Get yourself a copy of the library manual.
23.11.3.14 char * strcat ( char * dest, const char * src )
Concatenate two strings.
The strcat() function appends the src string to the dest string overwriting the '\0' character at the end of dest, and then adds a terminating '\0' character. The strings may not overlap, and the dest string must have enough space for the result.
Returns
The strcat() function returns a pointer to the resulting string dest.