Array of Char pointers (strings) crashes after input? - c

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;
}

Answer 1

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.

Answer 2

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