I have an double value that I need to add to a sting. I was doing the following messages below. Appreciate any ideas! stringstream out; double mark1 = 10; out << mark1; string str2 = "part1"; str2 = str2 + out.str(); There are no errors until I compile with: *** Using Compiler 'V5.06 (build 20)', folder: 'C:\Keil_v5\ARM\ARMCC\Bin' Build target 'mbed NUCLEO_F091RC' compiling main.cpp... main.cpp(304): warning: #177-D: variable "mark1" was declared but never referenced int mark1 = 10; main.cpp: 1 warning, 0 errors linking... .\build\Nucleo_blink_led.axf: Error: L6218E: Undefined symbol __fread_bytes_avail (referred from ios.o). .\build\Nucleo_blink_led.axf: Error: L6218E: Undefined symbol mbsinit (referred from ios.o). .\build\Nucleo_blink_led.axf: Error: L6218E: Undefined symbol wmemmove (referred from ios.o). Not enough information to list image symbols. Finished: 1 information, 0 warning and 3 error messages. ".\build\Nucleo_blink_led.axf" - 3 Error(s), 1 Warning(s). Target not created. Build Time Elapsed: 00:00:02