C interface for C++ Hashmap library not working correctly

Question

I am relatively new to C, although I have been developing with other languages for an extended period of time. In an attempt to "Learn by doing", whilst working through a coursera course on C & C++ I decided to challenge myself with both by writing a wrapper for an existing project, which would translate to what I would eventually be using them for in my real world application of them.

I am building a C interface for a C++ implementation of Google Sparse Hash Map, using a guide on C interfaces / wrapping (here), but seem to be having issues with either data not inserting, or not being able to read the data once inserted?

This is what I have.

The C++ Object

sparsehashmap.cpp

#include "sparsehashmap.h"
#include<string>

SparseHashMap::SparseHashMap(){_shash.set_deleted_key("");}

void SparseHashMap::insert(const char* arg1, const char* arg2)
{
    _shash[arg1] = arg2;
}

const char* SparseHashMap::read(const char* key)
{
    return _shash[key];
}

int SparseHashMap::exists(const char* key)
{

    if (_shash.find(key) == _shash.end())
    {
        return false;
    }
    else
    {
        return true;
    }
}

void SparseHashMap::remove(const char* key)
{
    _shash.erase(key);
}

int SparseHashMap::length()
{
    return (int) _shash.size();
}

void SparseHashMap::flush()
{
    _shash.clear();
}

sparsehashmap.h

#ifndef __CPPSPARSEHASH__
#define __CPPSPARSEHASH__

#include <iostream>
#include <sparsehash/sparse_hash_map> //https://github.com/sparsehash/sparsehash

typedef google::sparse_hash_map<const char*, const char*> _SPARSEHASH;

class SparseHashMap
{
    private:
        _SPARSEHASH _shash;

    public:

    SparseHashMap();
    void            insert(const char* arg1, const char* arg2);
    const char*     read(const char* key);
    int             exists(const char* key);
    void            remove(const char* key);
    int             length();
    void            flush();
};

#endif // __CPPSPARSEHASH__

The C Wrapper

sparse.h

#ifndef __FOSPARSE_H__
#define __FOSPARSE_H__

#ifdef __cplusplus
extern "C" {
#endif

    struct CSparseHashMap;
    typedef struct CSparseHashMap _SparseHashMap;

    _SparseHashMap *sparsehashmap_create();
    void            sparsehashmap_destroy(_SparseHashMap *shm);
    void            sparsehashmap_insert    (_SparseHashMap *shm, const char* arg1, const char* arg2);
    const char*     sparsehashmap_read      (_SparseHashMap *shm, const char* key);
    int             sparsehashmap_exists    (_SparseHashMap *shm, const char* key);
    void            sparsehashmap_remove    (_SparseHashMap *shm, const char* key);
    int             sparsehashmap_length    (_SparseHashMap *shm);
    void            sparsehashmap_flush     (_SparseHashMap *shm);

#ifdef __cplusplus
}
#endif

#endif

sparse.cpp

#include <stdlib.h>
#include "sparse.h"
#include "sparsehashmap.h"

struct CSparseHashMap {
    void *obj;
};

_SparseHashMap *sparsehashmap_create()
{
    _SparseHashMap *shm;
    SparseHashMap *obj;

    shm         = (__typeof__(shm))malloc(sizeof(*shm));
    obj         = new SparseHashMap();
    shm->obj    = obj;

    return shm;
}

void sparsehashmap_destroy(_SparseHashMap *shm)
{
    if (shm == NULL)
        return;
    delete static_cast<SparseHashMap *>(shm->obj);
    free(shm);
}

void sparsehashmap_insert(_SparseHashMap *shm,  const char* arg1, const char* arg2)
{
    SparseHashMap *obj;

    if (shm == NULL)
        return;
    obj = static_cast<SparseHashMap *>(shm->obj);
    obj->insert(arg1, arg2);
}

const char* sparsehashmap_read(_SparseHashMap *shm,  const char* key)
{
    SparseHashMap *obj;

    if (shm == NULL)
        return 0;
    obj = static_cast<SparseHashMap *>(shm->obj);
    
    return obj->read(key);
}

int sparsehashmap_exists(_SparseHashMap *shm,  const char* key)
{
    SparseHashMap *obj;

    if (shm == NULL)
        return 0;
    obj = static_cast<SparseHashMap *>(shm->obj);
    
    return obj->exists(key);
}

void sparsehashmap_remove(_SparseHashMap *shm,  const char* key)
{
    SparseHashMap *obj;

    if (shm == NULL)
        return;
    obj = static_cast<SparseHashMap *>(shm->obj);
    obj->remove(key);
}

int sparsehashmap_length(_SparseHashMap *shm)
{
    SparseHashMap *obj;

    if (shm == NULL)
        return 0;
    obj = static_cast<SparseHashMap *>(shm->obj);

    return obj->length();
}

void sparsehashmap_flush(_SparseHashMap *shm)
{
    SparseHashMap *obj;

    if (shm == NULL)
        return;
    obj = static_cast<SparseHashMap *>(shm->obj);
    obj->flush();
}

