printf(3C) Standard C Library Functions printf(3C)
printf, fprintf, sprintf, snprintf - print formatted output
#include <stdio.h>
int printf(const char *format, /* args*/ ...);
int fprintf(FILE *stream, const char *format, /* args*/ ...);
int sprintf(char *s, const char *format, /* args*/ ...);
int snprintf(char *s, size_t n, const char *format, /* args*/ ...);
The printf() function places output on the standard output stream std‐
out.
The fprintf() function places output on on the named output stream
stream.
The sprintf() function places output, followed by the null byte (\0),
in consecutive bytes starting at s; it is the user’s responsibility to
ensure that enough storage is available.
The snprintf() function is identical to sprintf() with the addition of
the argument n, which specifies the size of the buffer referred to by
s. The buffer is always terminated with the null byte.
Each of these functions converts, formats, and prints its arguments
under control of the format. The format is a character string, begin‐
ning and ending in its initial shift state, if any. The format is com‐
posed of zero or more directives: ordinary characters, which are simply
copied to the output stream and conversion specifications, each of
which results in the fetching of zero or more arguments. The results
are undefined if there are insufficient arguments for the format. If
the format is exhausted while arguments remain, the excess arguments
are evaluated but are otherwise ignored.
Conversions can be applied to the nth argument after the format in the
argument list, rather than to the next unused argument. In this case,
the conversion character % (see below) is replaced by the sequence %n$,
where n is a decimal integer in the range [1, NL_ARGMAX], giving the
position of the argument in the argument list. This feature provides
for the definition of format strings that select arguments in an order
appropriate to specific languages (see the EXAMPLES section).
In format strings containing the %n$ form of conversion specifications,
numbered arguments in the argument list can be referenced from the for‐
mat string as many times as required.
In format strings containing the % form of conversion specifications,
each argument in the argument list is used exactly once.
All forms of the printf() functions allow for the insertion of a lan‐
guage-dependent radix character in the output string. The radix charac‐
ter is defined by the program’s locale (category LC_NUMERIC). In the
POSIX locale, or in a locale where the radix character is not defined,
the radix character defaults to a period (.).
Conversion Specifications
Each conversion specification is introduced by the % character or by
the character sequence %n$, after which the following appear in
sequence:
· An optional field, consisting of a decimal digit string followed
by a $, specifying the next argument to be converted. If this
field is not provided, the args following the last argument con‐
verted will be used.
· Zero or more flags (in any order), which modify the meaning of
the conversion specification.
· An optional minimum field width. If the converted value has fewer
bytes than the field width, it will be padded with spaces by
default on the left; it will be padded on the right, if the left-
adjustment flag (-), described below, is given to the field
width. The field width takes the form of an asterisk (*),
described below, or a decimal integer.
If the conversion character is s, a standard-conforming applica‐
tion (see standards(5)) interprets the field width as the minimum
number of bytes to be printed; an application that is not stan‐
dard-conforming interprets the field width as the minimum number
of columns of screen display. For an application that is not
standard-conforming, %10s means if the converted value has a
screen width of 7 columns, 3 spaces would be padded on the right.
If the format is %ws, then the field width should be interpreted as
the minimum number of columns of screen display.
· An optional precision that gives the minimum number of digits to
appear for the d, i, o, u, x, and X conversions (the field is
padded with leading zeros); the number of digits to appear after
the radix character for the e, E, and f conversions, the maximum
number of significant digits for the g and G conversions; or the
maximum number of bytes to be printed from a string in s and S
conversions. The precision takes the form of a period (.) fol‐
lowed either by an asterisk (*), described below, or an optional
decimal digit string, where a null digit string is treated as 0.
If a precision appears with any other conversion character, the
behavior is undefined.
If the conversion character is s or S, a standard-conforming
application (see standards(5)) interprets the precision as the
maximum number of bytes to be written; an application that is not
standard-conforming interprets the precision as the maximum num‐
ber of columns of screen display. For an application that is not
standard-conforming, %.5s would print only the portion of the
string that would display in 5 screen columns. Only complete
characters are written.
For %ws, the precision should be interpreted as the maximum number of
columns of screen display. The precision takes the form of a period (.)
followed by a decimal digit string; a null digit string is treated as
zero. Padding specified by the precision overrides the padding speci‐
fied by the field width.
