You must check for the return value of fgets to see if you have successfully received something, if not then you should never use your buffer as a string as you are not NUL terminating the buffer.
/* Checking for buffer != NULL is of no use */
/* as buffer will always be not NULL since */
/* since you have allocated it as char buffer[BUFSIZ] */
if (fgets(buffer, BUFSIZ, stdin) == NULL) {
/* buffer may not be a valid string */
}
So what you can do it to initialize the buffer to a NUL string, as soon as you enter the function (after your declarations are done
buffer[0] = 0; /* initialize to NUL string */
now you can use buffer as a string anywhere.
Also note than if BUFSIZ is too big greater than a couple of KB, then your might get seg fault due to stack overflow. If they are too big you could make buffer as "static char" instead of "char".