Usage of struct in C++ caused unexpected behavior

phugoid · 04-29-2006, 09:10 AM

I could use some help understanding the output from the program below, which I compiled using gcc 4.0.2:

Code:

#include <iostream>
using namespace std;

struct charptr {
    char* m;
};

int main() {

    charptr foo;

    *(foo.m) = 'c';
    cout << (*(foo.m)) << '\n';

}

Output:

Code:

c
     23621:
     23621:     calling fini: /usr/lib/libstdc++.so.6 [0]
     23621:
     23621:
     23621:     calling fini: /lib/tls/libm.so.6 [0]
     23621:
     23621:
     23621:     calling fini: /lib/libgcc_s.so.1 [0]
     23621:
     23621:
     23621:     calling fini: /lib/tls/libc.so.6 [0]
     23621:

If I replace the line:

cout << (*(foo.m)) << '\n';

with

cout << "Stuff here" << (*(foo.m)) << '\n';

I get a segmentation fault at runtime intead.

I thought *(foo.m) in this case represents "the char value pointed to by foo.m", and I should set and read it this way.

Flesym · 04-29-2006, 09:40 AM

You can compile that with gcc? How do you do that?! Actually this is not C, but C++, so you should compile with g++. Hmmm..., however you have to allocate memory for 'foo.m' using the 'new' keyword. Otherwise you'll run into a segfault.

Code:

foo.m = new char;

phugoid · 04-29-2006, 11:59 AM

Thanks Flesym. I realize now that foo.m was pointing to an undefined location before I started using it.

I do compile with g++, I was just quoting the result of doing "c++ -v"... Sorry for confusion.

exvor · 04-29-2006, 12:00 PM

Code:

 
#include <iostream>
using namespace std;

struct charptr {
    char* m;
};

int main() {

    charptr foo;  //maybe this should be struct charptr foo; 

    *(foo.m) = 'c';
    cout << (*(foo.m)) << '\n';

}

Not sure if C++ uses struct the same way C does but if it does im suprized this even compiles.

in C it would look something like this

Code:

 

#include<stdio.h> 
  struct charptr { 
                  char *m; 
                 } 
int main () 
{ 
   struct charptr foo; // creates the pointer but it points to nothing
   char letter;  //Creates a place in memory to store the data 
   letter = 'c'; //assiens the data to the address space 

   foo.m = &letter;  // points the pointer to the address in         
                     //   memory that contains the data. 
   

   printf("%c\n",*foo.m); //derefrence the pointer to get the data 
   return 0; 
}

spooon · 04-29-2006, 03:48 PM

Yes it compiles and it is perfectly correct C++ syntax. C++ is very different from C.

Vagrant · 04-29-2006, 04:46 PM

Just follow Fleysm's advice. You didn't allocate any memory there for it. You'll get unpredictable results without allocating memory properly. As far as "gcc" or "g++," its the same thing. The real issue is the extension of the filename, compiling a .c with g++, invokes the C compiler, compiling a .cpp with gcc invokes the C++ compiler.