The Test

main.c

#include <stdio.h>
#include <string.h>
#include <stdbool.h>
#include "sparse.h"

#define MAXCHAR 256


int main(int argc, char* argv[])
{
    // Create Pointer to Sparsehashmap object
    _SparseHashMap *fshm = sparsehashmap_create();


    /*  
        Open File specified in command line argument <argv[1]> 
        and initialise array pointer to hold the line data  
    */
    
    FILE *fp;
    char row[MAXCHAR];
    int ct = 0;
    fp = fopen(argv[1], "r");

    /*  
        Loop through the file, split the row using a single space
        as a delimeter, and insert as <key> => <value> into the 
        sparsehashmap 
    */

    while (feof(fp) != true)
    {
        char key[MAXCHAR], value[MAXCHAR];
        fgets(row, MAXCHAR, fp);

        sscanf(row, "%s %s", key, value);

//      if( (ct % 100) == 0 )
//          printf("Key: %s\tValue: %s\n", key, value);

        sparsehashmap_insert(fshm, key, value);
        
        ct++;
    }

    // close the file as it is no longer needed.
    fclose(fp);

    // Print the total number of times the while loop ran
    printf("%d Total entries.\n", ct);

    // Print the length of the Sparsehashmap
    printf("C SparseHashMap: Length of Hashmap = %d\n", sparsehashmap_length(fshm));
    

    /*
        Read, Exists and Remove Examples
    */

    printf("C SparseHashMap: _SparseHashMap[\"63-5039337\"] = %s\n", sparsehashmap_read(fshm, "63-5039337"));
    printf("C SparseHashMap: _SparseHashMap[\"26-8663826\"] = %s\n", sparsehashmap_read(fshm, "26-8663826"));
    printf("C SparseHashMap: _SparseHashMap[\"87-9801138\"] = %s\n", sparsehashmap_read(fshm, "87-9801138"));


    printf("C SparseHashMap: Key Exists [\"15-8895492\"] = %d\n", sparsehashmap_exists(fshm, "15-8895492"));
    printf("C SparseHashMap: Key Exists [\"22-4942175\"] = %d\n", sparsehashmap_exists(fshm, "22-4942175"));
    

    printf("C SparseHashMap: Remove Key From Hashmap...\n");

    printf("C SparseHashMap: Remove Key From Hashmap [\"63-5039337\"]\n"); 
        sparsehashmap_remove(fshm, "63-5039337");
    printf("C SparseHashMap: Remove Key From Hashmap [\"20-6892893\"]\n"); 
        sparsehashmap_remove(fshm, "20-6892893");
    printf("C SparseHashMap: Remove Key From Hashmap [\"58-8150180\"]\n"); 
        sparsehashmap_remove(fshm, "58-8150180");


    printf("C SparseHashMap: Length of Hashmap = %d\n", sparsehashmap_length(fshm));    

    /*
        Destroy the Sparsehashmap to free its
        memory allocation.
    */
    printf("C SparseHashMap: Destroy Hashmap\n");
    sparsehashmap_destroy(fshm);

    printf("C SparseHashMap: Freed");
    return 0;
}

I have created a gist of the data that I am inserting and querying, which can be found here.

Once built, I run it against the file of mock data, but of the file of 2000 test entries, these are the results I get

2001 Total entries.                                     /* times file is looped with fgets() */
C SparseHashMap: Length of Hashmap = 1                  /* Size of the hashmap */
C SparseHashMap: _SparseHashMap["63-5039337"] = (null)  /* Read Function Response */
C SparseHashMap: _SparseHashMap["26-8663826"] = (null)  /* Read Function Response */
C SparseHashMap: _SparseHashMap["87-9801138"] = (null)  /* Read Function Response */
C SparseHashMap: Key Exists ["15-8895492"] = 0
C SparseHashMap: Key Exists ["22-4942175"] = 0
C SparseHashMap: Remove Key From Hashmap ["63-5039337"]
C SparseHashMap: Remove Key From Hashmap ["20-6892893"]
C SparseHashMap: Remove Key From Hashmap ["58-8150180"]
C SparseHashMap: Length of Hashmap = 3                  /* Size of the hashmap */
C SparseHashMap: Destroy Hashmap
C SparseHashMap: Freed%

The size of the Hash Map starts at one (after apparently inserting 2000 entries), it is unable to read any keys from the input data, and then after it performs a number of removals, the length jumps to three?

