strcat Function in c++ - c++

Question

I'm new to C and C++ programming, can anyone give me a hint on what I'm doing wrong here. I'm trying to write to concat function that takes to pointers to chars and concatenates the second to the first. The code does do that, but the problem is that it adds a bunch of junk at the end. For instance, when passing the arguments - "green" and "blue", the output will be "greenblue" plus a bunch of random characters. I also wrote the strlen function that strcat uses, which I will provide below it for reference. I'm using the online compiler at https://www.onlinegdb.com/online_c++_compiler
The exact instructions and specification is this:
The strcat(char *__s1, const char *__s2) functions concatenates the contents of __s2 onto __s1 beginning with the NULL character of __s1. Note: The concatenation includes the NULL character of __s2. The function returns __s1.
int main(int argc, char** argv)
{
const int MAX = 100;
char s1[MAX];
char s2[MAX];
cout << "Enter your first string up to 99 characters. ";
cin.getline(s1, sizeof(s1));
int size_s1 = strlen(s1);
cout << "Length of first string is " << size_s1 << "\n";
cout << "Enter your second string up to 99 characters. ";
cin.getline(s2, sizeof(s2));
int size_s2 = strlen(s2);
cout << "Length of second string is " << size_s2 << "\n";
cout << " Now the first string will be concatenated with the second
string ";
char* a = strcat(s1,s2);
for(int i = 0; i<MAX; i++)
cout <<a[i];
// system("pause");
return 0;
}
//strcat function to contatenate two strings
char* strcat(char *__s1, const char *__s2)
{
int indexOfs1 = strlen(__s1);
int s2L = strlen(__s2);
cout <<s2L << "\n";
int indexOfs2 = 0;
do{
__s1[indexOfs1] = __s2[indexOfs2];
indexOfs1++;
indexOfs2++;
}while(indexOfs2 < s2L);
return __s1;
}
//Returns length of char array
size_t strlen(const char *__s)
{
int count = 0;
int i;
for (i = 0; __s[i] != '\0'; i++)
count++;
return (count) / sizeof(__s[0]);
}

Answer 1

The behavior you are seeing is a result of the null terminator of __s1 being overwritten by data from __s2 and no new null terminator being appended. The extra characters you are seeing are just random values in RAM that happen to be after the end of your string. To prevent this a NULL character MUST be added at the end of your string.
A working version is as follows:
char* strcar(char *__s1, const char *__s2)
{
//check inputs for NULL
if(__s1 == NULL || __s2 == NULL)
return NULL;
int s1Length = strlen(__s1);
int s2Length = strlen(__s2);
//ensure strings do not overlap in memory
if(!(__s1 + s1Length < __s2 || __s2 + s2Length < __s1))
return NULL;
//append __s2 to __s1
//the "+ 1" here is necessary to copy the NULL from the end of __s2
for(int i = 0; i < s2Length + 1; i++)
result[s1Length + i] = __s2[i];
}

Answer 2

You Need to add a trailing "\0"-char at the end of __s1.
e.g. insert
__s1[indexOfs1] = 0;
before your return-line.