Question

what is the diference between the two statements and which is the correct one and what should be used?

fp is the file pointer used.

if(fp!= '\n')

getting a warning

C4047: '!=' : 'FILE *' differs in levels of indirection from 'int'

and

if(fp!= NULL)

Not getting any warnings.

Full code snippet as follows

if(fp!= NULL)
    {
        fgets(mystring,BUFSIZ,fp);
        puts(mystring);
    }

Answer 1

what is the diference between the two statements and which is the correct one and what should be used?

About the difference:
fp != '\n' tries to compare a pointer with an integer. This is meaningless.
fp != NULL checks whether fp is a null pointer. This is a sensible thing to do.

About which is the correct one:
This question assumes that one of them actually is correct. (But fp != '\n' is definitely incorrect.)

About what should be used:
That depends on what you’re trying to do.

This code:

fgets(mystring,BUFSIZ,fp);
puts(mystring);

is also buggy because you’re ignoring the return value of fgets, which would tell you whether fgets succeeded and hence whether mystring was set to a valid string.