I have a ino project and tried to add a C file and its associated h file.
The project would not build. So I added the C file as a ino file and deleted the included h file.
The project built, downloaded succesfully to my surprise.

I would like to know why adding the *.c and *.h files did not work and how the the build worked without having
the h file included.

I come from a C background.

Thanks

I figured it out by lots of reading on the  Arduino forum.

Adding *.c and *h files to an arduino project you have to change the extension of the *.c file to *.cpp and place them in the same folder of the *.ino file. 

There is another way the files can be added and that is to again change your *.c to a *.cpp and put both *.cpp and *.h files in their own folder (the folder must be named exactly the same as the *.h and  *.c files) then place the folder as a subfolder under the ... Arduino\libraries\    folder. 

Now building the file will not produce the errors and it runs as it is supposed to.

The files are located in the same folder as my *.ino file. I tried using "add existing item" and although added to
the project it did not build.

Then i put them in their own /src folder below the *.ino file and although it did add them to the project it also 
did not work.

Here are the errors. 
=============================
Compiling 'I2C' for 'Arduino/Genuino Uno'
Build folder: file:///C:/Users/All%20surface/AppData/Local/Temp/VMicroBuilds/I2C/uno
 
ccVqbBDf.ltrans0.ltrans.o*: In function main
ccVqbBDf.ltrans0.o*: (.text.startup+0xd0): undefined reference to i2c_init()
ccVqbBDf.ltrans0.o*: (.text.startup+0xd6): undefined reference to i2c_start(unsigned char)
ccVqbBDf.ltrans0.o*: (.text.startup+0xe2): undefined reference to i2c_stop()
ccVqbBDf.ltrans0.o*: (.text.startup+0xf2): undefined reference to i2c_write(unsigned char)
ccVqbBDf.ltrans0.o*: (.text.startup+0xfe): undefined reference to i2c_write(unsigned char)
ccVqbBDf.ltrans0.o*: (.text.startup+0x106): undefined reference to i2c_write(unsigned char)
ccVqbBDf.ltrans0.o*: (.text.startup+0x10e): undefined reference to i2c_write(unsigned char)
ccVqbBDf.ltrans0.o*: (.text.startup+0x116): undefined reference to i2c_write(unsigned char)
ccVqbBDf.ltrans0.ltrans.o*: ccVqbBDf.ltrans0.o:(.text.startup+0x11e): more undefined references to i2c_write(unsigned char) follow
 
ccVqbBDf.ltrans0.ltrans.o*: In function main
ccVqbBDf.ltrans0.o*: (.text.startup+0x144): undefined reference to i2c_rep_start(unsigned char)
ccVqbBDf.ltrans0.o*: (.text.startup+0x14a): undefined reference to i2c_write(unsigned char)
ccVqbBDf.ltrans0.o*: (.text.startup+0x150): undefined reference to i2c_rep_start(unsigned char)
ccVqbBDf.ltrans0.o*: (.text.startup+0x154): undefined reference to i2c_readAck()
ccVqbBDf.ltrans0.o*: (.text.startup+0x15c): undefined reference to i2c_readAck()
ccVqbBDf.ltrans0.o*: (.text.startup+0x164): undefined reference to i2c_readAck()
ccVqbBDf.ltrans0.o*: (.text.startup+0x16c): undefined reference to i2c_readAck()
ccVqbBDf.ltrans0.o*: (.text.startup+0x174): undefined reference to i2c_readAck()
ccVqbBDf.ltrans0.ltrans.o*: ccVqbBDf.ltrans0.o:(.text.startup+0x17c): more undefined references to i2c_readAck() follow
 
ccVqbBDf.ltrans0.ltrans.o*: In function main
ccVqbBDf.ltrans0.o*: (.text.startup+0x184): undefined reference to i2c_readNak()
 
collect2.exe*: error: ld returned 1 exit status

Error compiling for board Arduino/Genuino Uno
========================

Yes that makes sense

Thanks