Undefined reference to stdlib (strtof) while compiling
Its because the "strtof" function resides in "stdlib.h", 
"Stdio.h" deals with i/o tasks. 

Since you din't include you get that undefined reference error.
Correct that and compile again. Hopefully the error shall be resolved, else reply in the comments.


  Update1:
  
  After digging around standard gcc stdlib definitions and avr-gcc definitions found out that the above solution works for a gcc and not avr-gcc especially the 8-bit ones which the arduino IDE compiles for uno and the works as expected for 32bit processors like of Due.
  
  Cite:
  
  
  https://www.arduino.cc/reference/en/language/variables/data-types/double/
  https://www.arduino.cc/reference/en/language/variables/data-types/float/
  


Reason:

The size of the double and float is the same, ie: 4 bytes
and even if you use double its actually the float itself and wont gain any precision advantage. 

Solution:

Since the size of the double and the float is same there is not point in having two function definition doing the same job. 
so 


Use "strtod"  instead of "strtof" or 
Define a macro #define strtof(A, B) strtod(A, B) in the begning of your sketch so that the "strtod" handles the functioning of the "strtof".