· An optional h specifies that a following d, i, o, u, x, or X con‐
version character applies to a type short int or type unsigned
short int argument (the argument will be promoted according to
the integral promotions, and its value converted to type short
int or unsigned short int before printing); an optional h speci‐
fying that a following n conversion character applies to a
pointer to a type short int argument; an optional l (ell) speci‐
fying that a following d, i, o, u, x, or X conversion character
applies to a type long int or unsigned long int argument; an
optional l (ell) specifying that a following n conversion
character applies to a pointer to a type long int argument; an
optional ll (ell ell) specifying that a following d, i, o, u,
x, or X conversion character applies to a type long long or
unsigned long long argument; an optional ll (ell ell) specify‐
ing that a following n conversion character applies to a pointer
to a long long argument; or an optional L specifying that a fol‐
lowing e, E, f, g, or G conversion character applies to a type
long double argument. If an h, l, ll, or L appears with any other
conversion character, the behavior is undefined.
· An optional l (ell) specifying that a following c conversion
character applies to a wint_t argument; an optional l (ell) spec‐
ifying that a following s conversion character applies to a
pointer to a wchar_t argument.
· A conversion character (see below) that indicates the type of
conversion to be applied.
A field width, or precision, or both may be indicated by an asterisk
(*) . In this case, an argument of type int supplies the field width or
precision. Arguments specifying field width, or precision, or both must
appear in that order before the argument, if any, to be converted. A
negative field width is taken as a − flag followed by a positive field
width. A negative precision is taken as if the precision were omitted.
In format strings containing the %n$ form of a conversion specifica‐
tion, a field width or precision may be indicated by the sequence *m$,
where m is a decimal integer in the range [1, NL_ARGMAX] giving the
position in the argument list (after the format argument) of an integer
argument containing the field width or precision, for example:
printf("%1$d:%2$.*3$d:%4$.*3$d\n", hour, min, precision, sec);
The format can contain either numbered argument specifications (that
is, %n$ and *m$), or unnumbered argument specifications (that is, % and
*), but normally not both. The only exception to this is that %% can be
mixed with the %n$ form. The results of mixing numbered and unnumbered
argument specifications in a format string are undefined. When numbered
argument specifications are used, specifying the Nth argument requires
that all the leading arguments, from the first to the (N-1)th, are
specified in the format string.
Flag Characters
The flag characters and their meanings are:
’ The integer portion of the result of a decimal conversion (%i,
%d, %u, %f, %g, or %G) will be formatted with thousands’ grouping
characters. For other conversions the behavior is undefined. The
non-monetary grouping character is used.
− The result of the conversion will be left-justified within the
field. The conversion will be right-justified if this flag is not
specified.
+ The result of a signed conversion will always begin with a sign
(+ or -). The conversion will begin with a sign only when a nega‐
tive value is converted if this flag is not specified.
space If the first character of a signed conversion is not a sign or if
a signed conversion results in no characters, a space will be
placed before the result. This means that if the space and +
flags both appear, the space flag will be ignored.
# The value is to be converted to an alternate form. For c, d, i,
s, and u conversions, the flag has no effect. For an o conver‐
sion, it increases the precision (if necessary) to force the
first digit of the result to be a zero. For x or X conversion, a
non-zero result will have 0x (or 0X) prepended to it. For e, E,
f, g, and G conversions, the result will always contain a radix
character, even if no digits follow the radix character. Without
this flag, the radix character appears in the result of these
conversions only if a digit follows it. For g and G conversions,
trailing zeros will not be removed from the result as they nor‐
mally are.
0 For d, i, o, u, x, X, e, E, f, g, and G conversions, leading
zeros (following any indication of sign or base) are used to pad
to the field width; no space padding is performed. If the 0 and −
flags both appear, the 0 flag will be ignored. For d, i, o, u, x,
and X conversions, if a precision is specified, the 0 flag will
be ignored. If the 0 and ’ flags both appear, the grouping char‐
acters are inserted before zero padding. For other conversions,
the behavior is undefined.
Conversion Characters
Each conversion character results in fetching zero or more arguments.
The results are undefined if there are insufficient arguments for the
format. If the format is exhausted while arguments remain, the excess
arguments are ignored.
The conversion characters and their meanings are:
d,i The int argument is converted to a signed decimal in the style
[−]dddd. The precision specifies the minimum number of digits to
appear; if the value being converted can be represented in fewer
digits, it will be expanded with leading zeros. The default pre‐
cision is 1. The result of converting 0 with an explicit preci‐
sion of 0 is no characters.
o The unsigned int argument is converted to unsigned octal format
in the style dddd. The precision specifies the minimum number of
digits to appear; if the value being converted can be represented
in fewer digits, it will be expanded with leading zeros. The
default precision is 1. The result of converting 0 with an
explicit precision of 0 is no characters.
u The unsigned int argument is converted to unsigned decimal format
in the style dddd. The precision specifies the minimum number of
digits to appear; if the value being converted can be represented
in fewer digits, it will be expanded with leading zeros. The
default precision is 1. The result of converting 0 with an
explicit precision of 0 is no characters.
x The unsigned int argument is converted to unsigned hexadecimal
format in the style dddd; the letters abcdef are used. The preci‐
sion specifies the minimum number of digits to appear; if the
value being converted can be represented in fewer digits, it will
be expanded with leading zeros. The default precision is 1. The
result of converting 0 with an explicit precision of 0 is no
characters.
