Question

I’m writing a Trie Tree in C, with these structs:

typedef struct sNode Node;
struct sNode {
    int word;
    unsigned int nsons;
    Node *next[26];
};

typedef struct {
     Node *root;
} Trie;

But I’m having a problem in this insert function:

int insert(Trie* trie, char word[]){
    int i=0, posiletter, contletters=0;
    char letter=word[0];
    Node *newroot;
    Node *newNode=NULL;
    Node **copyroot;
    Node *current=NULL;
    Node *Nodefather=NULL;
    Node verif;


    if (trie->root=NULL){
        newroot=malloc(sizeof(Node));   
        for (i=0; i<=25; i++) newroot->next[i]=NULL;
        newroot->nsons=0;
        newroot->word=0;
        trie->root=newroot;
    }
    copyroot=&trie->root;
    current=*copyroot;
    while (word[contletters]!='\0'){
        posiletter=(word[contletters])-'a';

        while( (current->next[posiletter])!= NULL ){ // Error in this line
            Nodefather=current;
            current=current->next[posiletter];
        }
        if ( (current->next[letter-'a']) == NULL ){
            newNode=malloc(sizeof(Node));
            //newNode=malloc(sizeof(Node));   
            for (i=0; i<=25; i++) newNode->next[i]=NULL;
            newNode->nsons=0;
            newNode->word=0;
            Nodefather->next[letter-'a']=newNode;
            Nodefather->nsons=(Nodefather->nsons)+1;

        }
        contletters++;

    }
    if  (letter=='\0') Nodefather->next[letter-'a']->word=1; //end of word
    return 1;
}

The function receives a pointer to the tree and an string. If the root of the Trie is NULL, it creates a node, after this, it searchs on the array next a free (NULL) position to store.

The problem is in this line:

while( (current->next[posiletter])!= NULL ){ // Error in this line

It gives me Segmentation fault error when I execute this, and this error on GDB:

Program received signal SIGSEGV, Segmentation fault.
0x0804883f in insert (trie=0x804b008, word=0xbffff230 "abcdef") at trie.c:97
97          while( (current->next[posiletter])!= NULL ){ // Error in this line

And using Valgrind this way: valgrind –tool=memcheck –leak-check=full –track-origins=yes ./trie It give me these errors

==3409== Invalid read of size 4
==3409==    at 0x804883F: insert (trie.c:97)
==3409==    by 0x80485FD: main (triet.c:36)
==3409==  Address 0x8 is not stack'd, malloc'd or (recently) free'd
==3409== 
==3409== 
==3409== Process terminating with default action of signal 11 (SIGSEGV)
==3409==  Access not within mapped region at address 0x8
==3409==    at 0x804883F: insert (trie.c:97)
==3409==    by 0x80485FD: main (triet.c:36)
==3409==  If you believe this happened as a result of a stack
==3409==  overflow in your program's main thread (unlikely but
==3409==  possible), you can try to increase the size of the
==3409==  main thread stack using the --main-stacksize= flag.
==3409==  The main thread stack size used in this run was 8388608.
==3409== HEAP SUMMARY:
==3409==     in use at exit: 4 bytes in 1 blocks
==3409==   total heap usage: 1 allocs, 0 frees, 4 bytes allocated
==3409== 
==3409== LEAK SUMMARY:
==3409==    definitely lost: 0 bytes in 0 blocks
==3409==    indirectly lost: 0 bytes in 0 blocks
==3409==      possibly lost: 0 bytes in 0 blocks
==3409==    still reachable: 4 bytes in 1 blocks
==3409==         suppressed: 0 bytes in 0 blocks
==3409== Reachable blocks (those to which a pointer was found) are not shown.
==3409== To see them, rerun with: --leak-check=full --show-reachable=yes
==3409== 
==3409== For counts of detected and suppressed errors, rerun with: -v
==3409== ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 11 from 6)
Segmentation fault

I really appreciate any help. Thanks!

====Edit====
Values in the time of the crash:

(gdb) print current
$1 = (Node *) 0x0
(gdb) print current->next
Isn't possible acess the memory at the address 0x8
(gdb) print current->next[posiletter]
Isn't possible acess the memory at the address 0x8
(gdb) print posiletter
$2 = 0

===Edit2===
Other values requesteds:

(gdb) print contletters
$3 = 0
(gdb) print trie->root
$4 = (Node *) 0x0
(gdb) print trie->root->next[word[0]-'a']
Isn't possible acess the memory at the address 0x8

Answer 1

Ah how did I miss that!

Here is your problem:

if (trie->root=NULL)

should be

if (trie->root == NULL)

Note the ==.

The way you have written it, you set root to NULL which evaluates to false and the root initialization is never done. You could figure it out by the fact that your gdb outputs 0x0 for trie->root