Incorrect result printed when multiplying numbers in C

I've recently begun using C and I'm having some problems with the simple arithmetic operation of just being able to multiply two numbers.

Initially, I tried to print the result of 8 * 2 to a file. Currently this outputs 16. Next, I multiplied what I really wanted 8*0.000000123456789. Here is my code:

            fprintf( fp2, "%d\n", (long double)(8*0.000000123456789));

The result outputted to the file is : -1086490624. W开发者_如何学JAVAhereas really it should have been : 0.000000987654312. I intentionally type-casted the arithmetic as a long double to prevent any overflow. Not doing anything and using:

            fprintf( fp2, "%d\n", (8*0.000000123456789));

Yields 1719134127, again having no relation to the actual value of 0.000000987654312

Any suggestions please?

I believe that your problem is that you're using the %d placeholder in printf, which only prints out int values. Since you're passing a double or long double to printf, you probably want to use another modifier in place of %d. For example, if you're printing out a double, you could use the %f modifier:

fprintf( fp2, "%f\n", (8*0.000000123456789));

The more technical reason that you're getting random values back instead of anything close to the original value is that when you try printing something with printf, the function tries interpreting the bits of the argument in a way that depends on what specifier you use. Since floating-point values like floats and doubles are usually represented internally in the computer using a completely different encoding than integers (often as IEEE-754 values rather than signed two's complement), the results of trying to take bits of a float or double and representing them as an integer are usually meaningless.

Hope this helps!

Suggestion 1: Read the manual for printf.

Suggestion 2: Repeat to yourself over and over: "printf is not typesafe. I promise to be extra careful when using it."

Suggestion 3: Always always enable and take to heart all compiler warnings. This would have caught it right away.

Answer: You promise printf that you will provide an int by saying %d. However, in reality you provide a long double. The result of this failure to keep your promise is undefined behaviour. To print a long double, say %Lf (cf. (1)).