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)