printf

"printf" is a function in the C-standard library that outputs text to standard output. Standard output is typically the terminal or debug console, depending on the system the program is running on. The f stands for formatted, allowing the function to output not just fixed strings, but also text with variable data in it.

Hello world

The simplest of all C-programs is known as "Hello world" program. It outputs "Hello world", using printf.

#include <stdio.h>

int main(void) {
  printf("Hello world!\n");
  return 0;
}

Using parameters

Parameters need to be specified in the format string. A parameter definition starts with a % character. A simple example with one numerical parameter looks as below:

  printf("Total sum is: %d\n", Sum);

Parameter specification

Type field

The Type field can be any of:

Character	Description
%	Prints a literal % character (this type doesn't accept any flags, width, precision, length fields).
d, i	int as a signed decimal number. %d and %i are synonymous for output, but are different when used with scanf() for input (where using %i will interpret a number as hexadecimal if it's preceded by 0x, and octal if it's preceded by 0.)
u	Print decimal unsigned int.
f, F	double in normal (fixed-point) notation. f and F only differs in how the strings for an infinite number or NaN are printed (inf, infinity and nan for f; INF, INFINITY and NAN for F).
e, E	double value in standard form ([-]d.ddd e[+/-]ddd). An E conversion uses the letter E (rather than e) to introduce the exponent. The exponent always contains at least two digits; if the value is zero, the exponent is 00. In Windows, the exponent contains three digits by default, e.g. 1.5e002, but this can be altered by Microsoft-specific _set_output_format function.
g, G	double in either normal or exponential notation, whichever is more appropriate for its magnitude. g uses lower-case letters, G uses upper-case letters. This type differs slightly from fixed-point notation in that insignificant zeroes to the right of the decimal point are not included. Also, the decimal point is not included on whole numbers.
x, X	unsigned int as a hexadecimal number. x uses lower-case letters and X uses upper-case.
o	unsigned int in octal.
s	null-terminated string.
c	char (character).
p	void * (pointer to void) in an implementation-defined format.
a, A	double in hexadecimal notation, starting with 0x or 0X. a uses lower-case letters, A uses upper-case letters. (C++11 iostreams have a hexfloat that works the same).
n	Print nothing, but writes the number of characters successfully written so far into an integer pointer parameter. Java: indicates a platform neutral newline/carriage return. Note: This can be utilized in Uncontrolled format string exploits.