Question

Question 15.4: varargs/varargs1.html

In the example code there, they seem to think ‍‍‍va_arg returns NULL if the end of the list has been reached:

len = strlen(first);

va_start(argp, first);

while((p = va_arg(argp, char *)) != NULL)
    len += strlen(p);

va_end(argp);

But the documentation for va_arg explicitly says this is not the case, and that va_arg will happily go past the end of the list.

This agrees with my experience trying to imitate the above code and getting a segfault as a result.

Answer 1

Indeed, va_arg doesn’t mention returning NULL at the nd of the list.

If there is no next argument, or if
type is not compatible with the type
of the actual next argument (as
promoted according to the default
argument promotions), random errors
will occur.

But the FAQ says:

A call to vstrcat looks something like

char *str = vstrcat("Hello, ", "world!", (char *)NULL);

Which means they assume the caller will pass NULL as the last argument to signal the end of the list.