stack smashing in string and infinite loop - c

Question

stack smashing
stuck in loop.
enters an infinite loop randomly.
sometimes during first time sometimes during second.
The code is a mess however. the logic is to split a string into three strings alternatively three times.
#include<stdio.h>
#include<stdlib.h>
#include<string.h>
char mstr[21];
void split(char str1[21]);
void printx(char strxyz[7]);
void main()
{
int i,j,count=0;
char str[21];
for(i=1;i<=21;i++)
{
str[i-1]=i;
printf("%d\t",str[i-1]);
if(i%7==0)
{
printf("\n");
}
}
while(count<3)
{
split(str);
count++;
}
}
void split(char str1[21])
{
int i,j=0,k,ans,n;
char strx[7],stry[7],strz[7];
printf("\n\n* * * * * split * * * * *\n\n");
for(i=0;i<21;)
{
for(n=0;j<7;n++)
{
strx[j]=str1[i];i=i+1;
stry[j]=str1[i];i=i+1;
strz[j]=str1[i];i=i+1;
j=j+1;
}
}
printf("enter the group\n");
scanf("%d",&ans);
switch(ans)
{
case 1:
strcat(stry,strx);
strcat(stry,strz);
strcpy(mstr,stry);
break;
case 2:
strcat(strx,stry);
strcat(strx,strz);
strcpy(mstr,strx);
break;
case 3:
strcat(strx,strz);
strcat(strx,stry);
strcpy(mstr,strx);
break;
default:
printf("invalid\n");
}
printf("\n mstr values\n");
for(k=0;k<21;k++)
{
printf("%d\t",mstr[k]);
}
}
void printx(char strxyz[7])
{
int i;
printf("\n");
for(i=0;i<7;i++)
{
printf("%d\t",strxyz[i]);
}
}
2nd Output:
student#CSE-LAB3:~/Desktop/fdrive$ cc shuffle.c
student#CSE-LAB3:~/Desktop/fdrive$ ./a.out
1 2 3 4 5 6 7
8 9 10 11 12 13 14
15 16 17 18 19 20 21
split * * * * *
enter the group
2
mstr values
1 4 7 10 13 16 19 2 5 8 11 14 17 20 3 6 9 12 15 18 21
split * * * * *
enter the group
2
mstr values
1 4 7 10 13 16 19 2 5 8 11 14 17 20 3 6 2 5 8 11 14
split * * * * *
enter the group
1
Segmentation fault (core dumped)
edited:
the string
str = 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
now we have to split the string into three.
so.
strx = 1 2 3 4 5 6 7
stry = 8 9 10 11 12 13 14
srtrz = 15 16 17 18 19 20 21
now based on the input (1 or 2 or 3 ) merge the string into a single string.
if the input is 1. place the string strx( first sub string ) in the middle. so it becomes
like stry+strx+strz
> output
> 1 4 7 10 13 16 19
> 2 5 8 11 14 17 20
> 3 6 9 12 15 18 21
logic:
Imagine a real life problem.You have 21 cards. You divide it into three stacks of 7 cards. You ask your friend to think of a card from the three stacks.You then ask which stack has the card. The stack that contains the card is placed in the middle of other two stacks and merged without changing the order. Now You have to split the stack of 21 cards into 7 cards stacks . The process is repeated 3 times i.e. he has to choose the stack 3 times.

