mprintf: use snprintf if available

This is the single place in libcurl code where it uses the "native"
s(n)printf() function. Used for writing floats. The use has been
reviewed and vetted and uses a HUGE target buffer, but switching to
snprintf() still makes this safer and removes build-time warnings.

Reported-by: Philip Heiduck

Fixes curl#9569
Closes curl#9570

CMakeLists.txt

@@ -1079,6 +1079,11 @@ check_symbol_exists(getrlimit      "${CURL_INCLUDES}" HAVE_GETRLIMIT)
 check_symbol_exists(setlocale      "${CURL_INCLUDES}" HAVE_SETLOCALE)
 check_symbol_exists(setmode        "${CURL_INCLUDES}" HAVE_SETMODE)
 check_symbol_exists(setrlimit      "${CURL_INCLUDES}" HAVE_SETRLIMIT)
+
+if(NOT MSVC OR (MSVC_VERSION GREATER_EQUAL 1900))
+  # earlier MSVC compilers had faulty snprintf implementations
+  check_symbol_exists(snprintf       "${CURL_INCLUDES}" HAVE_SNPRINTF)
+endif()
 check_function_exists(mach_absolute_time HAVE_MACH_ABSOLUTE_TIME)
 check_symbol_exists(inet_pton      "${CURL_INCLUDES}" HAVE_INET_PTON)
 

configure.ac

@@ -3535,6 +3535,7 @@ AC_CHECK_FUNCS([fnmatch \
   setlocale \
   setmode \
   setrlimit \
+  snprintf \
   utime \
   utimes
 ],[

lib/config-win32.h

@@ -254,6 +254,11 @@
 /* Define to the function return type for send. */
 #define SEND_TYPE_RETV int
 
+/* Define to 1 if you have the snprintf function. */
+#if defined(_MSC_VER) && (_MSC_VER >= 1900)
+#define HAVE_SNPRINTF 1
+#endif
+
 /* ---------------------------------------------------------------- */
 /*                       TYPEDEF REPLACEMENTS                       */
 /* ---------------------------------------------------------------- */

lib/mprintf.c

@@ -964,7 +964,11 @@ static int dprintf_formatf(
 #endif
         /* NOTE NOTE NOTE!! Not all sprintf implementations return number of
            output characters */
+#ifdef HAVE_SNPRINTF
+        (snprintf)(work, sizeof(work), formatbuf, p->data.dnum);
+#else
         (sprintf)(work, formatbuf, p->data.dnum);
+#endif
 #ifdef __clang__
 #pragma clang diagnostic pop
 #endif