Excluding dprintf(): The functionality described on this reference page is aligned with the ISO C
standard. Any conflict between the requirements described here and the ISO C standard is unintentional.
This volume of POSIX.1‐2017 defers to the ISO C standard.
The fprintf() function shall place output on the named output stream. The printf() function shall place
output on the standard output stream stdout. The sprintf() function shall place output followed by the
null byte, '\0', in consecutive bytes starting at *s; it is the user's responsibility to ensure that
enough space is available.
The dprintf() function shall be equivalent to the fprintf() function, except that dprintf() shall write
output to the file associated with the file descriptor specified by the fildes argument rather than place
output on a stream.
The snprintf() function shall be equivalent to sprintf(), with the addition of the n argument which
states the size of the buffer referred to by s. If n is zero, nothing shall be written and s may be a
null pointer. Otherwise, output bytes beyond the n‐1st shall be 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.
If copying takes place between objects that overlap as a result of a call to sprintf() or snprintf(), the
results are undefined.
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: ordinarycharacters, which are simply copied to the output stream,
and conversionspecifications, each of which shall result 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 shall be 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 character % (see below) is replaced by the
sequence "%n$", where n is a decimal integer in the range [1,{NL_ARGMAX}], giving the position of the
argument in the argument list. This feature provides for the definition of format strings that select
arguments in an order appropriate to specific languages (see the EXAMPLES section).
The format can contain either numbered argument conversion specifications (that is, "%n$" and "*m$"), or
unnumbered argument conversion specifications (that is, % and *), but not both. The only exception to
this is that %% can be mixed with the "%n$" 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.
In format strings containing the "%n$" form of conversion specification, numbered arguments in the
argument list can be referenced from the format string as many times as required.
In format strings containing the % form of conversion specification, each conversion specification uses
the first unused argument in the argument list.
All forms of the fprintf() functions allow for the insertion of a language-dependent radix character in
the output string. The radix character is defined in the current locale (category LC_NUMERIC). In the
POSIX locale, or in a locale where the radix character is not defined, the radix character shall default
to a <period> ('.').
Each conversion specification is introduced by the '%' character or by the character sequence "%n$",
after which the following appear in sequence:
* Zero or more flags (in any order), which modify the meaning of the conversion specification.
* An optional minimum fieldwidth. If the converted value has fewer bytes than the field width, it
shall be padded with <space> characters by default on the left; it shall 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.
* An optional precision that gives the minimum number of digits to appear for the d, i, o, u, x, and X
conversion specifiers; the number of digits to appear after the radix character for the a, A, e, E,
f, and F conversion specifiers; the maximum number of significant digits for the g and G conversion
specifiers; or the maximum number of bytes to be printed from a string in the s and S conversion
specifiers. 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 zero.
If a precision appears with any other conversion specifier, the behavior is undefined.
* An optional length modifier that specifies the size of the argument.
* A conversionspecifier character 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. Applications shall ensure that arguments specifying
field width, or precision, or both 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 flag characters and their meanings are:
' (The <apostrophe>.) The integer portion of the result of a decimal conversion (%i, %d, %u, %f,
%F, %g, or %G) shall 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 shall be left-justified within the field. The conversion is right-
justified if this flag is not specified.
+ The result of a signed conversion shall always begin with a sign ('+' or '-'). The conversion
shall 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> shall be prefixed to the result. This means that if the <space> and '+'
flags both appear, the <space> flag shall be ignored.
# Specifies that the value is to be converted to an alternative form. For o conversion, it shall
increase the precision, if and only if necessary, to force the first digit of the result to be a
zero (if the value and precision are both 0, a single 0 is printed). For x or X conversion
specifiers, a non-zero result shall have 0x (or 0X) prefixed to it. For a, A, e, E, f, F, g, and
G conversion specifiers, the result shall always contain a radix character, even if no digits
follow the radix character. Without this flag, a radix character appears in the result of these
conversions only if a digit follows it. For g and G conversion specifiers, trailing zeros shall
not be removed from the result as they normally are. For other conversion specifiers, the
behavior is undefined.
