VS2005 and vswprintf

When you upgrade an existing C++ program to Visual Studio 2005, the compiler will tell you any calls to vswprintf lack an argument specified by the C++ standard. Through the magic of C++ function overloading, your code still works, because Microsoft provides both the old-style proprietary three-argument function and the new-style standard four-argument function. The compiler tells you about the new function by means of a warning, and if you're a smart programmer, you've told the compiler to treat warnings as if they were errors in the sense that they prevent an object module from being generated, so there is a fair amount of motivation to adopt the new function so that the warning will go away.

The new parameter is the count of characters in the output buffer. After reading the documentation for this parameter, you might assume that, like the return value, the parameter does not include the terminating 0 character. But that assumption would be wrong; if you pass the number of characters you expect as a return value, the function will return -1 and errno will be set to ERANGE. You need to add 1 to the parameter (after, of course, making sure your buffer is large enough for the terminating 0). MSDN does not document this, and after having guessed at it myself, I searched the web until I found a Linux man page which told me my guess actually coincided with somebody's reality. I've chosen to assume Microsoft plays by the same rules.