Comparing unsigned char and EOF

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The golden rule for writing this line is 

   while ((ch = fgetc(stdin)) != EOF)

ch should be int .Your cute trick of making ch unsigned fails because EOF is a signed int quantity.

Ok, let’s now go into the depth……

Step 1: 

ch=fgetc(fp)

fgetc() returns -1 (a signed int). By the golden rules of C ch gets the last octet of bits which is all 1‘s. And hence the value 255. The byte pattern of ch after the execution of 

ch = fgetc(fp);

would thus be 

11111111

Step 2:

ch != EOF

Now EOF is a signed integer and ch is an unsigned char …

Again I refer to the golden rule of C … the smaller guy ch is converted to big size int before comparision so its byte pattern is now 

00000000000000000000000011111111 = (255)10

while EOF is 

11111111111111111111111111111111 = (-1)10

There is no way they can be equal……. Hence the statement to steer the following while-loop

while ((ch = fgetc(stdin)) != EOF)

will never evaluate to false …

And hence the infinite loop .

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