Difference(s) between flushall() and _flushall()

This warning is nonsensical; flushall is not POSIX, and to my knowledge, never was. The correct ISO C way to flush all open stdio streams is fflush(NULL);.

To shed some more light on the issue, I believe this is a generic warning message Visual C uses for functions which are not part of the C standard, but which their standard library has for some minimal degree of compatibility. Most of these functions are things like open and read, and correspond in some vague way to the POSIX functions by the same name, and can be thought of as the lower-level primitives underlying stdio, etc. Some, like flushall, are not actually from POSIX, but just based on oddball functions copied from various legacy unices.

Since these functions are not actually part of the C (or C++) standards, and the C standard (and possibly also the C++ standard...?) reserves for the application identifiers which are not explicitly defined or reserved in the standard, it's non-conformant to expose in the standard headers names like open or flushall. POSIX gets around this by using feature test macros by which an application can make the namespace guarantees it wants from the implementation explict, but Microsoft instead took an approach of deprecating the POSIX-like names and offering an alternative set of functions, prefixed with underscores, which makes them part of the namespace reserved for the implementation. The warning is recommending that you use these Microsoft-specific names, so that your code would still compile if you turned on the options (probably controlled by macros of compiler switches) for a strict namespace conformance in the headers.

Unfortunately, writing code the Microsoft-recommended way with the underscore-prefixed names renders your code completely non-portable. Since these names are in the namespace reserved for the implementation, other platforms might happen to use the same names for functions which do completely different things, and being as they're reserved, you would not be allowed to replace or #define around them to remap them to other functions.

In short: I would find the warning flag that's generating this warning, and turn it off. Along with the one that spews FUD that such-and-such standard function is "insecure" and you should use the _s-suffixed function instead. But if the only function you need is fflushall(), simply replace it with fflush(NULL); and you're good to go.