Re: _sntprintf and _sntprintf_s
"Norman Diamond" <ndiamond@community.nospam> wrote in message
news:udOyprwgGHA.1856@TK2MSFTNGP03.phx.gbl
The search function of MSDN library United
States found the page for _snwprintf_s in MSDN library Korea but not
in MSDN library United States. Some manual searching found it in
MSDN library United States.
Now for some strange looking statements in page
http://msdn2.microsoft.com/en-us/library/f30dzcf6.aspx
First are two important statements which are only a bit strange:
sizeOfBuffer
The size of the storage location for output. Size in bytes for
_snprintf_s or size in words for _snwprintf_s.
Count
Maximum number of characters to store, or _TRUNCATE.
OK, the sizeOfBuffer parameter is the same as the count parameter of
the old _snwprintf function. This really specifies the size of the
buffer. The new Count parameter specifies a maximum number of
characters.
Wrong. The count parameter of the old function is the same as the count
parameter of the new function. The sizeOfBuffer parameter exists in the new
function and has no counterpart in the old.
In both the old and new functions, you use the count parameter to give you
manual control of the number of characters copied for reasons that may or
may not be related to buffer size, e.g., you may only want to copy 3
characters even though the source buffer is a thousand characters long and
the destination buffer could hold a million.
In the case of sprintf and swprintf, there is no count parameter at all.
Likewise, there is no count parameter in the new functions, sprintf_s and
swprintf_s. However, you have a new sizeOfBuffer parameter.
I suppose the dual limit is a convenience, but I don't
understand how the level of security improves. The size of the
buffer is still specified the same way even though the parameter name
has changed, and the new Count parameter specifies a different
alternative limit instead of the size of the buffer.
As noted above, the count parameter does not specify the size of the buffer.
Moreover, you are attempting to infer the total properties of the function
from its argument list. The differences go beyond that. In particular,
_snprintf/_snwprintf do not null terminate a string if the length of the
string is >= count, which creates the risk of buffer overruns.
Read the remarks under Return value and other remarks on the page you have
linked to.
_snprintf_s returns the number of characters stored in buffer, not
counting the terminating null character. _snwprintf_s returns the
number of wide characters stored in buffer, not counting the
terminating null wide character.
This looks terrible. Actually _snwprintf_s looks mostly OK, since the
programmer wants to do calculations on the number of wide characters
and keep track of how much space remains in the buffer. But why does
_snprintf_s return the number of characters instead of the number of
bytes?
Um...because a byte and a char have the same size?
If a programmer is using the ANSI version then the programmer
wants to do calculations on the number of bytes in order to keep
track of how much space remains in the buffer. I also hope that
_snwprintf_s doesn't have a split personality on the treatment of
surrogate pairs.
In C++, using these functions is simplified by template overloads;
the overloads can infer buffer length automatically (eliminating the
need to specify a size argument) and they can automatically replace
older, non-secure functions with their newer, secure counterparts.
For more information, see Secure Template Overloads.
I'm not sure how Secure Template Overloads can infer buffer length
automatically, but maybe they call some undocumented memory management
functions,
Try this.
#include <iostream>
using namespace std;
template<int n>
void foo(char (&buff)[n])
{
cout << "sizeof buffer is " << n << endl;
}
int main()
{
char buffer[12];
foo(buffer);
}
so OK, I read that page too.
http://msdn2.microsoft.com/en-us/library/ms175759.aspx
To enable template overloads for the count functions, define
_CRT_SECURE_CPP_OVERLOAD_STANDARD_NAMES_COUNT to be 1. Before doing
so, however, make sure that your code passes the count of
characters, not the size of the buffer (a common mistake).
Um, no wonder it's common for my code to pass the size of the buffer
instead of the count of characters. Look again at the sizeOfBuffer
parameter to _sntprintf_s and the count parameter to the old
_sntprintf function. In the Unicode version it's supposed to be the
size in WCHAR_Ts and in the ANSI version it's supposed to be the size
in bytes. In both cases it should be the size in TCHARs.
That is the same thing.
--
John Carson