Program to find length of string and concanate two strings

/*Serial No.121     [swami100.cpp]*/

/* C program which read two strings and concatenate the largest string to the smallest string (among the two strings).*/
#include<stdio.h>
#include<string.h>
#include<conio.h>

int leng(char str[20]);
void concat(char str4[20], char str5[20]);
void main()
{
char str1[20],str2[20],str3[20];
int i,j,len,first,second;
clrscr();
printf("Enter the first string:\n");
gets(str1);

printf("Enter the second string:\n");
gets(str2);
first=leng(str1);
second=leng(str2);
printf("\nLength of 1st string= %d",first);
printf("\nLength of 2nd string= %d",second);

if(first<=second)
    {
    concat(str1,str2);
    }
else
    {
    concat(str2,str1);
    }
getch();
}

int leng(char str[20])
    {
    int i=0;
    while(str[i]!='\0')
        {
        i++;
        }
    return(i);
    }

void concat(char str4[20], char str5[20])
    {
    int i=0,j;
    char str6[20];
    while(str4[i]!='\0')
        {
        str6[i]=str4[i];
        i++;
        }
    j=0;
    while(str5[j]!='\0')
        {
        str6[i]=str5[j];
        i++;j++;
        }
    str6[i]='\0';
    printf("\n\nConcanated string= %s",str6);
    }