Reuse of va_list - c

Question

I need to do two (or more) passes over a va_list. I have a buffer of some size, and I want to write a formatted string with sprintf into it. If the formatted string doesn't fit in the allocated space I want to double the allocated space and repeat until it fits.
(As a side-note, i would like be able to calculate the length of the formatted string first and allocate enough space, but the only function that I found that can do that is _snprintf, and it is deprecated in VS2005 ...)
Now, so far there are no problems: i use vsnprintf and call va_start before each invokation.
But I've also created a function that takes a va_list as a parameter, instead of "...". Then I cannot use va_start again! I've read about va_copy, but it is not supported in VS2005.
So, how would you do this?

Answer 1

A previous question about the lack of va_copy in MSVC had some decent enough suggestions, including to implement your own version of va_copy for use in MSVC:
#define va_copy(d,s) ((d) = (s))
You might want to throw that into a 'portability' header protected by an #ifndef va_copy and #ifdef _MSC_VER for use on VC.

Answer 2

The va_copy() is supposed to be part of the C99 specification; as with all variadic parameter support is very platform dependent. The va_list, va_copy(), va_start(), va_end() macros are defined in stdarg.h .
GCC: When attempting to reuse a va_list on GCC, one MUST use va_copy(), as the GCC implementation causes the va_list to be modified, causing the pointer to be positioned after last param after use by a v??printf() function.
SUN: When attempting to reuse a va_list in SunStudio (v11,v12), the va_list variable is untouched and can be reused as many times as desired without needing to va_copy().
MS_Visual C: Not certain, but looks like the 2010 VC++ docs do mention 'va_copy()' and may imply reuse of va_list is reasonable, but should be tested.
Example:
#include <stdio.h>
#include <stdarg.h>
/**
* Version of vsprintf that dynamically resizes the given buffer to avoid overrun.
* Uses va_copy() under GCC compile.
**/
int my_vsprintf(char **buffer, char *msg, va_list args)
{
int bufLen = 0;
va_list dupArgs; // localize args copy
#ifdef __GNUC__
va_copy(dupArgs,args); // Clone arguments for reuse by different call to vsprintf.
#else
dupArgs = args; // Simply ptr copy for GCC compatibility
#endif
// Perform 1st pass to calculate required buffer size. The vsnprintf() funct
// returns the number of chars (excluding \0 term) necessary to produce the output.
// Notice the NULL pointer, and zero length.
bufLen = vsnprintf(NULL,0,msg, dupArgs);
// NOTE: dupArgs on GCC platform is mangled by usage in v*printf() and cannot be reused.
#ifdef __GNUC__
va_end(dupArgs); // cleanup
#endif
*buffer = realloc(*buffer,bufLen + 1); // resize buffer, with \0 term included.
#ifdef __GNUC__
va_copy(dupArgs,args); // Clone arguments for reused by different call to vsprintf.
#endif
// Perform 2nd pass to populate buffer that is sufficient in size,
// with \0 term size included.
bufLen = vsnprintf(buffer, bufLen+1, msg, dupArgs);
// NOTE: dupArgs on GCC platform is mangled by usage in v*printf() and cannot be reused.
#ifdef __GNUC__
va_end(dupArgs); // cleanup
#endif
return(bufLen); // return chars written to buffer.
}
/**
* Version of sprintf that dynamically resizes the given buffer to avoid buffer overrun
* by simply calling my_vsprintf() with the va_list of arguments.
*
* USage:
**/
int my_sprintf(char **buffer, char *msg, ...)
{
int bufLen = 0;
va_list myArgs;
va_start(myArgs, msg); // Initialize myArgs to first variadic parameter.
// re-use function that takes va_list of arguments.
bufLen = my_vsprintf(buffer, msg, myArgs );
va_end(myArgs);
}

Answer 3

Late to reply but hopefully someone will find this useful.
I needed to use va_copy but being unavailable in vc2005, I searched and found myself on this page.
I was a little concerned by using the seemingly crude:
va_copy(d,s) ((d) = (s))
So I did a little digging around. I wanted to see how va_copy was implemented in vc2013, so I compiled a test program that uses va_copy and disassembled it:
First, its usage according to msdn:
void va_copy(
va_list dest,
va_list src
); // (ISO C99 and later)
disassembly of va_copy as implemented in msvcr120 (the whole 7 lines!):
PUSH EBP
MOV EBP,ESP
MOV EAX,DWORD PTR SS:[EBP+8] ;get address of dest
MOV ECX,DWORD PTR SS:[EBP+0C] ;get address of src
MOV DWORD PTR DS:[EAX],ECX ;move address of src to dest
POP EBP
RETN
So as you can see, it really is as simple as assigning the value of the src va_list to the dest va_list before parsing.
Since I only use ms compilers, and currently use vc2005 with the possibility of upgrading in the future, here's what I went with:
#if _MSC_VER < 1800 // va_copy is available in vc2013 and onwards
#define va_copy(a,b) (a = b)
#endif

Answer 4

I see of no portable way (and I think that va_copy has been introduced in C99 because there was no portable way to achieve its result in c89). A va_list can be a reference type mock up declared as
typedef struct __va_list va_list[1];
(see gmp for another user of that trick) and that explains a lot of the language constraints around them. BTW, don't forget the va_end if portability is important.
If portability is not important, I'd check stdard.h and see if I can hack something considering the true declaration.