+Arguments:
+ g a growable-string
+ func called-from function name, for debug
+ line called-from file line number, for debug
+ limit maximum string size
+ flags see below
+ format printf-like format string
+ ap variable-args pointer
+
+Flags:
+ SVFMT_EXTEND buffer can be created or exteded as needed
+ SVFMT_REBUFFER buffer can be recopied to tainted mem as needed
+ SVFMT_TAINT_NOCHK do not check inputs for taint
+