PRINTF(3) Linux Programmer's Manual PRINTF(3)
printf, fprintf, sprintf, snprintf, vprintf, vfprintf, vsprintf,
vsnprintf - formatted output conversion
int printf(const char *format, ...);
int fprintf(FILE *stream, const char *format, ...);
int sprintf(char *str, const char *format, ...);
int snprintf(char *str, size_t size, const char *format, ...);
int vprintf(const char *format, va_list ap);
int vfprintf(FILE *stream, const char *format, va_list ap);
int vsprintf(char *str, const char *format, va_list ap);
int vsnprintf(char *str, size_t size, const char *format, va_list ap);
The functions in the printf family produce output according to a format
as described below. The functions printf and vprintf write output to
stdout, the standard output stream; fprintf and vfprintf write output
to the given output stream; sprintf, snprintf, vsprintf and vsnprintf
write to the character string str.
The functions vprintf, vfprintf, vsprintf, vsnprintf are equivalent to
the functions printf, fprintf, sprintf, snprintf, respectively, except
that they are called with a va_list instead of a variable number of
arguments. These functions do not call the va_end macro. Consequently,
the value of ap is undefined after the call. The application should
call va_end(ap) itself afterwards.
These eight functions write the output under the control of a format
string that specifies how subsequent arguments (or arguments accessed
via the variable-length argument facilities of stdarg(3)) are converted
Upon successful return, these functions return the number of characters
printed (not including the trailing '\0' used to end output to
strings). The functions snprintf and vsnprintf do not write more than
size bytes (including the trailing '\0'). If the output was truncated
due to this limit then the return value is the number of characters
(not including the trailing '\0') which would have been written to the
final string if enough space had been available. Thus, a return value
of size or more means that the output was truncated. (See also below
under NOTES.) If an output error is encountered, a negative value is
Format of the format string
The format string is a character string, beginning and ending in its
initial shift state, if any. The format string is composed of zero or
more directives: ordinary characters (not %), which are copied
unchanged to the output stream; and conversion specifications, each of
which results in fetching zero or more subsequent arguments. Each con-
version specification is introduced by the character %, and ends with a
conversion specifier. In between there may be (in this order) zero or
more flags, an optional minimum field width, an optional precision and
an optional length modifier.
The arguments must correspond properly (after type promotion) with the
conversion specifier. By default, the arguments are used in the order
given, where each `*' and each conversion specifier asks for the next
argument (and it is an error if insufficiently many arguments are
given). One can also specify explicitly which argument is taken, at
each place where an argument is required, by writing `%m$' instead of
`%' and `*m$' instead of `*', where the decimal integer m denotes the
position in the argument list of the desired argument, indexed starting
from 1. Thus,
printf("%*d", width, num);
printf("%2$*1$d", width, num);
are equivalent. The second style allows repeated references to the same
argument. The C99 standard does not include the style using `$', which
comes from the Single Unix Specification. If the style using `$' is
used, it must be used throughout for all conversions taking an argument
and all width and precision arguments, but it may be mixed with `%%'
formats which do not consume an argument. There may be no gaps in the
numbers of arguments specified using `$'; for example, if arguments 1
and 3 are specified, argument 2 must also be specified somewhere in the
For some numeric conversions a radix character (`decimal point') or
thousands' grouping character is used. The actual character used
depends on the LC_NUMERIC part of the locale. The POSIX locale uses `.'
as radix character, and does not have a grouping character. Thus,
results in `1234567.89' in the POSIX locale, in `1234567,89' in the
nl_NL locale, and in `1.234.567,89' in the da_DK locale.
The flag characters
The character % is followed by zero or more of the following flags:
# The value should be converted to an ``alternate form''. For o
conversions, the first character of the output string is made
zero (by prefixing a 0 if it was not zero already). For x and X
conversions, a non-zero result has the string `0x' (or `0X' for
X conversions) prepended to it. For a, A, e, E, f, F, g, and G
conversions, the result will always contain a decimal point,
even if no digits follow it (normally, a decimal point appears
in the results of those conversions only if a digit follows).
For g and G conversions, trailing zeros are not removed from the
result as they would otherwise be. For other conversions, the
result is undefined.
0 The value should be zero padded. For d, i, o, u, x, X, a, A, e,
E, f, F, g, and G conversions, the converted value is padded on
the left with zeros rather than blanks. If the 0 and - flags
both appear, the 0 flag is ignored. If a precision is given
with a numeric conversion (d, i, o, u, x, and X), the 0 flag is
ignored. For other conversions, the behavior is undefined.
