C conversion specifiers and modifiers

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Tagged: C

A handy reference to the C conversion specifiers used with printf() and scanf(), like %d, %u, %s, and %f, plus the modifiers that set the field width.

In this post I want to create a helpful reference for the C conversion specifiers you can use, commonly with printf(), scanf() and similar I/O functions. See also basic I/O and C variables and types.

SpecifierMeaning
%d / %iSigned decimal integer
%uUnsigned decimal integer
%cUnsigned char
%sString
%pPointer in hexadecimal form
%oUnsigned octal integer
%x / %XUnsigned hexadecimal number
%eFloating point number in exponential format in e notation
%EFloating point number in exponential format in E notation
%fdouble number in decimal format
%g / %Gdouble number in decimal format or exponential format depending on the value
%%Literal %
%a / %AHexadecimal floating point (C99)
%b / %BBinary integer output (C23). printf("%b", 5) prints 101. Needs a recent libc (glibc since 2.35). macOS and MSVC support may vary.
%nWrites the number of characters printed so far into an int* argument

In addition to those specifiers, we have a set of modifiers.

Let’s start with digits. Using a digit between % and the format specifier, you can tell the minimum field width. Example: %3d will take 3 spaces regardless of the number printed.

This:

printf("%4d\n", 1);
printf("%4d\n", 12);
printf("%4d\n", 123);
printf("%4d\n", 1234);

should print

   1
  12
 123
1234

If you put a dot before the digit, you are now telling the precision: the number of decimal digits. This of course applies to decimal numbers. Example:

printf("%4.2f\n", 1.0);
printf("%4.3e\n", 12.232432442);
printf("%4.1e\n", 12.232432442);
printf("%4.1f\n", 123.22);

will print:

1.00
1.223e+01
1.2e+01
123.2

In addition to digits, we have length modifiers. The common ones are h, l and L, plus a few from C99:

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