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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ATTRIBUTES | STANDARDS | HISTORY | CAVEATS | BUGS | EXAMPLES | SEE ALSO | COLOPHON |
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printf(3) Library Functions Manual printf(3)
printf, fprintf, dprintf, sprintf, snprintf, vprintf, vfprintf,
vdprintf, vsprintf, vsnprintf - formatted output conversion
Standard C library (libc, -lc)
#include <stdio.h>
int printf(const char *restrict format, ...);
int fprintf(FILE *restrict stream,
const char *restrict format, ...);
int dprintf(int fd,
const char *restrict format, ...);
int sprintf(char *restrict str,
const char *restrict format, ...);
int snprintf(char str[restrict .size], size_t size,
const char *restrict format, ...);
int vprintf(const char *restrict format, va_list ap);
int vfprintf(FILE *restrict stream,
const char *restrict format, va_list ap);
int vdprintf(int fd,
const char *restrict format, va_list ap);
int vsprintf(char *restrict str,
const char *restrict format, va_list ap);
int vsnprintf(char str[restrict .size], size_t size,
const char *restrict format, va_list ap);
Feature Test Macro Requirements for glibc (see
feature_test_macros(7)):
snprintf(), vsnprintf():
_XOPEN_SOURCE >= 500 || _ISOC99_SOURCE
|| /* glibc <= 2.19: */ _BSD_SOURCE
dprintf(), vdprintf():
Since glibc 2.10:
_POSIX_C_SOURCE >= 200809L
Before glibc 2.10:
_GNU_SOURCE
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 function dprintf() is the same as fprintf() except that it
outputs to a file descriptor, fd, instead of to a stdio(3)
stream.
The functions snprintf() and vsnprintf() write at most size bytes
(including the terminating null byte ('\0')) to str.
The functions vprintf(), vfprintf(), vdprintf(), vsprintf(),
vsnprintf() are equivalent to the functions printf(), fprintf(),
dprintf(), 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
undefined after the call. See stdarg(3).
All of these 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 copying 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 CAVEATS.
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 precision and
an optional length modifier.
The overall syntax of a conversion specification is:
%[$][flags][width][.precision][length modifier]conversion
The arguments must correspond properly (after type promotion)
with the conversion specifier. By default, the arguments are
used in the order given, where each '*' (see Field width and
Precision below) and each conversion specifier asks for the next
argument (and it is an error if insufficiently many arguments are
given). One can also specify explicitly which argument is taken,
at each place where an argument is required, by writing "%m$"
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. (See
setlocale(3).) 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.
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 m, if errno
contains a valid error code, the output of
strerrorname_np(errno) is printed; otherwise, the value
stored in errno is printed as a decimal number. 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 an integer 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.) 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 C99 standard.
The Single UNIX Specification 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. (See setlocale(3).)
Note that many versions of gcc(1) cannot parse this option
and will issue a warning. (SUSv2 did not include %'F, but
SUSv3 added it.) Note also that the default locale of a C
program is "C" whose locale information indicates no
thousands' grouping character. Therefore, without a prior
call to setlocale(3), no thousands' grouping characters
will be printed.
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.
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 decimal digit string one may write "*"
or "*m$" (for some decimal integer m) to specify that the field
width is given in the next argument, or in the m-th argument,
respectively, which must be of type int. A negative field width
is taken as a '-' flag followed by a positive field width. In no
case does a nonexistent or small field width cause truncation of
a field; if the result of a conversion is wider than the field
width, the field is expanded to contain the conversion result.
Precision
An optional precision, in the form of a period ('.') followed by
an optional decimal digit string. Instead of a decimal digit
string one may write "*" or "*m$" (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 conversions,
or the maximum number of characters to be printed from a string
for s and S conversions.
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 or
unsigned short argument, or a following n conversion
corresponds to a pointer to a short argument.
l (ell) A following integer conversion corresponds to a long
or unsigned long argument, or a following n conversion
corresponds to a pointer to a long 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. On a following a, A, e, E, f, F, g, or
G conversion, this length modifier is ignored (C99; not in
SUSv2).
ll (ell-ell). A following integer conversion corresponds to
a long long or unsigned long long argument, or a following
n conversion corresponds to a pointer to a long long
argument.
q A synonym for ll. This is a nonstandard extension,
derived from BSD; avoid its use in new code.
