Correctly handle zero-length buffers.

diff --git a/src/core/vsprintf.c b/src/core/vsprintf.c
index ada1f13de853ad420a71e0606421c64bb7dd1536..481ca26d2824e28682d3d72892b9136edef32999 100644 (file)
--- a/src/core/vsprintf.c
+++ b/src/core/vsprintf.c
@@ -171,7 +171,7 @@ static char * format_decimal ( char *end, signed long num, int width ) {
  * @v args             Arguments corresponding to the format string
  * @ret len            Length of formatted string
  */
-int vcprintf ( struct printf_context *ctx, const char *fmt, va_list args ) {
+size_t vcprintf ( struct printf_context *ctx, const char *fmt, va_list args ) {
        int flags;
        int width;
        uint8_t *length;
@@ -296,14 +296,8 @@ static void printf_sputc ( struct printf_context *ctx, unsigned int c ) {
  */
 int vsnprintf ( char *buf, size_t size, const char *fmt, va_list args ) {
        struct printf_context ctx;
-       int len;
-
-       /* Ensure last byte is NUL if a size is specified.  This
-        * catches the case of the buffer being too small, in which
-        * case a trailing NUL would not otherwise be added.
-        */
-       if ( size != PRINTF_NO_LENGTH )
-               buf[size-1] = '\0';
+       size_t len;
+       size_t end;
 
        /* Hand off to vcprintf */
        ctx.handler = printf_sputc;
@@ -312,7 +306,12 @@ int vsnprintf ( char *buf, size_t size, const char *fmt, va_list args ) {
        len = vcprintf ( &ctx, fmt, args );
 
        /* Add trailing NUL */
-       printf_sputc ( &ctx, '\0' );
+       if ( size ) {
+               end = size - 1;
+               if ( len < end )
+                       end = len;
+               buf[end] = '\0';
+       }
 
        return len;
 }