printf By Example

C'stdio.h provides a set of function to print values on the screen following a provided format: printf, sprintf, fprintf, dprintf, snprintf and their counterparts to handle variadic arguments vprintf, vsprintf, vfprintf, vdprintf, vsnprintf.

This document is just a set of example to remember how to specify the format.

The format has the following format:

%[<flags>][<field's width>][.<precision>][<length modifier>]<conversion specifier>

Generic Examples

Complete documentation: man 3 printf

Strings/Characters

Format Values Display Comments
%c%c%c%c%c 'H', 'e', 'l', 'l', 'o' Hello  
%s %s "Hello", "World" Hello World  
%10s %s "Hello", "World" =     Hello World= Right justification by default
%-10s %s "Hello", "World" Hello      World Left justification on-demand
%10s "LongLongWord" LongLongWord Minimal width
%10.5s "LongLongWord" =     LongL= Limit the output
%-10.5s "LongLongWord" =LongL     = Limit the output left justified

Integers

Format Values Display Comments
%d 0xDEADBEEF -559038737 %d and %i are aliases
%+d 42 +42 Prefix with '+' for positive numbers
%u 0xDEADBEEF 3735928559 Unsigned representation
%o 0xDEADBEEF 33653337357 Octal representation
%x 0xDEADBEEF deadbeef Hexadecimal representation
%X 0xDEADBEEF DEADBEEF Hexadecimal upper case

Floats

Format Values Display Comments
%.3f double faraday 96485.332 Precision is the number of digits after
%f double faraday 96485.332123 Precision is 6 by default
%.9g double faraday 96485.3321 Precision is the total number of digits
%g double faraday 96485.3 Precision is still 6 by default
%.3g double faraday 9.65e+04  
%+.9f double faraday +96485.332123300 More decimals
%e double faraday 9.648533e+04 Exponent notation
%E double faraday 9.648533E+04 Exponent upper case
%015.2f double faraday 000000096485.33 Padding with 0
% 15.2f double faraday =       96485.33= Padding with spaces (default)
%0+15.2f double faraday +00000096485.33 Some flags combinations
%a double faraday 0x1.78e555060857cp+16 Hexadecimal notation

Pointers

Format Values Display Comments
%p &someFunction 0x559d6478a2f7  
%p &someVariable 0x559d6478d010  

Glibc extensions

Format Values Display Comments
%m nothing Success Display errno
%m nothing No such file or directory Display errno after an error

Go Extensions

Complete documentation: go doc fmt

Generic

Format Values Display Comments
%v {1 3.14} {1 3.14} Generic value
%+v {1 3.14} {a:1 b:3.14} Value with field names
%#v {1 3.14} main.S{a:1, b:3.14} Go-syntax
%T {1 3.14} main.S Type

Strings

Format Values Display Comments
%q "Hello World!" "Hello World!" Quoted strings
% x "\xde\xad\xbe\xbe" de ad be ef Spaced hexadecimal

Booleans

Format Values Display Comments
%t/%t true, false true/false Boolean

Integers

Format Values Display Comments
%b 127 1111111 Base 2
%q 127 '\u007f' Singly quoted character literal

Slices

Format Values Display Comments
%p ["Hello" "World"] 0xc0000c4020 Address of the first element in a slice

Emacs 29.4 (Org mode 9.6.15)

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