Why does scanf() need "%lf" for doubles, when printf() is okay with just "%f"?

c scanf length-modifiers
raldi picture raldi · Oct 17, 2008 · Viewed 304.5k times · Source

Why is it that scanf() needs the l in "%lf" when reading a double, when printf() can use "%f" regardless of whether its argument is a double or a float?

Example code:

double d;
scanf("%lf", &d);
printf("%f", d);

Answer

MSN picture MSN · Oct 17, 2008

Because C will promote floats to doubles for functions that take variable arguments. Pointers aren't promoted to anything, so you should be using %lf, %lg or %le (or %la in C99) to read in doubles.

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