X Behaves the same as the x conversion character except that let‐
ters ABCDEF are used instead of abcdef.
f The double argument is converted to decimal notation in the style
[−]ddd.ddd, where the number of digits after the radix character
(see setlocale(3C)) is equal to the precision specification. If
the precision is missing it is taken as 6; if the precision is
explicitly 0 and the # flag is not specified, no radix character
appears. If a radix character appears, at least 1 digit appears
before it. The value is rounded to the appropriate number of dig‐
its.
e,E The double argument is converted to the style [−]d.ddde±dd, where
there is one digit before the radix character (which is non-zero
if the argument is non-zero) and the number of digits after it is
equal to the precision. When the precision is missing it is taken
as 6; if the precision is 0 and the # flag is not specified, no
radix character appears. The E conversion character will produce
a number with E instead of e introducing the exponent. The expo‐
nent always contains at least two digits. The value is rounded to
the appropriate number of digits.
g,G The double argument is printed in style f or e (or in style E in
the case of a G conversion character), with the precision speci‐
fying the number of significant digits. If an explicit precision
is 0, it is taken as 1. The style used depends on the value con‐
verted: style e (or E) will be used only if the exponent result‐
ing from the conversion is less than -4 or greater than or equal
to the precision. Trailing zeros are removed from the fractional
part of the result. A radix character appears only if it is fol‐
lowed by a digit.
c The int argument is converted to an unsigned char, and the
resulting byte is printed.
If an l (ell) qualifier is present, the wint_t argument is con‐
verted as if by an ls conversion specification with no precision
and an argument that points to a two-element array of type
wchar_t, the first element of which contains the wint_t argument
to the ls conversion specification and the second element con‐
tains a null wide-character.
C Same as lc.
wc The int argument is converted to a wide character (wchar_t), and
the resulting wide character is printed.
s The argument must be a pointer to an array of char. Bytes from
the array are written up to (but not including) any terminating
null byte. If a precision is specified, a standard-conforming
application (see standards(5)) will write only the number of
bytes specified by precision; an application that is not stan‐
dard-conforming will write only the portion of the string that
will display in the number of columns of screen display speci‐
fied by precision. If the precision is not specified, it is taken
to be infinite, so all bytes up to the first null byte are
printed. An argument with a null value will yield undefined
results.
If an l (ell) qualifier is present, the argument must be a
pointer to an array of type wchar_t. Wide-characters from the
array are converted to characters (each as if by a call to the
wcrtomb(3C) function, with the conversion state described by an
mbstate_t object initialized to zero before the first wide-char‐
acter is converted) up to and including a terminating null wide-
character. The resulting characters are written up to (but not
including) the terminating null character (byte). If no preci‐
sion is specified, the array must contain a null wide-character.
If a precision is specified, no more than that many characters
(bytes) are written (including shift sequences, if any), and the
array must contain a null wide-character if, to equal the charac‐
ter sequence length given by the precision, the function would
need to access a wide-character one past the end of the array.
In no case is a partial character written.
S Same as ls.
ws The argument must be a pointer to an array of wchar_t. Bytes from
the array are written up to (but not including) any terminating
null character. If the precision is specified, only that portion
of the wide-character array that will display in the number of
columns of screen display specified by precision will be written.
If the precision is not specified, it is taken to be infinite, so
all wide characters up to the first null character are printed.
An argument with a null value will yield undefined results.
p The argument must be a pointer to void. The value of the pointer
is converted to a set of sequences of printable characters, which
should be the same as the set of sequences that are matched by
the %p conversion of the scanf(3C) function.
n The argument must be a pointer to an integer into which is writ‐
ten the number of bytes written to the output standard I/O stream
so far by this call to one of the printf() functions. No argument
is converted.
% Print a %; no argument is converted. The entire conversion speci‐
fication must be %%.
If a conversion specification does not match one of the above forms,
the behavior is undefined.
If a floating-point value is the internal representation for infinity,
the output is [±]Infinity, where Infinity is either Infinity or Inf,
depending on the desired output string length. Printing of the sign
follows the rules described above.
If a floating-point value is the internal representation for "not-a-
number," the output is [±]NaN. Printing of the sign follows the rules
described above.
