title: C compilation errors: undeclared (first use in this function)
body: <p>On the PI, I needed the i2c.so library using this git: <a href="https://github.com/silentbobbert/pi_sensors" rel="nofollow noreferrer">https://github.com/silentbobbert/pi_sensors</a>. When running makefile from this git to get the i2c.so, i received this error:</p>
  
  <p><img src="https://i.stack.imgur.com/qbi3S.png" alt="enter image description here">
  <img src="https://i.stack.imgur.com/OaUca.png" alt="enter image description here"></p>
  
  <p>Here are the .c and the .h files:
  <a href="https://github.com/silentbobbert/pi_sensors/tree/master/Info/LinuxInterface" rel="nofollow noreferrer">https://github.com/silentbobbert/pi_sensors/tree/master/Info/LinuxInterface</a></p>
  
  <p>For reference, here is the contents of makefile:</p>
  
  <pre><code>SHELL = /bin/sh
  CC    = gcc
  FLAGS        = -c -Wall -Werror -fpic
  DEBUGFLAGS   = -O0 -D _DEBUG
  RELEASEFLAGS = -O2 -D NDEBUG
  
  TARGET  = i2c.so
  SOURCES = $(shell echo *.c)
  HEADERS = $(shell echo *.h)
  OBJECTS = $(SOURCES:.c=.o)
  
  PREFIX = $(DESTDIR)/usr/local
  BINDIR = $(PREFIX)/bin
  
  all:
      $(CC) $(FLAGS) $(RELEASEFLAGS) $(SOURCES)
      $(CC) -shared -o $(TARGET) $(OBJECTS)
  </code></pre>