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182_printf.asciidoc

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printf

NAME

printf - Print formatted data to STDOUT.

SYNOPSIS
#include <stdio.h>

int printf( const char *format [ , argument , ...] );
DESCRIPTION

Prints to standard output (stdout) a sequence of arguments formatted as the format argument specifies.

PARAMETERS

  • format - String that contains the text to be printed.

Optionally it can contain format tags that are substituted by the values specified in subsequent argument(s) and formatted as requested. The number of format tags must correspond to the number of additional arguments that follows. The format tags follow this prototype: %[flags][width][.precision][modifiers]type where type is the most significant and defines how the value will be printed:

  • format - String that contains the text to be printed. Optionally it can contain format tags that are substituted by the values specified in subsequent argument(s) and formatted as requested. The number of format tags must correspond with the number of additional arguments that follows.

The format tags follow this prototype:

%[flags][width][.precision][modifiers]type where type is the most significant and defines how the value will be printed:

type Output Example

c

Character

'a'

d or i

Signed decimal integer

392

e

Scientific notation (mantise/exponent) using e character

3.9265e2

E

Scientific notation (mantise/exponent) using E character

3.9265E2

f

Decimal floating point

392.650000

g

Use shorter %e or %f

392.65

G

Use shorter %E or %f

392.65

o

Signed octal

610

s

String of characters

"sample"

u

Unsigned decimal integer

7235

x

Unsigned hexadecimal integer

7fa

X

Unsigned hexadecimal integer (capital letters)

7FA

p

Address pointed by the argument

B800:0000

n

Nothing printed. The argument must be a pointer to integer where the number of characters written so far will be stored.

The other parameters flags, width, precision and modifiers are optional and follow these specifications:

flags meaning

-

Left align within the given width. (right align is the default).

+

Forces to prepend the result with a sign (+ or -) if signed type. (by default only - (minus) is printed).

blank

If the argument is a positive signed value, a blank is inserted before the number.

#

Used with o, x or X type the value is prepended with 0, 0x or 0X respectively if non-zero.
Used with e, E or f forces the output value to contain a decimal point even if only zeros follow.
Used with g or G the result is the same as e or E but trailing zeros are not removed.
width meaning

number

Minimum number of characters to be printed. If the value to be printed is shorter than this number the result is padded with blanks. The value is never truncated even if the result is larger.

0number

Same as above but filled with 0s instead of blanks.

*

The width is not specified in the format string, it is specified by an integer value preceding the argument has has to be formatted.
.precision meaning

.number

for d, i, o, u, x, X types: precision specifies the minimum number of decimal digits to be printed. If the value to be printed is shorter than this number the result is padded with blanks. The value is never truncated even if the result is larger.(if nothing specified default is 1).
for e, E, f types: number of digits to be printed after the decimal point. (if nothing specified default is 6).
for g, G types: maximum number of significant numbers to be printed.
for s type: maximum number of characters to be printed. (default is to print until first null character is encountered).
for c type : (no effect).
modifier meaning (influences on how arguments are interpreted by the function)

h

argument is interpreted as short int (integer types).

l

argument is interpreted as long int (integer types) or double (floating point types).

L

argument is interpreted as long double (floating point types).

argument(s) - Optional parameter(s) that contain the data to be inserted instead of % tags specified in format parameter. There must be the same number of these parameter than the number of format tags.

RETURN VALUE

On success, the total number of characters printed is returned. On error, a negative number is returned.

SEE ALSO

fprintf, scanf

EXAMPLE
link:src/printf.c[role=include]
OUTPUT
$ gcc -Wall printf.c
$ ./a.out
Characters: a A
Decimals: 1977 6500000
Preceding with blanks:       1977
Preceding with zeros: 0000001977
Some different radices: 100 64 144 0x64 0144
floats: 3.14 +3e+00 3.141600E+00
Width trick:    10
A string