Question

Basically I am trying to build a hash structure of cstrings that I can re-assign the member stringnode_ref *nodes to a larger dimension array (malloc a new one, re-hash the existing values, and delete the old one) when the hash begins to fill up.

stringtable.c:

#include <stdio.h>
#include "stringtable.h"

struct stringnode{
    hashcode_t key;
    cstring value;
};

struct stringtable{
    size_t dim;
    size_t numEntries;
    stringnode_ref *nodes;
};

stringtable_ref new_stringtable(){
    size_t index = 0;
    stringtable_ref sTable = malloc(sizeof(struct stringtable));
    sTable->dim = 31;
    sTable->numEntries;
    sTable->nodes = malloc(31 * sizeof(struct stringnode));
    for( index = 0; index < 31; index++ ){
        sTable->nodes[index]->key = 0;
        sTable->nodes[index]->value = NULL;
    }
    return sTable;
}

stringtable.h:

#ifndef __STRINGTABLE_H__
#define __STRINGTABLE_H__

#include <stdlib.h>
#include <inttypes.h>

typedef char *cstring;

typedef uint32_t hashcode_t;

typedef stringtable *stringtable_ref;

typedef stringnode *stringnode_ref;

stringtable_ref new_stringtable();

#endif // __STRINGTABLE_H__

oc.c:

#include <stdlib.h>
#include <stdio.h>
#include "stringtable.h"

int main( int argc, char **argv ){
    stringtable_ref table = new_stringtable();
    return EXIT_SUCCESS;
}

compiling with:

gcc -g -O0 -Wall -Wextra -std=gnu99 -c stringtable.c
gcc -g -O0 -Wall -Wextra -std=gnu99 -c oc.c
gcc -g -O0 -Wall -Wextra -std=gnu99 -o oc stringtable.o oc.o

this all compiles fine with no errors but upon declaring stringtable_ref table = new_stringtable(); in the main(), the program segfaults at sTable->nodes[index]->key = 0;. Any thoughts?

Answer 1

sTable->nodes = malloc(31 * sizeof(struct stringnode));
for( index = 0; index < 31; index++ ){
    sTable->nodes[index]->key = 0;

here’s the problem: the segfault occurs because you don’t allocate memory for sTable-nodes[index], so it may be garbage (when I examined your code on GDB under OS X, it was actually NULL).

Solution: add the following line as the first line of the body of the for loop:

sTable->nodes[index] = malloc(sizeof(*(sTable->nodes[index])));