printf, fprintf, sprintf, snprintf, vprintf, vfprintf, vsprintf, vsnprintf — formatted output conversion
#include <stdio.h>
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, | |
...) ; |
#include <stdarg.h>
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
for output.
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
returned.
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 conversion 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);
and
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 format string.
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,
printf("%'.2f", 1234567.89);
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 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 boundary. (The default is right justification.)
Except for n
conversions, 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 −) should 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
information indicates any. Note that many versions of
gcc(1) cannot parse
this option and will issue a warning. SUSv2 does not
include %'F.
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, since glibc 2.2.3 this will give
Arabic-Indic digits in the Persian (`fa_IR')
locale.
An optional decimal digit string (with non-zero 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 decimal
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.
Here, `integer conversion' stands for d
, i
, o
, u
, x
, or X
conversion.
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.
A following integer conversion corresponds to a
short int or
unsigned short
int argument, or a following n
conversion
corresponds to a pointer to a short int argument.
(ell) A following integer conversion corresponds
to a long int
or unsigned long
int argument, or a following n
conversion
corresponds 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.
(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.)
(`quad'. 4.4BSD and Linux libc5 only. Don't use.)
This is a synonym for ll
.
A following integer conversion corresponds to an
intmax_t
or
uintmax_t
argument.
A following integer conversion corresponds to a
size_t
or
ssize_t
argument. (Linux libc5 has Z
with this meaning. Don't use
it.)
A following integer conversion corresponds to a
ptrdiff_t
argument.
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
, Lg
, LG
).
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.
o
,u
,x
,X
The unsigned
int argument is converted to unsigned
octal (o
),
unsigned decimal (u
), or unsigned
hexadecimal (x
and X
) notation. The letters abcdef
are used for
x
conversions; the letters ABCDEF
are used for X
conversions. 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.
e
,E
The double
argument is
rounded and converted in the style
[−]d.
ddde
±dd
where there is one digit before the decimal-point
character and the number of digits after it is equal
to the precision; if the precision is missing, it is
taken as 6; if the precision is zero, no
decimal-point character appears. An E
conversion uses the letter
E
(rather than
e
) to
introduce the exponent. The exponent always contains
at least two digits; if the value is zero, the
exponent is 00.
f
,F
The double
argument is
rounded and converted to decimal notation in the
style [−]ddd.
ddd,
where the number of digits after the decimal-point
character is equal to the precision specification. If
the precision is missing, it is taken as 6; if the
precision is explicitly zero, no decimal-point
character appears. If a decimal point appears, at
least one digit appears before it.
(The SUSv2 does not know about F
and says that character string
representations for infinity and NaN may be made
available. The C99 standard specifies `[−]inf'
or `[−]infinity' for infinity, and a string
starting with `nan' for NaN, in the case of
f
conversion, and `[−]INF' or `[−]INFINITY'
or `NAN*' in the case of F
conversion.)
g
,G
The double
argument is
converted in style f
or e
(or F
or E
for G
conversions). The precision
specifies the number of significant digits. If the
precision is missing, 6 digits are given; if the
precision is zero, it is treated as 1. Style
e
is used
if the exponent from its 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 decimal point appears only if it is
followed by at least one digit.
a
,A
(C99; not in SUSv2) For a
conversion, the
double
argument is converted to hexadecimal notation (using
the letters abcdef) in the style [−]0x
h.
hhhhp
±d; for
A
conversion the prefix
0X
, the letters ABCDEF,
and the exponent separator P
is used. There is one hexadecimal
digit before the decimal point, and the number of
digits after it is equal to the precision. The
default precision suffices for an exact
representation of the value if an exact
representation in base 2 exists and otherwise is
sufficiently large to distinguish values of type
double
. The
digit before the decimal point is unspecified for
non-normalized numbers, and non-zero but otherwise
unspecified for normalized numbers.
If no l
modifier is present, the int
argument is
converted to an unsigned
char, and the resulting character is
written. If an l
modifier is
present, the wint_t
(wide
character) argument is converted to a multibyte
sequence by a call to the wcrtomb(3)
function, with a conversion state starting in the
initial state, and the resulting multibyte string is
written.
If no l
modifier is present: The const char * argument is
expected to be a pointer to an array of character
type (pointer to a string). Characters from the array
are written up to (but not including) a terminating
null byte ('\0'); if a precision is specified, no
more than the number specified are written. If a
precision is given, no null byte need be present; if
the precision is not specified, or is greater than
the size of the array, the array must contain a
terminating null byte.
If an l
modifier is present: The const wchar_t * argument
is expected to be a pointer to an array of wide
characters. Wide characters from the array are
converted to multibyte characters (each by a call to
the wcrtomb(3)
function, with a conversion state starting in the
initial state before the first wide character), up to
and including a terminating null wide character. The
resulting multibyte characters are written up to (but
not including) the terminating null byte. If a
precision is specified, no more bytes than the number
specified are written, but no partial multibyte
characters are written. Note that the precision
determines the number of bytes
written, not
the number of wide
characters or screen positions. The
array must contain a terminating null wide character,
unless a precision is given and it is so small that
the number of bytes written exceeds it before the end
of the array is reached.
