Hash Help

This is a discussion on Hash Help within the C Programming forums, part of the General Programming Boards category; Hi, I seem to get a seg fault when i implement this hash function from 'Algorithms in C -Sedgewick' .... ...

  1. #1
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    Hash Help

    Hi,
    I seem to get a seg fault when i implement this hash function from 'Algorithms in C -Sedgewick' ....
    I'm think it's more of my structuring, and pointer problems..

    Code:
    typedef struct {
            char *key;
            char *value;
    } TableEntry;
    
    typedef struct {
            int table_size;
            int num_items;
            TableEntry *table;
    } HashTable;
    
    static HashTable *
    hashtable_init(size_t size)
    {
        int i;
    
        HashTable *newtable = safe_malloc(sizeof(HashTable));
        newtable->table = safe_malloc(sizeof(TableEntry)*size);
        newtable->table_size = size;
        newtable->num_items = (int)size;
    
        if(newtable == NULL)
        {
                fprintf(stderr, "hashtable_init() - NYI\n");
                exit(EXIT_FAILURE);
        }
    
        /*allocate memory for word and key for each TableEntry*/
        for(i=0; i<(int)size; i++){
            newtable->table[i].key = safe_malloc(sizeof(char)*WORD_LEN);
            newtable->table[i].value = safe_malloc(sizeof(char)*WORD_LEN);
        }
    
        /*Initialise all keys and values to be null character*/
        for(i=0; i<(int)size; i++)
        {
            newtable->table[i].key = '\0';
            newtable->table[i].value = '\0';
        }
    
        return newtable;
    }
    
    static void
    hashtable_insert(HashTable *ht, char *key, char *value)
    {
        int hashval = hash_func(value, ht);
        int M = ht->num_items;
    
    
        while(ht->table[hashval].key != '\0')
            hashval = (hashval+1) % M;
    
            strcpy(ht->table[hashval].key, key);
            strcpy(ht->table[hashval].value, value);
    
    }
    
    int hash_func(char *v, HashTable *ht)
    {
            int h = 0, a = 31415, b = 27183;
            int M = ht->num_items;
    
            /*Segmentation Fault happens in this loop*/
            for(h=0; *v != '\0'; v++, a = a*b % (M-1))
                    h = (a*h + *v) % M;
    
    
            return h;
    }
    
    static void
    read_records(FILE *fp, HashTable *ht, char chomp(char*))
    {
            char key_buffer[MAX_STRING_SIZE];
            char value_buffer[MAX_STRING_SIZE];
    
            while (fgets(key_buffer, MAX_STRING_SIZE, fp) != NULL)
            {
                    if (fgets(value_buffer, MAX_STRING_SIZE, fp) == NULL)
                    {
                            fprintf(stderr, "Badly formed input file");
                            exit(EXIT_FAILURE);
                    }
                    hashtable_insert(ht, chomp(key_buffer), chomp(value_buffer));
            }
    }
    The chomp() function just replaces '\n' with '\0' from the input file

    Thx!!!!

  2. #2
    Registered User ssharish2005's Avatar
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    are u getting the right key values here
    Code:
    int M = ht->num_items;
    try putting some printfs and find out. The loop itself looks to me ike right.

    ssharish2005
    Last edited by ssharish2005; 04-26-2007 at 08:46 AM.

  3. #3
    Captain Crash brewbuck's Avatar
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    Why do people never say what LINE it crashes in?

  4. #4
    Deathray Engineer MacGyver's Avatar
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    Quote Originally Posted by brewbuck View Post
    Why do people never say what LINE it crashes in?
    How can they possibly know what is causing it? I mean it's not like they can scatter printf() statements throughout their code.... Oh, wait.

  5. #5
    Registered User ssharish2005's Avatar
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    Code:
    /*Segmentation Fault happens in this loop*/
            for(h=0; *v != '\0'; v++, a = a*b &#37; (M-1))
                    h = (a*h + *v) % M;
    Perhaps he has already mentioned it.

    ssharish2005

  6. #6
    Deathray Engineer MacGyver's Avatar
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    Ah, good point, although I would have recommended that he put that in bold if he dropped it in the code. Either way, I admit I missed it.

    Then again, since I didn't get any sleep for the past 22 hours or so, I suppose that could be the reason why.

  7. #7
    Captain Crash brewbuck's Avatar
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    Quote Originally Posted by MacGyver View Post
    Ah, good point, although I would have recommended that he put that in bold if he dropped it in the code. Either way, I admit I missed it.
    If I'd had a hint that such a comment was in the code somewhere, I would have bothered trying to find it

    As it stands, there is far too much code missing to isolate the problem.

  8. #8
    Deathray Engineer MacGyver's Avatar
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    Agreed.

  9. #9
    and the hat of wrongness Salem's Avatar
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    What on earth is safe_malloc ?
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
    I support http://www.ukip.org/ as the first necessary step to a free Europe.

  10. #10
    Captain Crash brewbuck's Avatar
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    Quote Originally Posted by Salem View Post
    What on earth is safe_malloc ?
    There's no need to feign ignorance. It most likely calls malloc() and exits if it fails, or longjmps somewhere. So, "safe" in the sense that it never returns NULL.

  11. #11
    Registered User ssharish2005's Avatar
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    Quote Originally Posted by Salem View Post
    What on earth is safe_malloc ?
    May be a user defined function, but there is no need one. And more over he is sending a paramtere. May be depending on the size the function allocates blocks and return address may be. Wired

    ssharish2005

  12. #12
    ATH0 quzah's Avatar
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    Quote Originally Posted by brewbuck View Post
    There's no need to feign ignorance. It most likely calls
    If you have to add "most likely", then you are ignorant on how it works.


    Quzah.
    Hope is the first step on the road to disappointment.

  13. #13
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    >char chomp(char*)
    So chomp returns a single char, not a string?

    > hashtable_insert(ht, chomp(key_buffer), chomp(value_buffer));
    Then you're passing those single chars to hashtable_insert, which is expecting strings.

    chomp should be returning a string, and declared as:
    Code:
    char *chomp(char*)

  14. #14
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    Seg fault at hash func fixed

    Oh yep!...u're right, shouldn't be returning single character from chomp().... thanks
    i'ved fixed that, and now segfault, happens in the hashtable_insert() function at the strcpy part....

    but i'll try to see wat i can do to fix that first before disturbing ...

    anyway, here's chomp() and safe_malloc()
    Code:
    static char *
    chomp(char *str)
    {
            size_t i;
    
            assert(str != NULL);
    
            for (i = strlen(str) - 1; str[i] == '\n'; i--)
            {
                    str[i] = '\0';
            }
    
            return str;
    }
    
    
    static void *
    safe_malloc(size_t size)
    {
            void *mem;
    
            if ((mem = malloc(size)) == NULL)
            {
                    fprintf(stderr, "Cannot allocate memory.\n");
                    exit(EXIT_FAILURE);
            }
    
            return mem;
    }
    thx!

  15. #15
    Captain Crash brewbuck's Avatar
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    Quote Originally Posted by quzah View Post
    If you have to add "most likely", then you are ignorant on how it works.
    The rest of the code looks rather sane. I ask myself, "what would a sane individual do in a function called safe_malloc?" Having written and seen this a thousand times, I'm pretty sure what it is.

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