concatenate two strings in c

1char str[80];
2strcpy(str, "these ");
3strcat(str, "strings ");
4strcat(str, "are ");
5strcat(str, "concatenated.");

1#include <stdio.h>
2#include <string.h>
3int main() {
4	char str[80];
5strcpy(str, "these ");
6strcat(str, "strings ");
7strcat(str, "are ");
8strcat(str, "concatenated.");
9printf("%s\n", str);
10}

1#include <stdio.h>
2#include <string.h>
3int main()
4{
5  char a[1000], b[1000];
6
7  printf("Enter the first string\n");
8  gets(a);
9
10  printf("Enter the second string\n");
11  gets(b);
12
13  strcat(a, b);
14
15  printf("String obtained on concatenation: %s\n", a);
16
17  return 0;
18}

1// this program is to contenate strings without using string function
2
3#include <stdio.h>
4#include <string.h>
5
6void concatenate(char *str1, char *str2)
7{
8    int i = strlen(str1), j = 0;
9
10    while (str2[j] != '\0')
11    {
12        str1[i] = str2[j];
13        i++;
14        j++;
15    }
16
17    str1[i] = '\0';  // declaring the end of the string
18}
19
20int main()
21{
22  char string1[20], string2[20];
23  gets(string1);
24  gets(string2);
25  
26  concatenate(string1, string2);
27  
28  printf("String 1 : ");
29  puts(string1);
30  
31  return 0;
32}