This C program defines two functions: _strlen and _strcat. The _strlen function takes a string as input and returns its length. The _strcat function concatenates two strings and returns the resulting string. The main function demonstrates the use of the _strcat function.

The _strcat function takes two parameters: dest and src, which represents the destination and source strings, respectively. The function first finds the length of the destination string using the _strlen function. It then appends a space character to the end of the destination string and sets the index to the length of the destination string.

The function then uses a loop to concatenate the source string to the end of the destination string. It does this by copying each character of the source string to the destination string one by one. The loop continues as long as there are characters in both the destination and source strings.

Finally, the function adds a null character to the end of the concatenated string and returns a pointer to the destination string.

The main function creates two strings: str1 and str2. It then calls the _strcat function with str1 as the destination string and str2 as the source string. It stores the resulting string in a pointer called ptr and prints it to the console.

When run, the program will output the concatenated string "Hello Word!".