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Full Discussion: C++ program is crashing on re-assigning const static member variable using an int pointer
Top Forums Programming C++ program is crashing on re-assigning const static member variable using an int pointer Post 302490802 by DreamWarrior on Tuesday 25th of January 2011 05:16:10 PM
Old 01-25-2011
Code:
#include <stdio.h>

static char			 _g_testChar[10] = "-GLOBAL";
static int			 _g_testInt = 1;

static const char	*_g_testConstChar = "-CONSTGLOBAL";
static const int	 _g_testConstInt = 2;

void main(int argc, char **argv)
{
	char				*testConstChar = "-CONSTCHAR";
	char				 testChar[10] = "-STACKCHAR";
	
	const int			 testConstInt = 3;
	int			 		 testInt = 4;
	
	char				*cPtr;
	int					*iPtr;
	
	cPtr = (argc > 1) ?
			( (argv[1][0] == 'c') ? testConstChar :
			  (argv[1][0] == 'C') ? _g_testConstChar : 
			  (argv[1][0] == 'g') ? _g_testChar : testChar )
			: testChar;
			
	iPtr = (argc > 2) ?
			( (argv[2][0] == 'c') ? &testConstInt :
			  (argv[2][0] == 'C') ? &_g_testConstInt :
			  (argv[2][0] == 'g') ? &_g_testInt : &testInt )
			: &testInt;
	
	printf("               Char            Int\n"
		   "Constant:      %-15p %p\n"
		   "Stack:         %-15p %p\n"
		   "Const Global:  %-15p %p\n"
		   "Global:        %-15p %p\n",
		   testConstChar, &testConstInt,
		   testChar, &testInt,
		   _g_testConstChar, &_g_testConstInt,
		   _g_testChar, &_g_testInt);
	
	printf("Trying char [%s]...\n", cPtr);
	*cPtr = '!';
	
	printf("Trying int [%d]...\n", *iPtr);
	*iPtr = 0;
	
	printf("%s\n"
		   "%d\n",
		   cPtr, *iPtr);
}

Play with this...it'll show you what your compiler "protects" and what it doesn't. Then look closely at the address table printed and look for the address space of your OS (for Linux it's here). What you'll probably see is that the variables you aren't allowed to change get put into the text segment, which can't be modified during runtime (easily).

On an aside, I have history with this because HP-UX was much more relaxed in this area. When we ported our HP-UX code to Linux we found out the hard way because our programs started crashing. It seemed innocent enough to us to have the essence of this:

Code:
void do_something(char *d)
{
  char *p;

  p = strtok(p, ":");
}

main()
{
  char *data = "this:is:some:data";

  do_something(data);
}

in the code.... Of course, since "this:is:some:data" is stored in the text segment, Linux crashed with the, not so helpful in this case, "segment violation". It dawned on us that the pointer was always in the 0x08 segment when it crashed, then we looked at the Linux address space and found that we were being bit by the compiler putting the string data in the text segment...always. Even worse, for strings it doesn't matter to the compiler whether the data is const or not. For integers, it seems to use const as a hint to go in the text segment if it's global, however stack constants aren't protected (and I suppose really can't be).

As far as Linux is concerned, it seems any quoted string in a source file is immutable and any static const is immutable. Any non-static const is only immutable if you follow the compiler's warnings if you're assigning a const variable's address to a non-const pointer variable.
Last edited by DreamWarrior; 01-25-2011 at 06:23 PM..
 

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LEARN ABOUT REDHAT

mlm

FBB::mlm(3bobcat)                                             OFoldStream manipulator                                            FBB::mlm(3bobcat)

NAME

       FBB::mlm - Manipulator modifying left margins of OFoldStream objects

SYNOPSIS

       #include <bobcat/ofoldstream> or
       #include <bobcat/ofoldstreambuf>
       Linking option: -lbobcat

DESCRIPTION

       The  mlm  class  implements  a  manipulator that can be inserted into OFoldStream objects to modify the stream's left margin by a requested
       amount. The request cannot result in a negative left margin value. If a negative left margin would be the arithmetic result of the  request
       then left margin 0 will silently be used.

       Depending  on  the  tab-setting  of  the  OFoldStream  the  inserted value represents the number of blank space characters or the number of
       tab-characters that will be added to the left margin.

       The request will be processed at the next newline character or std::flush or std::endl manipulator that is inserted into the stream.  If  a
       line  is  still empty once an mlm object and a flush manipulator are inserted into the stream then the new left margin will be effective at
       the next word inserted into that line (cf., the example section below)

       A bad_cast exception is thrown when the manipulator is inserted into an ostream that is not using a OFoldStreambuf buffer.

NAMESPACE

       FBB
       All constructors, members, operators and manipulators, mentioned in this man-page, are defined in the namespace FBB.

INHERITS FROM

       -

CONSTRUCTOR

       o      mlm(int addValue):
              The standard copy constructor is available.

MEMBER FUNCTIONS

       There are no public or protected member functions in this class.

EXAMPLE

           #include <iostream>
           #include <bobcat/ofoldstream>

           using namespace std;
           using namespace FBB;

           int main()
           {
               OFoldStream out(cout, 0, 80);
               out << "hello world (left margin is 0)" << mlm(4) << "
"
                      "this uses a 4 character wide left margin
" <<
                       mlm(-10) << flush <<
                       "left margin -6 changed to 0, active on this line
";
               return 0;
           }

FILES

       bobcat/mlm - defines the class interface

SEE ALSO

       bobcat(7), manipulators(3bobcat), lm(3bobcat), ofoldstream(3bobcat)

BUGS

       None Reported.

DISTRIBUTION FILES

       o      bobcat_3.01.00-x.dsc: detached signature;

       o      bobcat_3.01.00-x.tar.gz: source archive;

       o      bobcat_3.01.00-x_i386.changes: change log;

       o      libbobcat1_3.01.00-x_*.deb: debian package holding the libraries;

       o      libbobcat1-dev_3.01.00-x_*.deb: debian package holding the libraries, headers and manual pages;

       o      http://sourceforge.net/projects/bobcat: public archive location;

BOBCAT

       Bobcat is an acronym of `Brokken's Own Base Classes And Templates'.

COPYRIGHT

       This is free software, distributed under the terms of the GNU General Public License (GPL).

AUTHOR

       Frank B. Brokken (f.b.brokken@rug.nl).

libbobcat1-dev_3.01.00-x.tar.gz                                      2005-2012                                                   FBB::mlm(3bobcat)
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