concatenate « string « C Array Q&A

1. concatenate char array in C    stackoverflow.com

I have a a char array:

char* name = "hello";

I want to add an extension to that name to make it

hello.txt

How can I do this? name += ".txt" won't work

2. Concatenate two char arrays?    stackoverflow.com

If I have two char arrays like so:

char one[200];
char two[200];

And I then want to make a third which concatenates these how could I do it? I have tried:

char three[400];
strcpy(three, one);
strcat(three, two);

But this ...