0 For d, i, o, u, x, X, a, A, e, E, f, F, g, and G conversion specifiers, leading zeros (following
any indication of sign or base) are used to pad to the field width rather than performing space
padding, except when converting an infinity or NaN. If the '0' and '-' flags both appear, the '0'
flag is ignored. For d, i, o, u, x, and X conversion specifiers, if a precision is specified, the
'0' flag shall be ignored. If the '0' and <apostrophe> flags both appear, the grouping
characters are inserted before zero padding. For other conversions, the behavior is undefined.
The length modifiers and their meanings are:
hh Specifies that a following d, i, o, u, x, or X conversion specifier applies to a signedchar or
unsignedchar argument (the argument will have been promoted according to the integer promotions,
but its value shall be converted to signedchar or unsignedchar before printing); or that a
following n conversion specifier applies to a pointer to a signedchar argument.
h Specifies that a following d, i, o, u, x, or X conversion specifier applies to a short or
unsignedshort argument (the argument will have been promoted according to the integer
promotions, but its value shall be converted to short or unsignedshort 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 unsignedlong 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 longlong or
unsignedlonglong argument; or that a following n conversion specifier applies to a pointer to a
longlong 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.
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 a 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 longdouble argument.
If a length modifier appears with any conversion specifier other than as specified above, the behavior is
undefined.
The conversion specifiers and their meanings are:
d, i The int argument shall be 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 shall be expanded with leading zeros. The default precision is 1. The result
of converting zero with an explicit precision of zero shall be no characters.
o The unsigned argument shall be 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 shall be expanded with leading zeros. The default precision is 1.
The result of converting zero with an explicit precision of zero shall be no characters.
u The unsigned argument shall be 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 shall be expanded with leading zeros. The default precision is 1.
The result of converting zero with an explicit precision of zero shall be no characters.
x The unsigned argument shall be 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 shall be expanded with leading
zeros. The default precision is 1. The result of converting zero with an explicit precision of
zero shall be no characters.
X Equivalent to the x conversion specifier, except that letters "ABCDEF" are used instead of
"abcdef".
f, F The double argument shall be converted to decimal notation in the style "[-]ddd.ddd", where the
number of digits after the radix character is equal to the precision specification. If the
precision is missing, it shall be taken as 6; if the precision is explicitly zero and no '#' flag
is present, no radix character shall appear. If a radix character appears, at least one digit
appears before it. The low-order digit shall be rounded in an implementation-defined manner.
A double argument representing an infinity shall be converted in one of the styles "[-]inf" or
"[-]infinity"; which style is implementation-defined. A double argument representing a NaN shall
be converted in one of the styles "[-]nan(n-char-sequence)" or "[-]nan"; which style, and the
meaning of any n-char-sequence, is implementation-defined. The F conversion specifier produces
"INF", "INFINITY", or "NAN" instead of "inf", "infinity", or "nan", respectively.
e, E The double argument shall be converted in 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; if the precision is missing, it shall be taken as 6;
if the precision is zero and no '#' flag is present, no radix character shall appear. The low-
order digit shall be rounded in an implementation-defined manner. The E conversion specifier
shall produce a number with 'E' instead of 'e' introducing the exponent. The exponent shall
always contain at least two digits. If the value is zero, the exponent shall be zero.
A double argument representing an infinity or NaN shall be converted in the style of an f or F
conversion specifier.
g, G The double argument representing a floating-point number shall be converted in the style f or e
(or in the style F or E in the case of a G conversion specifier), depending on the value
converted and the precision. Let P equal the precision if non-zero, 6 if the precision is
omitted, or 1 if the precision is zero. Then, if a conversion with style E would have an exponent
of X:
-- If P>X≥-4, the conversion shall be with style f (or F) and precision P-(X+1).
-- Otherwise, the conversion shall be with style e (or E) and precision P-1.
