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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);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
snprintf(), vsnprintf():
_BSD_SOURCE || _XOPEN_SOURCE >= 500 || _ISOC99_SOURCE ||
_POSIX_C_SOURCE >= 200112L;
or cc -std=c99
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 output to the given output stream; sprintf(), snprintf(),
vsprintf() and vsnprintf() write to the character string str.
The functions snprintf() and vsnprintf() write at most size bytes
(including the terminating null byte ('\0')) to str.
The functions vprintf(), vfprintf(), vsprintf(), vsnprintf() are equiv-
alent 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. Because they invoke the va_arg macro, the value of ap is unde-
fined after the call. See stdarg(3).
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.
C99 and POSIX.1-2001 specify that the results are undefined if a call
to sprintf(), snprintf(), vsprintf(), or vsnprintf() would cause copy-
ing to take place between objects that overlap (e.g., if the target
string array and one of the supplied input arguments refer to the same
buffer). See NOTES.
Return value
Upon successful return, these functions return the number of characters
printed (excluding the null byte used to end output to strings).
The functions snprintf() and vsnprintf() do not write more than size
bytes (including the terminating null byte ('\0')). If the output was
truncated due to this limit, then the return value is the number of
characters (excluding the terminating null byte) 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 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);
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 argu-
ment 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 nonzero 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 con-
versions, the converted value is padded on the right with
blanks, rather than on the left with blanks or zeros. A - over-
rides 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 neg-
ative 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(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.
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 nonexistent 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 '.', the precision is taken to be zero. A negative precision is
taken as if the precision were omitted. 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 con-
versions, 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". 4.4BSD 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 knows about only 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 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
[-]d.ddde+-dd where there is one digit before the decimal-point
character and the number of digits after it is equal to the pre-
cision; 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 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+-; 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 nonnormalized 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(3)
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 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 preci-
sion 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 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 inte-
ger indicated by the int * (or variant) pointer argument. No
argument is converted.
m (Glibc extension.) Print output of strerror(errno). No argu-
ment is required.
% A '%' is written. No argument is converted. The complete con-
version specification is '%%'.
CONFORMING TO
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 c, d, e, E, f, F, g, G,
i, n, o, p, s, u, x, and X, where F is a synonym for f. Additionally,
it accepts D, O, and U as synonyms for ld, lo, and 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, and q, but accepts L and q both for long double and for long long
int (this is a bug). It no longer recognizes F, D, O, and U, 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, and z and conversion charac-
ters a and A.
glibc 2.2 adds the conversion character F with C99 semantics, and the
flag character I.
NOTES
Some programs imprudently rely on code such as the following
sprintf(buf, "%s some further text", buf);
to append text to buf. However, the standards explicitly note that the
results are undefined if source and destination buffers overlap when
calling sprintf(), snprintf(), vsprintf(), and vsnprintf(). Depending
on the version of gcc(1) used, and the compiler options employed, calls
such as the above will not produce the expected results.
The glibc implementation of the functions snprintf() and vsnprintf()
conforms to the C99 standard, that is, behaves as described above,
since glibc version 2.1. Until glibc 2.0.6 they would return -1 when
the output was truncated.
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 produced is
locale-dependent and difficult to predict. Use snprintf() and
vsnprintf() instead (or asprintf(3) and vasprintf(3)).
Linux libc4.[45] does not have a snprintf(), but provides a libbsd that
contains an snprintf() equivalent to sprintf(), that is, 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 secu-
rity hole.
EXAMPLE
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 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, ...)
{
int n;
int size = 100; /* Guess we need no more than 100 bytes */
char *p, *np;
va_list ap;
p = malloc(size);
if (p == 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);
/* Check error code */
if (n < 0) {
free(p);
return NULL;
}
/* If that worked, return the string */
if (n < size)
return p;
/* Else try again with more space */
size = n + 1; /* Precisely what is needed */
np = realloc(p, size);
if (np == NULL) {
free(p);
return NULL;
} else {
p = np;
}
}
}
If truncation occurs in glibc versions prior to 2.0.6, this is treated
as an error instead of being handled gracefully.
SEE ALSO
printf(1), asprintf(3), dprintf(3), scanf(3), setlocale(3), wcrtomb(3),
wprintf(3), locale(5)
COLOPHON
This page is part of release 3.69 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
http://www.kernel.org/doc/man-pages/.
GNU 2013-12-30 PRINTF(3)
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