Question

please look through the following program..

#include <stdio.h>
#include <malloc.h>
#include <iostream>
#include <string.h>

using namespace std;

int main()
{
    char *key1 = (char*)malloc(sizeof(char)*26);
    char *key2 = (char*)malloc(sizeof(char)*26);


    int intkey1 = 10;
    int intkey2 = 10;

    float floatkey1 = 13.5f;
    float floatkey2 = 13.5f;

    double doublekey1 = 18.5;
    double doublekey2 = 18.5;

    char charkey1[10] = "nikhil";
    char charkey2[10] = "nikhil";

    //cout<<sizeof(int)<<" "<<sizeof(float)<<" "<<sizeof(double)<<endl;

    memcpy(key1,&intkey1,sizeof(int));
    memcpy(key2,&intkey2,sizeof(int));

    int offset = sizeof(int);

    memcpy(&key1[offset],&floatkey1,sizeof(float));
    memcpy(&key2[offset],&floatkey2,sizeof(float));

    offset = offset + sizeof(float);

    memcpy(&key1[offset],&doublekey1,sizeof(double));
    memcpy(&key2[offset],&doublekey2,sizeof(double));

    offset = offset + sizeof(double);

    memcpy(&key1[offset],charkey1,sizeof(charkey1));
    memcpy(&key2[offset],charkey2,sizeof(charkey2));


    int diff = memcmp(key1,key2,26);
    if( diff > 0)
        cout<<"Key1 is greater than key2"<<endl;
    else if(diff < 0)
        cout<<"Key1 is smaller than key2"<<endl;
    else
        cout<<"Both keys are equal"<<endl;
}

I am developing a B+ tree for a database engine for which i need a generic keyCompare method…Is this way of comparing byte streams by memcpy always the safe and foolproof or should i need to compare based on the types of the fields present in char key that i get from scanning column information present in the database engine? IS this fastest?

Answer 1

Is this way of comparing byte streams by memcpy always the safe and foolproof

It doesn’t produce the correct result with integers and floats on little-endian architectures. And it can’t handle special floating point values like inf and nan.

or should i need to compare based on the types of the fields present in char key that i get from scanning column information present in the database engine?

It should compare types using type-specific methods. For strings that involves using locales.