Struct pointer array segmentation fault - c

Question

I'm building an autocomplete program that takes a few characters of input and gives back suggested words to complete the characters. I have an AutoComplete_AddWord function that adds words for suggestion. However, whenever I try to access my structs completions array(holds up to 10 suggested words for given host table's letters) a segmentation fault is thrown. Not sure where I'm going wrong. Thanks for any help.
struct table {
struct table *nextLevel[26];
char *completions[10]; /* 10 word completions */
int lastIndex;
};
static struct table Root = { {NULL}, {NULL}, 0 }; //global representing the root table containing all subsequent tables
void AutoComplete_AddWord(const char *word){
int i; //iterator
char *w = (char*)malloc(100*(sizeof(char));
for(i = 0; w[i]; i++){ // make lowercase version of word
w[i] = tolower(word[i]);
}
char a = 'a';
if(w[0] < 97 || w[0] > 122)
w++;
int index = w[0] - a; // assume word is all lower case
if(Root.nextLevel[index] == NULL){
Root.nextLevel[index] = (struct table*) malloc(sizeof(struct table));
TotalMemory += sizeof(table);
*Root.nextLevel[index] = (struct table){{NULL},{NULL},0};
}
else
// otherwise, table is already allocated
struct table *pointer = Root.nextLevel[index];
pointer->completions[0] = strdup(word); //Here is where seg fault keeps happening
}

Answer 1

OK, so there are a lot of errors with this, and you obviously didn't test it and compile it. But I was curious so I took a closer look, and the problem stems from here:
for(i = 0; w[i]; i++){ // make lowercase version of word
w[i] = tolower(word[i]);
}
You are diving right into a loop checking w[0], a fresh, uninitialized block of memory.
Changing it to this:
for(i = 0; word[i]; i++){ // make lowercase version of word
w[i] = tolower(word[i]);
}
Will solve that problem. Fixing the other miscellaneous problems mentioned above, a non-segfaulting version of the code looks like this:
#include <stdio.h>
#include <ctype.h>
#include <stdlib.h>
#include <string.h>
struct table {
struct table *nextLevel[26];
char *completions[10]; /* 10 word completions */
int lastIndex;
};
int TotalMemory = 0;
static struct table Root = { {NULL}, {NULL}, 0 }; //global representing the root table containing all subsequent tables
void AutoComplete_AddWord(const char *word){
int i; //iterator
char *w = (char*)malloc(100*(sizeof(char)));
for(i = 0; word[i]; i++){ // make lowercase version of word
w[i] = tolower(word[i]);
}
char a = 'a';
if(w[0] < 97 || w[0] > 122) w++;
int index = w[0] - a; // assume word is all lower case
if(Root.nextLevel[index] == NULL){
Root.nextLevel[index] = (struct table*) malloc(sizeof(struct table));
TotalMemory += sizeof(struct table);
*Root.nextLevel[index] = (struct table){{NULL},{NULL},0};
}
struct table *pointer = Root.nextLevel[index];
pointer->completions[0] = strdup(word); //Here is where seg fault keeps happening
}
int main(int argc, char **argv)
{
AutoComplete_AddWord("testing");
return 0;
}
I can't speak for what happens next with this program, but at least this gets you past the segfault.