Question

I am going through a previously written code and I found StringCbPrintf() function

I found declaration on msdn site like this :

HRESULT StringCbPrintf(
  _Out_  LPTSTR pszDest,
  _In_   size_t cbDest,
  _In_   LPCTSTR pszFormat,
  _In_    ...
);

What is _in_ and _out_ here ?

And why is it needed when we already have sprintf() ?

Answer 1

_In_ and _Out_ (note: neither _in_/_out_ as you wrote, nor __In__/__Out__ with double underscores, as written in some other answer) are so called SAL Annotations. They can be used with /analyze compiler option, and can help identify bugs and problems like buffer overruns etc. with raw C buffers and pointers. In addition to MSDN documentation on SAL, you can read also this blog post.

Someone ironically (and wrongly) wrote that:

“In the rest of the world, inputs are const pointers but I guess that
was too simple. :)”

missing the fact that SAL is more powerful than that. In fact, with SAL you can also specify the maximum size of a destination buffer, indicating which parameter contains the destination buffer size; e.g. if you open <strsafe.h> header, you can read that the actual SAL annotations used for StringCbPrintfW (the Unicode version of StringCbPrintf) is something like that:

STRSAFEAPI
StringCbPrintfW(
    __out_bcount(cbDest) STRSAFE_LPWSTR pszDest,
    __in size_t cbDest,
    __in __format_string STRSAFE_LPCWSTR pszFormat,
    ...)
{
    ....

Note how the __out_bcount(cbDest) SAL annotation applied to the pszDest parameter specifies that this is a pointer to an output buffer (__out), which size is expressed in bytes (_bcount) by the parameter cbDest. As you can see, this is a rich annotation (richer than simple “const” or “non const“).

In my opinion, SAL is kind of useless if you write C++ code with robust container classes like std::vector or std::string, which know their own size, etc. But SAL can be useful in C-ish code with raw pointers (like several Win32 APIs).

About the second part of your question:

“Why we need StringCbPrintf if we already have sprintf“

the main reason is that sprintf is an unsafe and buffer overruns-prone function; instead with StringCbPrintf you must specify the maximum size of the destination buffer, and this can help preventing buffer overruns (which are security enemies).