Why Am I Getting Segmentation Fault When I Am Taking String Input?

Question

Well, i was trying to make a message encryption and decryption program. So, why am i getting segmentation fault? If anyone could please help me i will be very very grateful! I had run only the encryption function. It produced proper results. Any clue what happened?

        #include <iostream>
        using namespace std;
        #define max 1000
        #include <string.h>
        #include <cstdlib>
        #include <stdlib.h>
        #include <math.h>
        #include <stdio.h>


        char * encrypt(char *s)
        {
            int x = (rand()/((RAND_MAX+1u)/5));
            char *res;
            int ascii;
            switch(x)
            {
                case 0:
                for(int i=0;i<strlen(s);i++)
                {
                    ascii = (int)s[i];
                    ascii = ascii-19;
                    res[i+2] = (char)ascii;

                }
                res[0]='a';
                res[1]='$';
                break; 
                case 1:
                for(int i=0;i<strlen(s);i++)
                {
                    ascii = (int)s[i];
                    ascii = ascii+sqrt(strlen(s));
                    res[i+2] = (char)ascii;
                }
                res[0]='x';
                res[1]='&';
                break;
                case 2:
                case 3:
                for(int i=0;i<strlen(s);i++)
                {
                    ascii = (int)s[i];
                    ascii = ascii-sqrt(strlen(s));
                    res[i+2] = (char)ascii;
                }
                res[0]='z';
                res[1]='^';
                break;
                case 4: 
                for(int i=0;i<strlen(s);i++)
                {
                    ascii = (int)s[i];
                    ascii = ascii+13;
                    res[i+2] = (char)ascii;
                }
                res[0]='a';
                res[1]='j';
                break;
            }
            return res;


        }

        char * decrypt(char *s)
        {
            int ascii;
            char *res;
            if(s[0]=='a' &&s[1]=='$')
            {
                for(int i=0;i<strlen(s);i++)
                {
                    ascii = (int)s[i+2];
                    ascii += 19;
                    res[i] = ascii;
                }   
            }
            else if(s[0]=='b' &&s[1]=='&')
            {
                for(int i=0;i<strlen(s);i++)
                {
                    ascii = (int)s[i+2];
                    ascii -= (strlen(s)*strlen(s));
                    res[i] = ascii;
                }   
            }
            else if(s[0]=='z' &&s[1]=='^')
            {
                for(int i=0;i<strlen(s);i++)
                {
                    ascii = (int)s[i+2];
                    ascii +=(strlen(s)*strlen(s));
                    res[i] = ascii;
                }   
            }
            else if(s[0]=='a' &&s[1]=='j')
            {
                for(int i=0;i<strlen(s);i++)
                {
                    ascii = (int)s[i+2];
                    ascii -= 13;
                    res[i] = ascii;
                }   
            }

            return res;

        }


        int main()
        {
            int ch;
            int i=0;
            char *s;
            char *res;
            while(1) {

            cout<<endl<<"1.Encrypt\n2.Decrypt";
            cout<<endl<<"Choice: ";

            cin>>ch;
            switch(ch)
            {
                case 1: 
                cout<<"\nEnter a message: ";
                cin>>s;

                res=encrypt(s);
                cout<<"\nEncrypted message is: "<<res<<endl;
                break;

                case 2:
                cout<<"\nEnter an Encrypted message: ";
                cin>>s;

                res=decrypt(s);
                cout<<"\nDecrypted message is: "<<res<<endl;
                break; 
                default: exit(0);
            }
        }
            return 0;
        }

The gdb gives me this message:

Starting program: /home/prasanna/encdec 

1.Encrypt
2.Decrypt
Choice: 1

Enter a message: Test Line

Program received signal SIGSEGV, Segmentation fault.
0xb7f41aab in std::basic_istream<char, std::char_traits<char> >& std::operator>><char,     std::char_traits<char> >(std::basic_istream<char, std::char_traits<char> >&, char*) ()
   from /usr/lib/i386-linux-gnu/libstdc++.so.6

Becasue `res` is indeterminate (i.e. it is never set to a valid memory location). — WhozCraig, Mar 13 '13 at 14:57
If, for example, you changed it to `char res[256] = "";`, it would work fine. — TheBuzzSaw, Mar 13 '13 at 15:00
@TheBuzzSaw in this case, no it wouldn't. The `return res;` is a dead-mans-giveaway of that. — WhozCraig, Mar 13 '13 at 15:04
@WhozCraig Ah, you're right. I didn't see the full scope of what was going on. Yeah, it'd have to be a heap allocation (or preferably `std::string`). — TheBuzzSaw, Mar 13 '13 at 15:50

score 7 · Accepted Answer · answered Mar 13 '13 at 14:58

7

The immediate reason is that char *s; declares an uninitialized pointer into which you attempt to read something - cin>>s;.

The real reason is that you're writing C code and calling it C++.

answered Mar 13 '13 at 14:58

Luchian Grigore

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Yeah I know that! Its c, but well, i drifted from my original idea. This is just to make sure my algo is working well... So, new operator is my solution? – Prasanna Choudhari Mar 13 '13 at 15:00
@PrasannaChoudhari no. `std::string` and `std::cin` is. – Luchian Grigore Mar 13 '13 at 15:01
While I rarely agree with the last statement, I do in this case, but not for the reasons likely intended. In this case, the *`#include`* list gives me the creeps. – WhozCraig Mar 13 '13 at 15:01
Well, i did char *res = new char [max] everywhere and also for *s. Worked well... Encryption is perfect. Decryption needs tuning.. – Prasanna Choudhari Mar 13 '13 at 15:11
@PrasannaChoudhari: I'm sure it works well, except for the probable memory leaks and possible buffer overflows. – Fred Larson Mar 13 '13 at 15:13
Well, i think i hit one of them! :-( Getting some weird outputs for some test cases. So, what exactly do u guys suggest? Well, this is not exactly my forte. – Prasanna Choudhari Mar 13 '13 at 15:20
@PrasannaChoudhari don't take this the wrong way - I suggest starting simple, using a book. – Luchian Grigore Mar 13 '13 at 15:32
Ok man! btw, i know it might sound ironic, but im a pretty good coder...its just that i goof up a lot! – Prasanna Choudhari Mar 13 '13 at 15:51
Yes, a book from the [C++ Definitive Book Guide and List](http://stackoverflow.com/q/388242/10077). – Fred Larson Mar 13 '13 at 15:52

score 3 · Answer 2 · answered Mar 13 '13 at 14:57

3

You have not allocated memory to

char *res;

but you

res[i+2] = (char)ascii;

write to that memory. That is undefined behaviour, and most often leads to a crash.

You're doing the same in main to char *s;:

cin>>s;

answered Mar 13 '13 at 14:57

Daniel Fischer

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Thanks!!! Got it! Well, i hate it when the solutions are silly! Even the answer to my previous question was way too silly! :-( – Prasanna Choudhari Mar 13 '13 at 15:01

score 0 · Answer 3 · answered Mar 13 '13 at 15:08

You have not allocated any memory to char* s in main. There is no space decided to store the string you received as input. You have only declared a pointer to some area (which is not yet decided)

Use std::string

or you can use new

char *s = new char[15];

Why Am I Getting Segmentation Fault When I Am Taking String Input?

3 Answers3