PRINTF(3C) Standard C Library Functions PRINTF(3C)

NAME

printf, fprintf, sprintf, snprintf, asprintf - print formatted output

SYNOPSIS

#include <stdio.h>

int printf(const char *restrict format,
/* args*/ ...);


int fprintf(FILE *restrict stream, const char *restrict format,
/* args*/ ...);


int sprintf(char *restrict s, const char *restrict format,
/* args*/ ...);


int snprintf(char *restrict s, size_t n,
const char *restrict format, /* args*/ ...);


int asprintf(char ** ret, const char *restrict format,
/* args*/ ...);

DESCRIPTION

The printf() function places output on the standard output stream
stdout.


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. If n is 0, nothing is written and s can be a null pointer.
Otherwise, output bytes beyond the n-1st are discarded instead of
being written to the array and a null byte is written at the end of
the bytes actually written into the array.


The asprintf() function is the same as the sprintf() function except
that it returns, in the ret argument, a pointer to a buffer
sufficiently large to hold the output string. This pointer should be
passed to free(3C) to release the allocated storage when it is no
longer needed. If sufficient space cannot be allocated, the
asprintf() function returns -1 and sets ret to be a NULL pointer.


Each of these functions converts, formats, and prints its arguments
under control of the format. The format is a character string,
beginning and ending in its initial shift state, if any. The format
is composed 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 specifier % (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 format 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
language-dependent radix character in the output string. The radix
character 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:

o 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 converted will be used.

o Zero or more flags (in any order), which modify the
meaning of the conversion specification.

o 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 specifier is s, a standard-conforming
application (see standards(7)) interprets the field width
as the minimum number of bytes to be printed; an
application that is not standard-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.

o 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 a, A,
e, E, f, 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 (.) followed 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 specifier, the behavior is undefined.

If the conversion specifier is s or S, a standard-
conforming application (see standards(7)) 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 number 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 specified
by the field width.

o An optional length modifier that specified the size of the
argument.

o A conversion specifier that indicates the type of
conversion to be applied.


A field width, or precision, or both can 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 specification, 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, %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 negative 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
conversion, 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 a, A, e, E, f, 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
normally are.


0
For d, i, o, u, x, X, a, A, e, E, f, 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
characters are inserted before zero padding. For other
conversions, the behavior is undefined.

Length Modifiers

The length modifiers and their meanings are:

hh
Specifies that a following d, i, o, u, x, or X
conversion specifier applies to a signed char or
unsigned char argument (the argument will have been
promoted according to the integer promotions, but its
value will be converted to signed char or unsigned
char before printing); or that a following n
conversion specifier applies to a pointer to a signed
char argument.


h
Specifies that a following d, i, o, u, x, or X
conversion specifier applies to a short or unsigned
short argument (the argument will have been promoted
according to the integer promotions, but its value
will be converted to short or unsigned short before
printing); or that a following n conversion specifier
applies to a pointer to a short argument.


l (ell)
Specifies that a following d, i, o, u, x, or X
conversion specifier applies to a long or unsigned
long argument; that a following n conversion
specifier applies to a pointer to a long argument;
that a following c conversion specifier applies to a
wint_t argument; that a following s conversion
specifier applies to a pointer to a wchar_t argument;
or has no effect on a following a, A, e, E, f, F, g,
or G conversion specifier.


ll (ell-ell)
Specifies that a following d, i, o, u, x, or X
conversion specifier applies to a long long or
unsigned long long argument; or that a following n
conversion specifier applies to a pointer to a long
long argument.


j
Specifies that a following d, i, o, u, x, or X
conversion specifier applies to an intmax_t or
uintmax_t argument; or that a following n conversion
specifier applies to a pointer to an intmax_t
argument. See NOTES.


z
Specifies that a following d, i, o, u, x, or X
conversion specifier applies to a size_t or the
corresponding signed integer type argument; or that a
following n conversion specifier applies to a pointer
to a signed integer type corresponding to size_t
argument.


t
Specifies that a following d, i, o, u, x, or X
conversion specifier applies to a ptrdiff_t or the
corresponding unsigned type argument; or that a
following n conversion specifier applies to a pointer
to a ptrdiff_t argument.


L
Specifies that a following a, A, e, E, f, F, g, or G
conversion specifier applies to a long double
argument.


If a length modifier appears with any conversion specifier other than
as specified above, the behavior is undefined.

Conversion Specifiers

Each conversion specifier 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 specifiers 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 precision is 1. The result of converting 0
with an explicit precision 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 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
Behaves the same as the x conversion specifier except that
letters ABCDEF are used instead of abcdef.


f, 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 converted
value is rounded to fit the specified output format according
to the prevailing floating point rounding direction mode. If
the conversion is not exact, an inexact exception is raised.

For the f specifier, a double argument representing an
infinity or NaN is converted in the style of the e conversion
specifier, except that for an infinite argument, "infinity"
or "Infinity" is printed when the precision is at least 8 and
"inf" or "Inf" is printed otherwise.

For the F specifier, a double argument representing an
infinity or NaN is converted in the SUSv3 style of the E
conversion specifier, except that for an infinite argument,
"INFINITY" is printed when the precision is at least 8 and or
"INF" is printed otherwise.


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 specifier will produce a number with E instead of
e introducing the exponent. The exponent always contains at
least two digits. The converted value is rounded to fit the
specified output format according to the prevailing floating
point rounding direction mode. If the conversion is not
exact, an inexact exception is raised.

