Add the sqlite3_vsnprintf() interface.

src/printf.c

@@ -934,21 +934,28 @@ char *sqlite3_mprintf(const char *zFormat, ...){
 ** current locale settings.  This is important for SQLite because we
 ** are not able to use a "," as the decimal point in place of "." as
 ** specified by some locales.
+**
+** Oops:  The first two arguments of sqlite3_snprintf() are backwards
+** from the snprintf() standard.  Unfortunately, it is too late to change
+** this without breaking compatibility, so we just have to live with the
+** mistake.
+**
+** sqlite3_vsnprintf() is the varargs version.
 */
-char *sqlite3_snprintf(int n, char *zBuf, const char *zFormat, ...){
-  char *z;
-  va_list ap;
+char *sqlite3_vsnprintf(int n, char *zBuf, const char *zFormat, va_list ap){
   StrAccum acc;
-
-  if( n<=0 ){
-    return zBuf;
-  }
+  if( n<=0 ) return zBuf;
   sqlite3StrAccumInit(&acc, zBuf, n, 0);
   acc.useMalloc = 0;
-  va_start(ap,zFormat);
   sqlite3VXPrintf(&acc, 0, zFormat, ap);
+  return sqlite3StrAccumFinish(&acc);
+}
+char *sqlite3_snprintf(int n, char *zBuf, const char *zFormat, ...){
+  char *z;
+  va_list ap;
+  va_start(ap,zFormat);
+  z = sqlite3_vsnprintf(n, zBuf, zFormat, ap);
   va_end(ap);
-  z = sqlite3StrAccumFinish(&acc);
   return z;
 }
 

src/sqlite.h.in

@@ -1861,6 +1861,8 @@ void sqlite3_free_table(char **result);
 ** the zero terminator.  So the longest string that can be completely
 ** written will be n-1 characters.
 **
+** ^The sqlite3_vsnprintf() routine is a varargs version of sqlite3_snprintf().
+**
 ** These routines all implement some additional formatting
 ** options that are useful for constructing SQL statements.
 ** All of the usual printf() formatting options apply.  In addition, there
@@ -1924,6 +1926,7 @@ void sqlite3_free_table(char **result);
 char *sqlite3_mprintf(const char*,...);
 char *sqlite3_vmprintf(const char*, va_list);
 char *sqlite3_snprintf(int,char*,const char*, ...);
+char *sqlite3_vsnprintf(int,char*,const char*, va_list);
 
 /*
 ** CAPI3REF: Memory Allocation Subsystem