Standard C Library Functions printf(3C)
NAME
printf, fprintf, sprintf, snprintf, asprintf - print format-
ted output
SYNOPSIS
#include
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, sizet 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.
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Standard C Library Functions printf(3C)
The asprintf() function is the same as the sprintf() func-
tion 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 NUL pointer.
Each of these functions converts, formats, and prints its
arguments under control of the format. The format is a char-
acter string, beginning and ending in its initial shift
state, if any. The format is composed of zero or more direc-
tives: 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$, where n is a decimal
integer in the range [1, NLARGMAX], giving the position of
the argument in the argument list. This feature provides for
the definition of format strings that select arguments in an
order appropriate to specific languages (see the EXAMPLES
section).
In format strings containing the %n$ 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 LCNUMERIC). In the POSIX locale, or in a
locale where the radix character is not defined, the radix
character defaults to a period (.).
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Standard C Library Functions printf(3C)
Conversion Specifications
Each conversion specification is introduced by the % charac-
ter or by the character sequence %n$, after which the fol-
lowing appear in sequence:
o An optional field, consisting of a decimal digit
string followed by a $, specifying the next argu-
ment to be converted. If this field is not pro-
vided, the args following the last argument con-
verted 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 aster-
isk (*), described below, or a decimal integer.
If the conversion specifier is s, a standard-
conforming application (see standards(5)) inter-
prets 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.
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Standard C Library Functions printf(3C)
If the conversion specifier is s or S, a standard-
conforming application (see standards(5)) inter-
prets 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 sup-
plies 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 preci-
sion 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$, where m is a decimal
integer in the range [1, NLARGMAX] giving the position in
the argument list (after the format argument) of an integer
argument containing the field width or precision, for exam-
ple:
printf("%1$d:%2$.*3$d:%4$.*3$d\n", hour, min, precision, sec);
The format can contain either numbered argument specifica-
tions (that is, %n$ and *m$), or unnumbered argument specif-
ications (that is, % and *), but normally not both. The only
exception to this is that %% can be mixed with the %n$ form.
The results of mixing numbered and unnumbered argument
specifications in a format string are undefined. When num-
bered argument specifications are used, specifying the Nth
argument requires that all the leading arguments, from the
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Standard C Library Functions printf(3C)
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 pre-
cision (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 indica-
tion 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
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Standard C Library Functions printf(3C)
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 conver-
sion specifier applies to a wintt argument;
that a following s conversion specifier
applies to a pointer to a wchart 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
intmaxt or uintmaxt argument; or that a
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Standard C Library Functions printf(3C)
following n conversion specifier applies to
a pointer to an intmaxt argument. See
NOTES.
z Specifies that a following d, i, o, u, x, or
X conversion specifier applies to a sizet
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 sizet argu-
ment.
t Specifies that a following d, i, o, u, x, or
X conversion specifier applies to a
ptrdifft or the corresponding unsigned type
argument; or that a following n conversion
specifier applies to a pointer to a
ptrdifft 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 insuffi-
cient 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 speci-
fies the minimum number of digits to appear; if the
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Standard C Library Functions printf(3C)
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 con-
verted can be represented in fewer digits, it will
be expanded with leading zeros. The default preci-
sion is 1. The result of converting 0 with an expli-
cit 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 preci-
sion 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 out-
put 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
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Standard C Library Functions printf(3C)
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 oth-
erwise.
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 miss-
ing 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 out-
put 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 infin-
ity 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 signifi-
cant digits. If an explicit precision is 0, it is
taken as 1. The style used depends on the value
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Standard C Library Functions printf(3C)
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. Trail-
ing 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 conver-
sion. 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 wintt argu-
ment is converted as if by an ls conversion specifi-
cation with no precision and an argument that points
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Standard C Library Functions printf(3C)
to a two-element array of type wchart, the first
element of which contains the wintt 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
(wchart), 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
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 yield
undefined results.
If an l (ell) qualifier is present, the argument
must be a pointer to an array of type wchart.
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
mbstatet object initialized to zero before the
first wide-character is converted) up to and includ-
ing 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.
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Standard C Library Functions printf(3C)
ws The argument must be a pointer to an array of
wchart. 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 argu-
ment 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 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 con-
verted.
% 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) func-
tion had been called.
The stctime and stmtime 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
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Standard C Library Functions printf(3C)
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.
ERORS
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 INTMAX or the
number of bytes needed to hold the output
excluding the terminating null is greater than
INTMAX.
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
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Standard C Library Functions printf(3C)
If the application calling the printf() functions has any
objects of type wintt or wchart, it must also include the
header 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 charac-
ter 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 posi-
tion.
In addition, the C language supports character sequences of
the form
\octal-number
and
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Standard C Library Functions printf(3C)
\hex-number
which translates into the character represented by the octal
or hexadecimal number. For example, if ASCI 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' (upper-
case 'X' is not allowed in this context). Support for hexa-
decimal sequences is an ANSI extension. See standards(5).
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
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Standard C Library Functions printf(3C)
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);
ATRIBUTES
See attributes(5) for descriptions of the following attri-
butes:
ATRIBUTE TYPE ATRIBUTE 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(5) for the standards conformance of printf(),
fprintf(), sprintf(), and snprintf(). The asprintf() func-
tion is modeled on the one that appears in the FreeBSD,
NetBSD, and GNU C libraries.
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Standard C Library Functions printf(3C)
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(5), environ(5), stan-
dards(5)
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 ini-
tial SUSv2 specification, where snprintf() when n = 0
returns an unspecified value less than 1.
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