In no case does a non-existent or small field width cause truncation of
a field; if the result of a conversion is wider than the field width,
the field is simply expanded to contain the conversion result. Charac‐
ters generated by printf() and fprintf() are printed as if the
putc(3C) function had been called.
The st_ctime and st_mtime fields of the file will be marked for update
between the call to a successful execution of printf() or fprintf() and
the next successful completion of a call to fflush(3C) or fclose(3C) on
the same stream or a call to exit(3C) or abort(3C).
The printf(), fprintf(), and sprintf() functions return the number of
bytes transmitted (excluding the terminating null byte in the case of
sprintf()).
The snprintf() function returns the number of characters formatted,
that is, the number of characters that would have been written to the
buffer if it were large enough. If the value of n is 0 on a call to
snprintf(), an unspecified value less than 1 is returned.
Each function returns a negative value if an output error was encoun‐
tered.
For the conditions under which printf() and fprintf() will fail and may
fail, refer to fputc(3C) or fputwc(3C).
In addition, all forms of printf() may fail if:
EILSEQ
A wide-character code that does not correspond to a valid charac‐
ter has been detected.
EINVAL
There are insufficient arguments.
In addition, printf() and fprintf() may fail if:
ENOMEM
Insufficient storage space is available.
If the application calling the printf() functions has any objects of
type wint_t or wchar_t, it must also include the header <wchar.h> to
have these objects defined.
The sprintf() and snprintf() functions are MT-Safe in multithreaded
applications. The printf() and fprintf() functions can be used safely
in multithreaded applications, as long as setlocale(3C) is not being
called to change the locale.
Escape Character Sequences
It is common to use the following escape sequences built into the C
language when entering format strings for the printf() functions, but
these sequences are processed by the C compiler, not by the printf()
function.
\a Alert. Ring the bell.
\b Backspace. Move the printing position to one character
before the current position, unless the current position
is the start of a line.
\f Form feed. Move the printing position to the initial
printing position of the next logical page.
\n Newline. Move the printing position to the start of the
next line.
\r Carriage return. Move the printing position to the start
of the current line.
\t Horizontal tab. Move the printing position to the next
implementation-defined horizontal tab position on the cur‐
rent line.
\v Vertical tab. Move the printing position to the start of
the next implementation-defined vertical tab position.
In addition, the C language supports character sequences of the form
\octal-number
and
\hex-number
which translates into the character represented by the octal or hex‐
adecimal number. For example, if ASCII representations are being used,
the letter ’a’ may be written as ’\141’ and ’Z’ as ’\132’. This syntax
is most frequently used to represent the null character as ’\0’. This
is exactly equivalent to the numeric constant zero (0). Note that the
octal number does not include the zero prefix as it would for a normal
octal constant. To specify a hexadecimal number, omit the zero so that
the prefix is an ’x’ (uppercase ’X’ is not allowed in this context).
Support for hexadecimal sequences is an ANSI extension. See stan‐
dards(5).
Example 1: To print the language-independent date and time format, the
following statement could be used:
printf (format, weekday, month, day, hour, min);
For American usage, format could be a pointer to the string:
"%s, %s %d, %d:%.2d\n"
producing the message:
Sunday, July 3, 10:02
whereas for German usage, format could be a pointer to the string:
"%1$s, %3$d. %2$s, %4$d:%5$.2d\n"
producing the message:
Sonntag, 3. Juli, 10:02
Example 2: To print a date and time in the form Sunday, July 3, 10:02,
where weekday and month are pointers to null-terminated strings:
printf("%s, %s %i, %d:%.2d", weekday, month, day, hour, min);
Example 3: To print pi to 5 decimal places:
printf("pi = %.5f", 4 * atan(1.0));
Default
Example 4: The following example applies only to applications which are
not standard-conforming (see standards(5)). To print a list of names in
columns which are 20 characters wide:
printf("%20s%20s%20s", lastname, firstname, middlename);
See attributes(5) for descriptions of the following attributes:
+-----------------------------+-----------------------------+
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+-----------------------------+-----------------------------+
|MT-Level |MT-Safe with exceptions |
+-----------------------------+-----------------------------+
|CSI |Enabled |
+-----------------------------+-----------------------------+
exit(2), lseek(2), write(2), abort(3C), ecvt(3C), exit(3C), fclose(3C),
fflush(3C), fputwc(3C), putc(3C), scanf(3C), setlocale(3C), stdio(3C),
wcstombs(3C), wctomb(3C), attributes(5), environ(5), standards(5)
SunOS 5.9 7 Oct 1999 printf(3C)