StringHashTable中使用的静态函数线程安全？

Question

I have got an StringHashTable class from
http://preshing.com/20110603/hash-table-performance-tests/

The following are parts of source :

class StringHashTable
{
    static uint fnv1Hash(const char *key)
    {
        unsigned int hash = 2166136261ul;
        for (const char *s = key; *s; s++)
            hash = (16777619 * hash) ^ (*s);
        return hash;
    };

    uint &operator[](const char *key)
    {
        uint hash = fnv1Hash(key) & (m_tableSize - 1);
        Bucket *firstBucket = m_table + hash;
        Bucket *b = firstBucket;
        if (b->key)
        {
            do
            {
                if (strcmp(b->key, key) == 0)
                    return b->value;// Found existing bucket
                b = b->next;
            } while (b);
        }
        ..........
    }
} 

Suppose that I have the global var :

StringHashTable hashtable(1024) ; //m_tableSize now 1024

And then the following is in main :

hashtable["0000"] = 0 ;
....
hashtable["9999"] = 9999 ;

After fill in all the data I need , thread 1 to n will get value according to key

while(1)
{
    s = get(); //return string like "0000" ... "9999"
    echo << hashtable[s.c_str()] << endl ;
}

I wonder if the StringHashTable would work fine in thread at first , because the function fnv1Hash is static , on second thought , there is no static member data in this StringHashTable , so while thread1 is doing hashtable["0000"] and thread2 is doing hashtable["9999"] at the very same time both thread1 are calling fnv1Hash they will both get the right hash returned !!!

My question is : different thread call static uint fnv1Hash(const char *key) with different key at the very same time still work fine ? In StringHashTable , fnv1Hash is static for any reason ?!

Answer 1

The function fnv1Hash() doesn't access any non-local state other than the data pointed to by key . Assuming the content of the array key points to isn't written to concurrently, there is no threading issue. Of course, if another thread writes to the array pointed to by key , all bets are off.

Given that fnv1Hash() does access any of the object's data it doesn't need a this pointer. Thus, it is made static to indicate both to the human reader and the compiler that the objects won't be accessed implicitly. For the compiler the upshot is that it doesn't need to pass a this pointer.