Answer 1

You have a couple of fundamental problems. The worst is that you are not terminating your strings with '\0' characters. This leads to undefined behavior. Also, you are concatenating, for example, strx to stry. But there is no room in the character array strx[] to hold the additional characters.
You can fix these problems by changing all declarations for arrays of size 21 to 22, e.g., char mstr[22];, and changing all declarations for arrays of size 7 to 8, e.g., char strx[8];. Then you need to be sure to add NUL terminators to the end of each string. One way to accomplish this would be to zero-initialize each of the arrays. Another method is to explicitly add the NUL character, and this is what I have done in the code below.
Then, you need to change the code that combines the strings. This code should copy the first substring into mstr[], and then concatenate the next two substrings:
case 1:
strcpy(mstr,stry);
strcat(mstr,strx);
strcat(mstr,strz);
There is also a significant logic error in the code that splits the strings. The double loop that you have is going out of array bounds. Consider that when i reaches 20, you will have: strz[j]=str1[22];i=22+1;, since i has already been incremented twice. This should be simplified to:
for(i=0, j=0; j < 7; j++, i += 3)
{
strx[j] = str1[i];
stry[j] = str1[i + 1];
strz[j] = str1[i + 2];
}
strx[j] = '\0';
stry[j] = '\0';
strz[j] = '\0';
It sounds like you want to preserve the changes made during each call to split(), so you should remove str[] from main() and instead use the global mstr[] here, since this is the string that you are storing the shuffled results in. It would be much better to remove the global entirely, though, as it is best to only use globals when you have to. Since the split() function distributes the contents of str1 among each of the strings strx, stry, and strz, you can store the combined results back in str1. And since str1 is a pointer to the first element of mstr[], the change is visible in the calling function. To accomplish this, the declaration for mstr[] needs to be moved into main(), and the code to combine strings should look like:
case 1:
strcpy(str1,stry);
strcat(str1,strx);
strcat(str1,strz);
Here is modified code that does more or less what I think you intended. I also added some code to print the resulting array after three calls to split():
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
void split(char str1[22]);
void printx(char strxyz[8]);
int main(void)
{
int i,count=0;
char mstr[22];
for(i=0;i<21;i++)
{
mstr[i]=i+1;
printf("%d\t",mstr[i]);
if((i+1)%7==0)
{
printf("\n");
}
}
mstr[i] = '\0';
while(count<3)
{
split(mstr);
count++;
}
printf("\nResults:\n");
for(i=0;i<21;i++)
{
printf("%d\t",mstr[i]);
if((i+1)%7==0)
{
printf("\n");
}
}
}
void split(char str1[22])
{
int i,j,k,ans;
char strx[8],stry[8],strz[8];
printf("\n\n* * * * * split * * * * *\n\n");
for(i=0, j=0;j<7;j++, i+=3)
{
strx[j]=str1[i];
stry[j]=str1[i+1];
strz[j]=str1[i+2];
}
strx[j] = '\0';
stry[j] = '\0';
strz[j] = '\0';
printf("enter the group\n");
scanf("%d",&ans);
switch(ans)
{
case 1:
strcpy(str1,stry);
strcat(str1,strx);
strcat(str1,strz);
break;
case 2:
strcpy(str1,strx);
strcat(str1,stry);
strcat(str1,strz);
break;
case 3:
strcpy(str1,strx);
strcat(str1,strz);
strcat(str1,stry);
break;
default:
printf("invalid\n");
}
printf("\n str1 values\n");
for(k=0;k<21;k++)
{
printf("%d\t",str1[k]);
}
}
void printx(char strxyz[8])
{
int i;
printf("\n");
for(i=0;i<7;i++)
{
printf("%d\t",strxyz[i]);
}
}

Answer 2

In procedure split, you have j<=7 but it should be j<7 or j<=6 to prevent out of bound problem.

Answer 3

What it sounds to me is there may be an uninitialized value or a wrong condition for terminating the loop.
I've spotted a problem, for example in the function,
void split(char str1[21])
{
//i, k and n are later initialized but j is not.
int i,j,k,ans,n;
char strx[7],stry[7],strz[7];
printf("\n\n* * * * * split * * * * *\n\n");
for(i=0;i<21;)
{
//j is not initialized, how do you know compiler will initialize it to 0 for sure?
//j <= 7 should be j < 7
for(n=0;j<=7;n++)
{
strx[j]=str1[i];i=i+1;
stry[j]=str1[i];i=i+1;
strz[j]=str1[i];i=i+1;
j=j+1;
}
}
printf("enter the group\n");
scanf("%d",&ans);
switch(ans)
{
case 1:
strcat(stry,strx);
strcat(stry,strz);
strcpy(mstr,stry);
break;
case 2:
strcat(strx,stry);
strcat(strx,strz);
strcpy(mstr,strx);
break;
case 3:
strcat(strx,strz);
strcat(strx,stry);
strcpy(mstr,strx);
break;
default:
printf("invalid\n");
}
printf("\n mstr values\n");
for(k=0;k<21;k++)
{
printf("%d\t",mstr[k]);
}
}
You declare j without initialization then it is compared against 7 in the for loop where n is incremented. Each time the program is started, they may be initialized to a random value. That's why you might receive undefined behavior. Also j <= 7 should be j < 7