Mr.C64 Mr.C64 - 1 year ago 123
C Question

Is va_end required after a vsnprintf_s call?

MSDN shows this sample code snippet for


// crt_vsnprintf_s.cpp
#include <stdio.h>
#include <wtypes.h>

void FormatOutput(LPCSTR formatstring, ...)
int nSize = 0;
char buff[10];
memset(buff, 0, sizeof(buff));
va_list args;
va_start(args, formatstring);
nSize = vsnprintf_s( buff, _countof(buff), _TRUNCATE, formatstring, args);
printf("nSize: %d, buff: %s\n", nSize, buff);

int main() {
FormatOutput("%s %s", "Hi", "there");
FormatOutput("%s %s", "Hi", "there!");
FormatOutput("%s %s", "Hi", "there!!");

In this sample,
is called without a matching

Is this a doc bug in MSDN, or should we just call
before invoking
and then let this function do the cleanup (i.e. calling
) for us?

BTW: I tried the above code and it works with VS2015 with Update 3, but I don't know if it's just undefined behavior...

Answer Source

va_end needs to be called for every va_start. From

If there is no corresponding call to va_start or va_copy, or if va_end is not called before a function that calls va_start or va_copy returns, the behavior is undefined.

Not only do you need va_end but you also need to make sure that your function does not return before va_end is executed if va_start or va_copy is executed.

To answer your question - yes, this is a documentation bug in MSDN.

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