C language Essay _fopen

A classmate asked me, the following code will output "= = =", why AH?

if (fp = fopen ("Data.dat", "R")) ==null) {printf ("= = =");}

I looked at the code is to "read" the way to open the Data.dat file, but the code does not specify where Data.dat, the program cannot find the file, so it is considered an error.
What do we do? Either create a Data.dat file and specify the path to the file, or open the file as "write", the Data.dat file does not exist, and the program creates a new Data.dat file.

C language Essay _fopen