fprintf
PRINTF(3) Linux Programmer’s Manual PRINTF(3)
NAME
printf, fprintf, sprintf, snprintf, vprintf, vfprintf, vsprintf,
vsnprintf - formatted output conversion
SYNOPSIS
#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);
DESCRIPTION
The functions in the printf family produce output according to a for-
mat as described below. The functions printf and vprintf write output
to stdout, the standard output stream; fprintf and vfprintf write out-
put 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 con-
verted for output.
Return value
Upon successful return, these functions return the number of charac-
ters 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.
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
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 pre-
cision 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 start-
ing 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 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 unde-
fined.
- The converted value is to be left adjusted on the field bound-
ary. (The default is right justification.) Except for n con-
versions, 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
information indicates any. Note that many versions of gcc can-
not 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.
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 dec-
imal 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 neg-
ative 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.
The precision
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 num-
ber of significant digits for g and G conversions, or the maximum num-
ber 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 follow-
ing 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
uintmax_t argument.
z A following integer conversion corresponds to a size_t or
ssize_t argument. (Linux libc5 has Z with this meaning. Don’t
use it.)
t A following integer conversion corresponds to a ptrdiff_t argu-
ment.
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).
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.
o,u,x,X
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
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 nota-
tion in the style [-]ddd.ddd, where the number of digits after
the decimal-point character is equal to the precision specifi-
cation. 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 con-
version, 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 signifi-
cant 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 [-]0xh.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 pre-
cision suffices for an exact representation of the value if an
exact representation in base 2 exists and otherwise is suffi-
ciently large to distinguish values of type double. The digit
before the decimal point is unspecified for non-normalized num-
bers, and nonzero but otherwise unspecified for normalized num-
bers.
c 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
function, with a conversion state starting in the initial
state, and the resulting multibyte string is written.
s 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 NUL character; if a precision is
specified, no more than the number specified are written. If a
precision is given, no null character need be present; if the
precision is not specified, or is greater than the size of the
array, the array must contain a terminating NUL character.
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 function, with a conversion
state starting in the initial state before the first wide char-
acter), 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 speci-
fied, no more bytes than the number specified are written, but
no partial multibyte characters are written. Note that the pre-
cision 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.
p The void * pointer argument is printed in hexadecimal (as if by
%#x or %#lx).
n The number of characters written so far is stored into the
integer indicated by the int * (or variant) pointer argument.
No argument is converted.
% A ‘%’ is written. No argument is converted. The complete con-
version specification is ‘%%’.
EXAMPLES
To print pi 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 international-
ized 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 cor-
rect 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;
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 ((p = realloc (p, size)) == NULL)
return NULL;
}
}
NOTES
The glibc implementation of the functions snprintf and vsnprintf con-
forms 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.
CONFORMING TO
The fprintf, printf, sprintf, vprintf, vfprintf, and vsprintf func-
tions conform to ANSI X3.159-1989 (‘‘ANSI C’’) and ISO/IEC 9899:1999
(‘‘ISO C99’’). The snprintf and vsnprintf functions conform to
ISO/IEC 9899:1999.
Concerning the return value of snprintf, the SUSv2 and the C99 stan-
dard 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 thou-
sands’ 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 a new conver-
sion 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.
HISTORY
Unix V7 defines the three routines printf, fprintf, sprintf, and has
the flag -, the width or precision *, the length modifier l, and the
conversions doxfegcsu, and also D,O,U,X as synonyms for ld,lo,lu,lx.
This is still true for BSD 2.9.1, but BSD 2.10 has the flags #, + and
<space> and no longer mentions D,O,U,X. BSD 2.11 has vprintf,
vfprintf, vsprintf, and warns not to use D,O,U,X. BSD 4.3 Reno has
the flag 0, the length modifiers h and L, and the conversions n, p, E,
G, X (with current meaning) and deprecates D,O,U. BSD 4.4 introduces
the functions snprintf and vsnprintf, and the length modifier q.
FreeBSD also has functions asprintf and vasprintf, that allocate a
buffer large enough for sprintf. In glibc there are functions dprintf
and vdprintf that print to a file descriptor instead of a stream.
BUGS
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 pro-
duced 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.
SEE ALSO
printf(1), asprintf(3), dprintf(3), wcrtomb(3), wprintf(3), scanf(3),
locale(5)
Linux Manpage 2000-10-16 PRINTF(3)
