Reuse of va_list

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);
}