Input/Output I

printf()

Input and output routines are provided in the C language. To access the functions for using formatted input and output, the following compiler control line must be included at the beginning of the source file:

#include <stdio.h>

This include file contains the header information for STandarD Input/Output functions. This documentation only explains the basics; check the fprintf man page for further information.

printf() prints output to standard output, the text screen being the default. Its syntax is as follows:

printf(control_string, arg1, arg2, ...)

The control string is a double-quoted text string which contains conversion and formatting information about how the remaining arguments are to be handled. An example call would take the form:

printf("A height of %d and a base of %.1f would ...", h, b)

and would generate the following output if h and b had the values 5 and 7.5, respectively.

A height of 5 and a base of 7.5 would ...

The Formatting Information

Between the % and the conversion character (d in the case above), it is possible to include various formatting information, in the following order. All of these are optional.

- A minus sign to specify that the resulting text should be quadded to the left.

mm A numeric value to specify the minimum field width. The converted number will be printed in a field that is at least this wide; if the field is larger than needed, it will be padded with blanks.

. A period to separate the field width from the precision width.

nn A numeric value to specify the number of digits to be printed to the right of the decimal point for a float or double (its precision), or the maximum number of characters to be printed from a string.

The Conversion Characters

d	The argument is converted to decimal notation.
o	The argument is converted to unsigned octal notation (without a leading zero).
x	The argument is converted to unsigned hexadecimal notation (without a leading 0x).
u	The argument is converted to unsigned decimal notation.
c	The argument is taken to be a single character.
s	The argument is a string; characters from the string are printed until a null character is reached or the number of characters indicated by the precision specification is exhausted.
f	The argument is assumed to be a float or double and converted to decimal notation of the form [-]mmm.nnnnn where the length of the string of n's is specified by the precision (the default is 6).

String Examples

Strings are handled somewhat differently. The precision numeric indicates how many of characters starting from the beginning of a string will be printed. The following examples show the results when using various format specifications for the string "hello, world", which consists of 12 characters. A colon marks the end of the field.

Output                  Format
hello, world:	        %s or %10s or %-10s
        hello, world:	%20s
hello, world        :	%-20s
          hello, wor:	%20.10s
hello, wor          :	%-20.10s
hello, wor:	        %.10s

-	A minus sign to specify that the resulting text should be quadded to the left.
mm	A numeric value to specify the minimum field width. The converted number will be printed in a field that is at least this wide; if the field is larger than needed, it will be padded with blanks.
.	A period to separate the field width from the precision width.
nn	A numeric value to specify the number of digits to be printed to the right of the decimal point for a float or double (its precision), or the maximum number of characters to be printed from a string.