Program to find length of string and concanate two strings using function by passing them as parameter

/*Serial No.117     [swami94.cpp]*/

#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;
clrscr();
printf("Enter the first string:\n");
gets(str1);

printf("Enter the second string:\n");
gets(str2);

concat(str1,str2);

len=leng(str1);
printf("\n\n\nlength of first string using return=%d",len);

getch();
}

int leng(char str[20])
    {
    int i=0;
    while(str[i]!='\0')
    {
    i++;
    }
printf("\n\n\nlength of first string=%d",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\nConcanate string= %s",str6);
    }