Infinity and NaN values are handled in one of the following
ways:

SUSv3
For the e specifier, a double argument
representing an infinity is printed as
"[-]infinity", when the precision for the
conversion is at least 7 and as "[-]inf"
otherwise. A double argument representing a NaN is
printed as "[-]nan". For the E specifier, "INF",
"INFINITY", and "NAN" are printed instead of
"inf", "infinity", and "nan", respectively.
Printing of the sign follows the rules described
above.


Default
A double argument representing an infinity is
printed as "[-]Infinity", when the precision for
the conversion is at least 7 and as "[-]Inf"
otherwise. A double argument representing a NaN is
printed as "[-]NaN". Printing of the sign follows
the rules described above.


g, G
The double argument is printed in style f or e (or in style E
in the case of a G conversion specifier), with the precision
specifying the number of significant digits. If an explicit
precision is 0, it is taken as 1. The style used depends on
the value converted: style e (or E) will be used only if the
exponent resulting 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 followed by a digit.

A double argument representing an infinity or NaN is
converted in the style of the e or E conversion specifier,
except that for an infinite argument, "infinity", "INFINITY",
or "Infinity" is printed when the precision is at least 8 and
"inf", "INF", or "Inf" is printed otherwise.


a, A
A double argument representing a floating-point number is
converted in the style "[-]0xh.hhhhp+-d", where the single
hexadecimal digit preceding the radix point is 0 if the value
converted is zero and 1 otherwise and the number of
hexadecimal digits after it is equal to the precision; if the
precision is missing, the number of digits printed after the
radix point is 13 for the conversion of a double value, 16
for the conversion of a long double value on x86, and 28 for
the conversion of a long double value on SPARC; if the
precision is zero and the '#' flag is not specified, no
decimal-point character will appear. The letters "abcdef" are
used for a conversion and the letters "ABCDEF" for A
conversion. The A conversion specifier produces a number with
'X' and 'P' instead of 'x' and 'p'. The exponent will always
contain at least one digit, and only as many more digits as
necessary to represent the decimal exponent of 2. If the
value is zero, the exponent is zero.

The converted value is rounded to fit the specified output
format according to the prevailing floating point rounding
direction mode. If the conversion is not exact, an inexact
exception is raised.

A double argument representing an infinity or NaN is
converted in the SUSv3 style of an e or E conversion
specifier.


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
converted 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 contains 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(7)) will write
only the number of bytes specified by precision; an
application that is not standard-conforming will write only
the portion of the string that will display in the number of
columns of screen display specified 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 print (null). Note, while this
behavior is common across many operating systems, POSIX does
not guarantee this behavior and portable applications should
avoid passing NULL.

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-character 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 precision 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 character
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 print (null).


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
written 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
specification must be %%.


If a conversion specification does not match one of the above forms,
the behavior is undefined.


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.
Characters 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).

RETURN VALUES

The printf(), fprintf(), sprintf(), and asprintf() functions return
the number of bytes transmitted (excluding the terminating null byte
in the case of sprintf() and asprintf()).


The snprintf() function returns the number of bytes that would have
been written to s if n had been sufficiently large (excluding the
terminating null byte.) If the value of n is 0 on a call to
snprintf(), s can be a null pointer and the number of bytes that
would have been written if n had been sufficiently large (excluding
the terminating null byte) is returned.


Each function returns a negative value if an output error was
encountered.

ERRORS

For the conditions under which printf() and fprintf() will fail and
may fail, refer to fputc(3C) or fputwc(3C).


The snprintf() function will fail if:

EOVERFLOW
The value of n is greater than INT_MAX or the number of
bytes needed to hold the output excluding the
terminating null is greater than INT_MAX.


The printf(), fprintf(), sprintf(), and snprintf() functions may fail
if:

EILSEQ
A wide-character code that does not correspond to a valid
character has been detected.


EINVAL
There are insufficient arguments.


The printf(), fprintf(), and asprintf() functions may fail due to an
underlying malloc(3C) failure if:

EAGAIN
Storage space is temporarily unavailable.


ENOMEM
Insufficient storage space is available.

USAGE

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.

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 current
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
hexadecimal 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 standards(7).

EXAMPLES

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 that are

not standard-conforming. To print a list of names in columns which
are 20 characters wide:

printf("%20s%20s%20s", lastname, firstname, middlename);

ATTRIBUTES

See attributes(7) for descriptions of the following attributes:


+--------------------+-----------------+
|ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+--------------------+-----------------+
|CSI | Enabled |
+--------------------+-----------------+
|Interface Stability | Committed |
+--------------------+-----------------+
|MT-Level | See below. |
+--------------------+-----------------+
|Standard | See below. |
+--------------------+-----------------+


All of these functions can be used safely in multithreaded
applications, as long as setlocale(3C) is not being called to change
the locale. The sprintf() and snprintf() functions are Async-Signal-
Safe.


See standards(7) for the standards conformance of printf(),
fprintf(), sprintf(), and snprintf(). The asprintf() function is
modeled on the one that appears in the FreeBSD, NetBSD, and GNU C
libraries.

SEE ALSO

exit(2), lseek(2), write(2), abort(3C), ecvt(3C), exit(3C),
fclose(3C), fflush(3C), fputwc(3C), free(3C), malloc(3C), putc(3C),
scanf(3C), setlocale(3C), stdio(3C), vprintf(3C), wcstombs(3C),
wctomb(3C), attributes(7), environ(7), standards(7)

NOTES

If the j length modifier is used, 32-bit applications that were
compiled using c89 on releases prior to Solaris 10 will experience
undefined behavior.


The snprintf() return value when n = 0 was changed in the Solaris 10
release. The change was based on the SUSv3 specification. The
previous behavior was based on the initial SUSv2 specification, where
snprintf() when n = 0 returns an unspecified value less than 1.

August 19, 2020 PRINTF(3C)