Array of pointers using malloc : The code runs into a segmentation fault. Help! Thanks

Question

it seems I'm a bit confused here. As with the use of a pointer to an array of pointers. if I could fetch more info regarding the error, I would have. All I received is a code for a segmentation fault. Thanks :)

a program to find the longest input string.

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

int main()
{
 char **array;
 int i, n, *L, maxlen;


 printf("Enter the number of strings that you wish to store : ");
 scanf("%d", &n);

 array=malloc(n*sizeof(char*)); //acquire storage for an array of pointes of size n//
 L=malloc(sizeof(int)*n);// for the length array//
 for(i=0;i<n;i++)
 {
    printf("Enter string %d : ", i+1); //sometimes, prints upto this//
    gets(array[i]); //sometimes skips the first string input and jumps to the second and stops at the third//
    L[i]=strlength(array[i]);
 }
 maxlen=0;
 for(i=0;i<n;i++)
 {
    if(L[i]>maxlen)
        maxlen=L[i];
 }

  printf("The string(s) with the maximum length with length %d are : ", maxlen);
 for(i=0;i<n;i++)
 {
     if(L[i]==maxlen)
    {
        printf("\n%s.", array[i]);
    }
 }
  return 0;
 }

int strlength(char *array)
{
int j=0;
while(array[j]!='\0')
  {

    j++;
  }
return j;
}

Answer 1

you allocated memory for pointer to pointer, but forgot to allocate for individual pointers( ie the strings).

it can be modified as,

array=malloc(n*sizeof(char*));

to allocate memory for storing all the pointers for the strings.

and

for( i = 0; i<n; i++)
{
     array[i] = malloc(<max length of string>);
}

then only you will have memory space allocated to store the strings

Answer 2

Your memory allocation for:

char **array;
array=malloc(n*sizeof(char*)); //acquire storage for an array of pointes of size n//

only allocates memory for a set of pointers, but none for the space that is needed for the strings the pointers will point to. A function can be defined to encapsulate these steps:

char **array = Create2DStr(numberOfStrings, maxLengthOfString);

Where Create2DStr can be implemented as follows:

char ** Create2DStr(ssize_t numStrings, ssize_t maxStrLen)
{
    int i;
    char **a = {0};
    a = calloc(numStrings, sizeof(char *));// Creates a set of pointers.
    for(i=0;i<numStrings; i++)
    {
      a[i] = calloc(maxStrLen + 1, 1);//creates space for strings pointed to.
    }
    return a;
}

Caution:
- If creating memory in a loop using the same pointer, you should be familiar with realloc()
- maxStrLen in example should contain 1 byte more than the length of the longest string you need to accommodate.
- Do not forget to free everything you created in the reverse order they were created, ie free space for strings first ( maxStrLen calls to free() ), then free the pointers to each string last (1 call to free() )). See following example:

free2DStr(&array,numStrings);

The implementation of free2DStr can be done many ways, see below for an example.

void free2DStr(char *** a, ssize_t numStrings)
{
    int i;
    if(!(*a)) return;
    for(i=0;i<numStrings; i++)
    {
        if((*a)[i]) 
        {
            free((*a)[i]);
            (*a)[i] = NULL;
        }
    }
    free((*a));
    (*a) = NULL;
}

Note that in this implementation the address of the object is used in the first argument to provide access to free the pointers residing at that location.