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.)
j A following integer conversion corresponds to an intmax_t
or uintmax_t argument, or a following n conversion
corresponds to a pointer to an intmax_t argument.
z A following integer conversion corresponds to a size_t or
ssize_t argument, or a following n conversion corresponds
to a pointer to a size_t argument.
Z A nonstandard synonym for z that predates the appearance
of z. Do not use in new code.
t A following integer conversion corresponds to a ptrdiff_t
argument, or a following n conversion corresponds to a
pointer to a ptrdiff_t argument.
SUSv3 specifies all of the above, except for those modifiers
explicitly noted as being nonstandard extensions. SUSv2
specified 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).
As a nonstandard extension, the GNU implementations treats ll and
L as synonyms, so that one can, for example, write llg (as a
synonym for the standards-compliant Lg) and Ld (as a synonym for
the standards compliant lld). Such usage is nonportable.
Conversion specifiers
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 (which is nonzero if
the argument is nonzero) 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.
(SUSv2 does not know about F and says that character
string representations for infinity and NaN may be made
available. SUSv3 adds a specification for F. 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, but added in SUSv3) 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
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 nonnormalized numbers, and nonzero but
otherwise unspecified for normalized numbers. The
exponent always contains at least one digit; if the value
is zero, the exponent is 0.
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 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 or C11, but in SUSv2, SUSv3, and SUSv4.)
Synonym for lc. Don't use.
S (Not in C99 or C11, but in SUSv2, SUSv3, and SUSv4.)
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 pointed to by the corresponding argument. That
argument shall be an int *, or variant whose size matches
the (optionally) supplied integer length modifier. No
argument is converted. (This specifier is not supported
by the bionic C library.) The behavior is undefined if
the conversion specification includes any flags, a field
width, or a precision.
m (glibc extension; supported by uClibc and musl.) Print
output of strerror(errno) (or strerrorname_np(errno) in
the alternate form). No argument is required.
% A '%' is written. No argument is converted. The complete
conversion specification is '%%'.
Upon successful return, these functions return the number of
bytes 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 CAVEATS.)
If an output error is encountered, a negative value is returned.
For an explanation of the terms used in this section, see
attributes(7).
┌──────────────────────────────┬───────────────┬────────────────┐
│ Interface │ Attribute │ Value │
├──────────────────────────────┼───────────────┼────────────────┤
│ printf(), fprintf(), │ Thread safety │ MT-Safe locale │
│ sprintf(), snprintf(), │ │ │
│ vprintf(), vfprintf(), │ │ │
│ vsprintf(), vsnprintf() │ │ │
└──────────────────────────────┴───────────────┴────────────────┘
fprintf()
printf()
sprintf()
vprintf()
vfprintf()
vsprintf()
snprintf()
vsnprintf()
C11, POSIX.1-2008.
dprintf()
vdprintf()
GNU, POSIX.1-2008.
fprintf()
printf()
sprintf()
vprintf()
vfprintf()
vsprintf()
C89, POSIX.1-2001.
snprintf()
vsnprintf()
SUSv2, C99, POSIX.1-2001.
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. POSIX.1-2001 and later
align their specification of snprintf() with C99.
dprintf()
vdprintf()
GNU, POSIX.1-2008.
glibc 2.1 adds length modifiers hh, j, t, and z and conversion
characters a and A.
glibc 2.2 adds the conversion character F with C99 semantics, and
the flag character I.
glibc 2.35 gives a meaning to the alternate form (#) of the m
conversion specifier, that is %#m.
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 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(3) and
vasprintf(3)).
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 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
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, ...)
{
int n = 0;
size_t size = 0;
char *p = NULL;
va_list ap;
/* Determine required size. */
va_start(ap, fmt);
n = vsnprintf(p, size, fmt, ap);
va_end(ap);
if (n < 0)
return NULL;
size = (size_t) n + 1; /* One extra byte for '\0' */
p = malloc(size);
if (p == NULL)
return NULL;
va_start(ap, fmt);
n = vsnprintf(p, size, fmt, ap);
va_end(ap);
if (n < 0) {
free(p);
return NULL;
}
return p;
}
If truncation occurs in glibc versions prior to glibc 2.0.6, this
is treated as an error instead of being handled gracefully.
printf(1), asprintf(3), puts(3), scanf(3), setlocale(3),
strfromd(3), wcrtomb(3), wprintf(3), locale(5)
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