I understand that the guide I am working from is a bit more basic than working with a hash map, but was wondering if anyone could point me to where I am going wrong with this? and how I should be looking at modifying it to have the operations working as expected?

Any guidance would be greatly appreciated.

======== EDIT ========

The issue seemed to be mapping pointers to pointers. After amending the *.cpp files to take / return std::string, and performing the relevant conversions before passing from the C instructions the interface began working as intended

./run MOCK_DATA.txt 
2001 Total entries.
C SparseHashMap: Length of Hashmap = 2000
C SparseHashMap: _SparseHashMap["63-5039337"] = {"id":1,"first_name":"Ilario","last_name":"Thickins","email":"ithickins0@nbcnews.com","gender":"Male","ip_address":"54.113.8.103"}
C SparseHashMap: _SparseHashMap["26-8663826"] = {"id":35,"first_name":"Mignonne","last_name":"Oakden","email":"moakdeny@ifeng.com","gender":"Female","ip_address":"41.14.68.148"}
C SparseHashMap: _SparseHashMap["87-9801138"] = {"id":1989,"first_name":"Mavis","last_name":"Collingwood","email":"mcollingwoodrg@google.it","gender":"Female","ip_address":"56.70.97.65"}
C SparseHashMap: Key Exists ["15-8895492"] = 1
C SparseHashMap: Key Exists ["22-4942175"] = 1
C SparseHashMap: Remove Key From Hashmap ["63-5039337"]
C SparseHashMap: Remove Key From Hashmap ["20-6892893"]
C SparseHashMap: Remove Key From Hashmap ["58-8150180"]
C SparseHashMap: Length of Hashmap = 1997
C SparseHashMap: Destroy Hashmap
C SparseHashMap: Freed

And when you used your debugger to run this code, what did you see? This is what a debugger is for and if you don't know how to use it, this is a good opportunity to learn running your program one line at a time in a debugger, monitoring all variables and their values as they change and analysing your program's logic and execution. It should be possible for you to use your debugger to find all simple problems in this and all future programs you write, all by yourself. — Sam Varshavchik, Apr 11 '22 at 14:44
Future bug: [Why is “while ( !feof (file) )” always wrong?](https://stackoverflow.com/questions/5431941/why-is-while-feof-file-always-wrong) — user4581301, Apr 11 '22 at 14:52
Future bugs: [What are the rules about using an underscore in a C++ identifier?](https://stackoverflow.com/questions/228783/what-are-the-rules-about-using-an-underscore-in-a-c-identifier) You usually get away with breaking these rules which makes the carnage when you don't even more inscrutable. — user4581301, Apr 11 '22 at 15:00

score 1 · Accepted Answer · answered Apr 11 '22 at 15:28

You are mapping pointers to pointers, but you likely intended to map strings to strings.

typedef google::sparse_hash_map<const char*, const char*> _SPARSEHASH;

With a std::map that might be:

std::map<const char*, const char*>

That just means that you are mapping pointers to pointers.

All of your pointers come from these two objects.

char key[MAXCHAR], value[MAXCHAR];

Every time you add a key to your hash map, you are using a key which is a pointer to your local key array (which is always the same on every iteration of the loop). And the value is a pointer to your local value array (again, which is always the same on every iteration of the loop). That's why you have only 1 entry in your map. You have successfully mapped &key[0] to &value[0].

What you probably want is to use std::string.

std::map<std::string, std::string>

Or

typedef google::sparse_hash_map<std::string, std::string> _SPARSEHASH;

Then you have bigger fish to fry

Now you have to worry about the fact that your implementation of read just returns a pointer, rather than returning a string. You just inherited all the problems of managing strings in C. You could treat it like std::string::c_str where you return the hash entry's c_str, but this will be invalidated if you add another entry to the map or remove one, so your semantics will have to be clearly documented, or else you'll have to allocate a buffer for a copy of your returned string and permit freeing that buffer again as is the way in C. (I'm glad C++ is a thing.)

You were completely correct. It seems that it was pointer's that were throwing me off. After going through the `*.cpp` files and replacing the `const char*` variables with `std::string` I'm now getting ALL results as I expected (amended to original question). All is well converting `const char*` to `std::string`, however, the other way round was a little more complicated, but managed it. Thank you for your assistance, and 'pointing' me in the right direction :) — guyver4mk, Apr 12 '22 at 08:56

C interface for C++ Hashmap library not working correctly

The C++ Object

The C Wrapper

The Test

1 Answers1

Then you have bigger fish to fry