- The converted value is to be left adjusted on the field bound-
ary. (The default is right justification.) Except for n conver-
sions, the converted value is padded on the right with blanks,
rather than on the left with blanks or zeros. A - overrides a 0
if both are given.
' ' (a space) A blank should be left before a positive number (or
empty string) produced by a signed conversion.
+ A sign (+ or -) always be placed before a number produced by a
signed conversion. By default a sign is used only for negative
numbers. A + overrides a space if both are used.
The five flag characters above are defined in the C standard. The
SUSv2 specifies one further flag character.
' For decimal conversion (i, d, u, f, F, g, G) the output is to be
grouped with thousands' grouping characters if the locale infor-
mation indicates any. Note that many versions of gcc cannot
parse this option and will issue a warning. SUSv2 does not
glibc 2.2 adds one further flag character.
I For decimal integer conversion (i, d, u) the output uses the
locale's alternative output digits, if any (for example, Arabic
digits). However, it does not include any locale definitions
with such outdigits defined.
The field width
An optional decimal digit string (with nonzero first digit) specifying
a minimum field width. If the converted value has fewer characters
than the field width, it will be padded with spaces on the left (or
right, if the left-adjustment flag has been given). Instead of a deci-
mal digit string one may write `*' or `*m$' (for some decimal integer
m) to specify that the field width is given in the next argument, or in
the m-th argument, respectively, which must be of type int. A negative
field width is taken as a `-' flag followed by a positive field width.
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 expanded to contain the conversion result.
An optional precision, in the form of a period (`.') followed by an
optional decimal digit string. Instead of a decimal digit string one
may write `*' or `*m$' (for some decimal integer m) to specify that the
precision is given in the next argument, or in the m-th argument,
respectively, which must be of type int. If the precision is given as
just `.', or the precision is negative, the precision is taken to be
zero. This gives the minimum number of digits to appear for d, i, o,
u, x, and X conversions, the number of digits to appear after the radix
character for a, A, e, E, f, and F conversions, the maximum number of
significant digits for g and G conversions, or the maximum number of
characters to be printed from a string for s and S conversions.
The length modifier
Here, `integer conversion' stands for d, i, o, u, x, or X conversion.
hh A following integer conversion corresponds to a signed char or
unsigned char argument, or a following n conversion corresponds
to a pointer to a signed char argument.
h A following integer conversion corresponds to a short int or
unsigned short int argument, or a following n conversion corre-
sponds to a pointer to a short int argument.
l (ell) A following integer conversion corresponds to a long int
or unsigned long int argument, or a following n conversion cor-
responds to a pointer to a long int argument, or a following c
conversion corresponds to a wint_t argument, or a following s
conversion corresponds to a pointer to wchar_t argument.
ll (ell-ell). A following integer conversion corresponds to a long
long int or unsigned long long int argument, or a following n
conversion corresponds to a pointer to a long long int argument.
L A following a, A, e, E, f, F, g, or G conversion corresponds to
a long double argument. (C99 allows %LF, but SUSv2 does not.)
q (`quad'. BSD 4.4 and Linux libc5 only. Don't use.) This is a
synonym for ll.
j A following integer conversion corresponds to an intmax_t or
z A following integer conversion corresponds to a size_t or
ssize_t argument. (Linux libc5 has Z with this meaning. Don't
t A following integer conversion corresponds to a ptrdiff_t argu-
The SUSv2 only knows about the length modifiers h (in hd, hi, ho, hx,
hX, hn) and l (in ld, li, lo, lx, lX, ln, lc, ls) and L (in Le, LE, Lf,
The conversion specifier
A character that specifies the type of conversion to be applied. The
conversion specifiers and their meanings are:
d,i The int argument is converted to signed decimal notation. The
precision, if any, gives the minimum number of digits that must
appear; if the converted value requires fewer digits, it is
padded on the left with zeros. The default precision is 1. When
0 is printed with an explicit precision 0, the output is empty.
The unsigned int argument is converted to unsigned octal (o),
unsigned decimal (u), or unsigned hexadecimal (x and X) nota-
tion. The letters abcdef are used for x conversions; the let-
ters ABCDEF are used for X conversions. The precision, if any,
gives the minimum number of digits that must appear; if the con-
verted value requires fewer digits, it is padded on the left
with zeros. The default precision is 1. When 0 is printed with
an explicit precision 0, the output is empty.
e,E The double argument is rounded and converted in the style
Linux Manpage 2000-10-16 PRINTF(3)