Read access violation for a hashtable entry immediately after creation - c

Question

I'm trying to write a table-lookup package that would work for a symbol table management.
The names and replacement texts are held in a struct nlist type, and I have an array of pointers to the name and replacement text.
#define HASHSIZE 101
struct nlist { // table entry
struct nlist *next; // next entry in chain
char *name; // defined name
char *defn; // replacement text
};
static struct nlist *hashtab[HASHSIZE] = { NULL }; // pointer table
The lookup routine searches for s in the table.
// hash: form hash value for string s
unsigned hash(char *s)
{
unsigned hashval;
for (hashval = 0; *s != '\0'; s++)
hashval = *s + 31 * hashval;
return hashval % HASHSIZE;
}
struct nlist *lookup(char *s)
{
struct nlist *np;
for (np = hashtab[hash(s)]; np != NULL; np = np->next)
if (strcmp(s, np->name) == 0)
return np; // found
return NULL; // not found
}
The install routine uses lookup to determine whether the name being installed is already present; if so, the new definition will supersede the old. Otherwise, a new entry is created.
// install: put (name, defn) in hashtab
struct nlist *install(char *name, char *defn)
{
struct nlist *np;
unsigned hashval;
if ((np = lookup(name)) == NULL) { // not found
np = (struct nlist *) malloc(sizeof(*np));
if (np == NULL || (np->name = strdup(name)) == NULL)
return NULL;
hashval = hash(name);
np->next = hashtab[hashval];
hashtab[hashval] = np;
} else // already there
free(np->defn); // free previous defn
if ((np->defn = strdup(defn)) == NULL)
return NULL;
return np;
}
In my main routine, "Access violation reading location" exception is thrown on the call of printf on pointer p. What is the cause of this error and how can I fix it?
int main()
{
struct nlist *install(char *name, char *defn);
struct nlist *p;
void undef(char *name);
p = install("DEFAULT", "ON");
printf("%s\n",p->name);
return 0;
}
I'm not certain of the correctness of the assignment of np within the install routine i.e. taking the size of *np instead of just struct nlist *.
Additionally, I have read advice that the additional cast isn't necessary.
struct nlist *np;
...
np = (struct nlist *) malloc(sizeof(*np));

Answer 1

PICNIC. I didn't include <string.h>