Array of Char pointers (strings) crashes after input?

Question

I want to store an array of strings , count their length and re-arrange them with length-increasing-order (smallest->larger) using the algorithm mentioned below //
Swap holds a relatively big string to replace the order (when another min is found)
I could use realloc() but I am not considering defend programming yet

#include <stdio.h>
#include <stdlib.h>



int main()
    {
        int i,j,N;
        printf("\n Input amount of alphanumericals: ");
        scanf("%d",&N);
            {
                int min;
                char *swap=(char*)malloc(sizeof(char)*150);
                char *A[N],**temp;
                temp=A;
                for(i=0;i<N;i++){
                    printf("\nInput %d element:",i+1); 
                    fgets(temp+i,150,STDIN);
                }


    printf("\n\nData [");
    for(i=0;i<N;i++)
        printf(" %s",A[i]);
    printf(" ]\n\n");

    //Ins sort
    for(i=0;i<N;i++){
        min=i;//Assume current is min
        for(j=i+1;j<N;j++){
            //Compare assuming min with current
            if(strcmp(A[j],A[min])<0){
                min=j;
            }
            //If next is min replace in current position
        if(min!=i){
            swap=A[i];
            A[i]=A[min];
            A[min]=swap;
        }   
    }
    free(swap);
    printf("\nAfter insertion point algorithm\n");
    printf("\n\nInsertion Sorted Data [");
    for(i=0;i<N;i++)
        printf(" %s",A[i]);
    printf(" ]");
    }
    return 0;
}

Does it even compiled for you(gets(temp+i);)? Anyway you are declaring pointers here and using it without allocating memory char *A[N],**temp; temp=A; — Digital_Reality
– Digital_Reality, Commented Jan 16, 2014 at 16:26
Besides temp which is just a shortcut for doing gets(A+i) or store alphanumericals in all cell , yeah blcc is pretty elastic sometimes :/ — solid.py
– solid.py, Commented Jan 16, 2014 at 16:27

Sergey Kalinichenko · Accepted Answer · 2014-01-16 16:46:04Z

2

You are tying to free memory that has not been allocated using malloc:

char *A[N],**temp;
temp = A;   // A is an automatic (AKA "stack") variable; now temp points to it as well
free(temp); // Undefined behavior

Inside the loop you are reading with gets into strings that have not been allocated:

gets(temp+i); // That's the same as gets(A[i]); Your A[i]s are unallocated.

As a side note, you should not use gets, because it is a prime source of buffer overruns. Use fgets instead, and pass stdin as the FILE* parameter. scanf with %20s is another alternative to limit the size.

In addition, since i goes from 1 to N, inclusive, this expression references one element past the A array on the last iteration:

gets(temp+i); // Undefined behavior when i==N

EDIT : Why is the code below crashes?

for(i=0;i<N;i++){
    printf("\nInput %d element:",i+1); 
    fgets(temp+i,150,STDIN);
}

The code below crashes because you did not allocate memory for the individual strings:

for(i=0;i<N;i++){
    printf("\nInput %d element:",i+1);
    temp+i = malloc(151); // No need to multiply by sizeof(char) or cast to char*
    fgets(temp+i,150,STDIN);
}

Note that you need to allocate one extra char for the null terminator.

edited Jan 16, 2014 at 16:46

answered Jan 16, 2014 at 16:26

Sergey Kalinichenko

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3 Comments

solid.py Over a year ago

It outputs:Input 1 element:<br/> Input 2 element:<I can type here><br/> Input 3 element:<here too><br/> Then crashes

Sergey Kalinichenko Over a year ago

@niCk That is because it's undefined behavior: it may not crash for two or three times. It may even run to completion. However, such program may crash the next time you run it. If you would like to see a report of your memory errors, run valgrind.

solid.py Over a year ago

edited to fgets() to avoid buffer errors but could you explain me why its still crashes I think I did legit allocations and assignements

lbolla · Accepted Answer · 2014-01-16 16:31:28Z

0

You are not using gets correctly: take a look at its manual page [1]. Moreover, using gets is not recommended: better use fgets.

Also, gets is not allocating the string for you, so when you pass it a char * pointer, the pointer has to point to a valid memory location. Instead, your A[n] is an array of dangling pointers.

Then, why free(temp)? You must call free for each pointer allocated with malloc: not the case for temp.

Finally, please format your code: use indent.

[1] http://linux.die.net/man/3/gets

answered Jan 16, 2014 at 16:31

lbolla

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2 Comments

solid.py Over a year ago

I did use indent the format became crappy when I copy-code-paste certain parts , thanks for the documentation

ryyker Over a year ago

@niCk - Once you paste formatted code from your editor into the edit box here, Select your newly pasted code, then click the {} tool above the editor. If your code was formatted correctly when pasted, this will help it to be formatted here as well.

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