Weird segfault involving popen
 

Hi guys

It has been quite a long time since my last post, but I am having some very weird behaviour on a program I am writing and I was hoping to get a little community feedback ;)

I am writing a class that basically opens a pipe to ksh runs a script gets the results and passes it back to the main program, the weird thing is that as the program terminates it segfaults I know it is related to the class that includes popen but I can't figure out why it is happening:

This is the popen class:
cpp

#include <iostream>

#include <cstdlib>

#include <stdio.h>

#include <string>



#include "shell_interp.h"



using namespace std;



Shell_Interp::Shell_Interp()

{

}





Shell_Interp::~Shell_Interp()

{

}



int Shell_Interp::runshellscript(char *filename, std::string& returnme)

{

   FILE *kshfd;



   char line[200];  // Returned values shouldn't be more than char

   char *files_to_open;  

   

   sprintf(files_to_open, "/usr/bin/ksh %s", filename);

      

   if( (kshfd = popen(files_to_open, "r")) == NULL )

   {

      return 1; // Damn file descriptor is invalid

   }   

      

   fgets( line, sizeof(line), kshfd); // Get line not in a loop cause we know the output

   std::string return_result(line);   // Convert to string to return

   returnme = return_result;          // Return this

   

   pclose(kshfd); // Close file descriptor         

   return 0;

}

h

#ifndef SHELL_INTERP_H

#define SHELL_INTERP_H



#include <string>



/**

@author Ben Maynard

*/

class Shell_Interp{

public:

    Shell_Interp();

    ~Shell_Interp();

    int runshellscript(char *filename, std::string& returnme);

};



#endif

And this is the calling code in the main function (sorry it is very dirty at the moment it is just test code

     Shell_Interp my_shell;     

     

     char *filename;

     std::string resulting;

     filename = "/home/work/test.sh";

     

     std::string getthisdude;

     

     int a = my_shell.runshellscript(filename, getthisdude);

          

     cout << "Ran shell script to get: " << getthisdude.c_str() << " return was " << a << endl;

     my_shell.~ Shell_Interp();



      return 0;

As it stand the returned string is exactly as expected, it runs everything before this point and everything after this point without a problem, but when it exits I get a segfault, if I remove the popen code no more segfaults so it is within this code here.

Any advice, help, pointers, questions, gratefully recieved.

Thanks

Ben

Re: Weird segfault involving popen
 

files_to_open has no space reserved for it. You declared it as a char* but didn't 'new' it. So do one of these two things:

char* files_to_open = new char[ kMaxFileSize ]; // don't forget to delete it!
-or-
char files_to_open[ kMaxFileSize ];

Re: Weird segfault involving popen
 

Damn you are good lol:

char files_to_open[ kMaxFileSize ];

Causes failure to compile:
shell_interp.cpp: In member function `int Shell_Interp::runshellscript(char*,
std::string&)':
shell_interp.cpp:31: error: cannot convert `char**' to `char*' for argument `1'
to `int sprintf(char*, const char*, ...)'
shell_interp.cpp:33: error: cannot convert `char**' to `const char*' for
argument `1' to `FILE* popen(const char*, const char*)'

However, char* files_to_open = new char[ kMaxFileSize ]; is perfect gets rid of the segfault completely.

I hate to impose further but any chance you could give me an indication as to why this happened? I would like to learn why this works for future reference instead of just that it does :D Thanks

Ben

Re: Weird segfault involving popen
 

It sounds like you changed

sprintf(files_to_open, "/usr/bin/ksh %s", filename);

to

sprintf(&files_to_open, "/usr/bin/ksh %s", filename);

because that would be a char**, not a char*. Possible?

Re: Weird segfault involving popen
 

Strangely enough no I didn't touch the sprintf() I just changed the declaration of file_to_open.

Ben