error: use of undeclared identifier 'tolower' compiling code

Error:

clang++  -I.  -MMD -I/usr/local/Cellar/verilator/3.900/share/verilator/include -I/usr/local/Cellar/verilator/3.900/share/verilator/include/vltstd -DVL_PRINTF=printf -DVM_COVERAGE=0 -DVM_SC=0 -DVM_TRACE=0 -Wno-char-subscripts -Wno-parentheses-equality -Wno-sign-compare -Wno-uninitialized -Wno-unused-parameter -Wno-unused-variable -fbracket-depth=4096 -Qunused-arguments     -std=c++11 -I../../../../common -lncurses   -c -o blinky.o ../../../simulator/blinky.cpp
../../../simulator/blinky.cpp:24:12: error: use of undeclared identifier
      'tolower'
    switch(tolower(userCh)) {
           ^

My CFLAGS are -std=c++11 -I../../../../common -lncurses

What am I doing wrong?