正确使用 printf 中的 double 格式说明符

What is the correct format specifier for double in printf? Is it %f or is it %lf? I believe it's %f, but I am not sure.

Code sample

#include <stdio.h>

int main()
{
   double d = 1.4;
   printf("%lf", d); // Is this wrong?
}

"%f" is the (or at least one) correct format for a double. There is no format for a float, because if you attempt to pass a float to printf, it'll be promoted to double before printf receives it¹. "%lf" is also acceptable under the current standard -- the l is specified as having no effect if followed by the f conversion specifier (among others).

Note that this is one place that printf format strings differ substantially from scanf (and fscanf, etc.) format strings. For output, you're passing a value, which will be promoted from float to double when passed as a variadic parameter. For input you're passing a pointer, which is not promoted, so you have to tell scanf whether you want to read a float or a double, so for scanf, %f means you want to read a float and %lf means you want to read a double (and, for what it's worth, for a long double, you use %Lf for either printf or scanf).

_{1. C99, §6.5.2.2/6: "If the expression that denotes the called function has a type that does not include a prototype, the integer promotions are performed on each argument, and arguments that have type float are promoted to double. These are called the default argument promotions." In C++ the wording is somewhat different (e.g., it doesn't use the word "prototype") but the effect is the same: all the variadic parameters undergo default promotions before they're received by the function.}

It can be %f, %g or %e depending on how you want the number to be formatted. See here for more details. The l modifier is required in scanf with double, but not in printf.

%Lf (note the capital L) is the format specifier for long doubles.

For plain doubles, either %e, %E, %f, %g or %G will do.

For double you can simply use %lf or you may use any of following as per your preference

%e or %E for values in exponential format

%g or %G for either normal or exponential notation, whichever is more appropriate for its magnitude.

Read more at here List of all Format Specifier in C

The correct printf format for double is %lf, exactly as you used it. There's nothing wrong with your code.

Format %lf in printf was not supported in old (pre-C99) versions of C language, which created superficial "inconsistency" between format specifiers for double in printf and scanf. That superficial inconsistency has been fixed in C99.

So in modern C it makes perfect sense to prefer to use %f with float, %lf with double and %Lf with long double consistently in both printf and scanf.