Finally, unless the '#' flag is used, any trailing zeros shall be removed from the fractional
portion of the result and the decimal-point character shall be removed if there is no fractional
portion remaining.
A double argument representing an infinity or NaN shall be converted in the style of an f or F
conversion specifier.
a, A A double argument representing a floating-point number shall be converted in the style
"[-]0xh.hhhhp±d", where there is one hexadecimal digit (which shall be non-zero if the argument
is a normalized floating-point number and is otherwise unspecified) before the decimal-point
character and the number of hexadecimal digits after it is equal to the precision; if the
precision is missing and FLT_RADIX is a power of 2, then the precision shall be sufficient for an
exact representation of the value; if the precision is missing and FLT_RADIX is not a power of 2,
then the precision shall be sufficient to distinguish values of type double, except that trailing
zeros may be omitted; if the precision is zero and the '#' flag is not specified, no decimal-
point character shall appear. The letters "abcdef" shall be 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 shall 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
shall be zero.
A double argument representing an infinity or NaN shall be converted in the style of an f or F
conversion specifier.
c The int argument shall be converted to an unsignedchar, and the resulting byte shall be written.
If an l (ell) qualifier is present, the wint_t argument shall be 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.
s The argument shall be a pointer to an array of char. Bytes from the array shall be written up to
(but not including) any terminating null byte. If the precision is specified, no more than that
many bytes shall be written. If the precision is not specified or is greater than the size of the
array, the application shall ensure that the array contains a null byte.
If an l (ell) qualifier is present, the argument shall be a pointer to an array of type wchar_t.
Wide characters from the array shall be converted to characters (each as if by a call to the
wcrtomb() 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 shall be written up to (but not including) the terminating
null character (byte). If no precision is specified, the application shall ensure that the array
contains a null wide character. If a precision is specified, no more than that many characters
(bytes) shall be written (including shift sequences, if any), and the array shall 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 shall a partial
character be written.
p The argument shall be a pointer to void. The value of the pointer is converted to a sequence of
printable characters, in an implementation-defined manner.
n The argument shall be a pointer to an integer into which is written the number of bytes written
to the output so far by this call to one of the fprintf() functions. No argument is converted.
C Equivalent to lc.
S Equivalent to ls.
% Print a '%' character; no argument is converted. The complete conversion specification shall be
%%.
If a conversion specification does not match one of the above forms, the behavior is undefined. If any
argument is not the correct type for the corresponding conversion specification, the behavior is
undefined.
In no case shall a nonexistent or small field width cause truncation of a field; if the result of a
conversion is wider than the field width, the field shall be expanded to contain the conversion result.
Characters generated by fprintf() and printf() are printed as if fputc() had been called.
For the a and A conversion specifiers, if FLT_RADIX is a power of 2, the value shall be correctly rounded
to a hexadecimal floating number with the given precision.
For a and A conversions, if FLT_RADIX is not a power of 2 and the result is not exactly representable in
the given precision, the result should be one of the two adjacent numbers in hexadecimal floating style
with the given precision, with the extra stipulation that the error should have a correct sign for the
current rounding direction.
For the e, E, f, F, g, and G conversion specifiers, if the number of significant decimal digits is at
most DECIMAL_DIG, then the result should be correctly rounded. If the number of significant decimal
digits is more than DECIMAL_DIG but the source value is exactly representable with DECIMAL_DIG digits,
then the result should be an exact representation with trailing zeros. Otherwise, the source value is
bounded by two adjacent decimal strings L < U, both having DECIMAL_DIG significant digits; the value of
the resultant decimal string D should satisfy L <= D <= U, with the extra stipulation that the error
should have a correct sign for the current rounding direction.
The last data modification and last file status change timestamps of the file shall be marked for update:
1. Between the call to a successful execution of fprintf() or printf() and the next successful
completion of a call to fflush() or fclose() on the same stream or a call to exit() or abort()
2. Upon successful completion of a call to dprintf()
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