C
(Not in C99, but in SUSv2.) Synonym for lc
. Don't use.
S
(Not in C99, but in SUSv2.) Synonym for ls
. Don't use.
The void *
pointer argument is printed in hexadecimal (as if by
%#x
or
%#lx
).
The number of characters written so far is stored into the integer indicated by the int * (or variant) pointer argument. No argument is converted.
(Glibc extension.) Print output of strerror(errno)
. No
argument is required.
A `%' is written. No argument is converted. The complete conversion specification is `%%'.
The fprintf
(), printf
(), sprintf
(), vprintf
(), vfprintf
(), and vsprintf
() functions conform to C89 and
C99. The snprintf
() and
vsnprintf
() functions conform
to C99.
Concerning the return value of snprintf
(), SUSv2 and C99 contradict each
other: when snprintf
() is
called with size
=0
then SUSv2 stipulates an unspecified return value less than
1, while C99 allows str
to be NULL in this case,
and gives the return value (as always) as the number of
characters that would have been written in case the output
string has been large enough.
Linux libc4 knows about the five C standard flags. It knows about the length modifiers h,l,L, and the conversions cdeEfFgGinopsuxX, where F is a synonym for f. Additionally, it accepts D,O,U as synonyms for ld,lo,lu. (This is bad, and caused serious bugs later, when support for %D disappeared.) No locale-dependent radix character, no thousands' separator, no NaN or infinity, no %m$ and *m$.
Linux libc5 knows about the five C standard flags and the
' flag, locale, %m$ and *m$. It knows about the length
modifiers h,l,L,Z,q, but accepts L and q both for long
doubles and for long long integers (this is a bug). It no
longer recognizes FDOU, but adds the conversion character
m
, which outputs
strerror(errno)
.
glibc 2.0 adds conversion characters C and S.
glibc 2.1 adds length modifiers hh,j,t,z and conversion characters a,A.
glibc 2.2 adds the conversion character F with C99 semantics, and the flag character I.
The glibc implementation of the functions snprintf
() and vsnprintf
() conforms to the C99 standard,
i.e., behaves as described above, since glibc version 2.1.
Until glibc 2.0.6 they would return −1 when the output
was truncated.
Because sprintf
() and
vsprintf
() assume an
arbitrarily long string, callers must be careful not to
overflow the actual space; this is often impossible to
assure. Note that the length of the strings produced is
locale-dependent and difficult to predict. Use snprintf
() and vsnprintf
() instead (or asprintf
() and vasprintf
).
Linux libc4.[45] does not have a snprintf
(), but provides a libbsd that
contains an snprintf
()
equivalent to sprintf
(), i.e.,
one that ignores the size
argument. Thus, the use of
snprintf
() with early libc4
leads to serious security problems.
Code such as
printf
(foo
);
often indicates a bug, since foo
may contain a %
character. If foo
comes from untrusted user input, it may contain %n, causing
the printf
() call to write to
memory and creating a security hole.
To print π to five decimal places:
#include <math.h> #include <stdio.h> fprintf(stdout, "pi = %.5f\n", 4 * atan(1.0));
To print a date and time in the form `Sunday, July 3,
10:02', where weekday
and month
are pointers to
strings:
#include <stdio.h> fprintf(stdout, "%s, %s %d, %.2d:%.2d\n", weekday, month, day, hour, min);
Many countries use the day-month-year order. Hence, an internationalized version must be able to print the arguments in an order specified by the format:
#include <stdio.h> fprintf(stdout, format, weekday, month, day, hour, min);
where format
depends on locale, and may permute the arguments. With the
value
"%1$s, %3$d. %2$s, %4$d:%5$.2d\n"
one might obtain `Sonntag, 3. Juli, 10:02'.
To allocate a sufficiently large string and print into it (code correct for both glibc 2.0 and glibc 2.1):
#include <stdio.h> #include <stdlib.h> #include <stdarg.h> char * make_message(const char *fmt, ...) { /* Guess we need no more than 100 bytes. */ int n, size = 100; char *p, *np; va_list ap; if ((p = malloc(size)) == NULL) return NULL; while (1) { /* Try to print in the allocated space. */ va_start(ap, fmt); n = vsnprintf(p, size, fmt, ap); va_end(ap); /* If that worked, return the string. */ if (n > −1 && n < size) return p; /* Else try again with more space. */ if (n > −1) /* glibc 2.1 */ size = n+1; /* precisely what is needed */ else /* glibc 2.0 */ size *= 2; /* twice the old size */ if ((np = realloc (p, size)) == NULL) { free(p); return NULL; } else { p = np; } } }
printf(1), asprintf(3), dprintf(3), scanf(3), setlocale(3), wcrtomb(3), wprintf(3), locale(5)
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