#include <stdio.h>
  int printf( const char *format, ... );

The printf() function prints output to STDOUT, according to format and other arguments passed to printf(). The string format consists of two types of items - characters that will be printed to the screen, and format commands that define how the other arguments to printf() are displayed. Basically, you specify a format string that has text in it, as well as "special" characters that map to the other arguments of printf(). For example, this code

   char name[20] = "Bob";
   int age = 21;
   printf( "Hello %s, you are %d years old\n", name, age );           

displays the following output:

   Hello Bob, you are 21 years old              

The %s means, "insert the first argument, a string, right here." The %d indicates that the second argument (an integer) should be placed there. There are different %-codes for different variable types, as well as options to limit the length of the variables and whatnot.

Code	Format
%c	character
%d	signed integers
%i	signed integers
%e	scientific notation, with a lowercase "e"
%E	scientific notation, with a uppercase "E"
%f	floating point
%g	use %e or %f, whichever is shorter
%G	use %E or %f, whichever is shorter
%o	octal
%s	a string of characters
%u	unsigned integer
%x	unsigned hexadecimal, with lowercase letters
%X	unsigned hexadecimal, with uppercase letters
%p	a pointer
%n	the argument shall be a pointer to an integer into which is placed the number of characters written so far
%%	a '%' sign

An integer placed between a % sign and the format command acts as a minimum field width specifier, and pads the output with spaces or zeros to make it long enough. If you want to pad with zeros, place a zero before the minimum field width specifier:

   %012d                

You can also include a precision modifier, in the form of a .N where N is some number, before the format command:

   %012.4d              

The precision modifier has different meanings depending on the format command being used:

• With %e, %E, and %f, the precision modifier lets you specify the number of decimal places desired. For example, %12.6f will display a floating number at least 12 digits wide, with six decimal places.
• With %g and %G, the precision modifier determines the maximum number of significant digits displayed.
• With %s, the precision modifer simply acts as a maximumfield length, to complement the minimum field length that precedes the period.

All of printf()'s output is right-justified, unless you place a minus sign right after the % sign. For example,

   %-12.4f              

will display a floating point number with a minimum of 12 characters, 4 decimal places, and left justified. You may modify the %d, %i, %o, %u, and %x type specifiers with the letter l and the letter h to specify long and short data types (e.g. %hd means a short integer). The %e, %f, and %g type specifiers can have the letter l before them to indicate that a double follows. The %g, %f, and %e type specifiers can be preceded with the character '#' to ensure that the decimal point will be present, even if there are no decimal digits. The use of the '#' character with the %x type specifier indicates that the hexidecimal number should be printed with the '0x' prefix. The use of the '#' character with the %o type specifier indicates that the octal value should be displayed with a 0 prefix.

You can also include constant escape sequences in the output string.

The return value of printf() is the number of characters printed, or a negative number if an error occurred.