
ssh preserving args When running a remote command with ssh, ssh doesn't preserve the separate arguments it receives. It just joins them with spaces and passes them to "$SHELL -c". This doesn't work as intended: ssh host touch 'hi there' # fails It creates 2 files, hi and there. Instead, do this: cmd=`shell-quote touch 'hi there'` ssh host "$cmd" This gives you just 1 file, hi there. process find output It's not ordinarily possible to process an arbitrary list of files output by find with a shell script. Anything you put in $IFS to split up the output could legitimately be in a file's name. Here's how you can do it using shell-quote: eval set -- `find -type f -print0 | xargs -0 shell-quote --` debug shell scripts shell-quote is better than echo for debugging shell scripts. debug() { [ -z "$debug" ] || shell-quote "debug:" "$@" } With echo you can't tell the difference between "debug 'foo bar'" and "debug foo bar", but with shell-quote you can. save a command for later shell-quote can be used to build up a shell command to run later. Say you want the user to be able to give you switches for a command you're going to run. If you don't want the switches to be re-evaluated by the shell (which is usually a good idea, else there are things the user can't pass through), you can do something like this: user_switches= while [ $# != 0 ] do case x$1 in x--pass-through) [ $# -gt 1 ] || die "need an argument for $1" user_switches="$user_switches "`shell-quote -- "$2"` shift;; # process other switches esac shift done # later eval "shell-quote some-command $user_switches my args" OPTIONS
--debug Turn debugging on. --help Show the usage message and die. --version Show the version number and exit. AVAILABILITY
The code is licensed under the GNU GPL. Check http://www.argon.org/~roderick/ or CPAN for updated versions. AUTHOR
Roderick Schertler <roderick@argon.org> perl v5.16.3 2010-06-11 SHELL-QUOTE(1)
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