printf(3) Library Functions Manual printf(3)
NAME
printf, fprintf, dprintf, sprintf, snprintf, vprintf, vfprintf, vd-
printf, vsprintf, vsnprintf - formatted output conversion
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#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
DESCRIPTION
The functions in the printf() family produce output according to a for-
mat as described below. The functions printf() and vprintf() write out-
put 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 (in-
cluding 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 ar-
ray 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 re-
sults in fetching zero or more subsequent arguments. Each conversion
specification is introduced by the character %, and ends with a conver-
sion specifier. In between there may be (in this order) zero or more
flags, an optional minimum field width, an optional precision and an op-
tional length modifier.
The overall syntax of a conversion specification is:
%[$][flags][width][.precision][length modifier]conversion
The arguments must correspond properly (after type promotion) with the
conversion specifier. By default, the arguments are used in the order
given, where each '*' (see Field width and Precision below) and each
conversion specifier asks for the next argument (and it is an error if
insufficiently many arguments are given). One can also specify explic-
itly which argument is taken, at each place where an argument is re-
quired, by writing "%m$" 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 de-
pends 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 con-
versions, the first character of the output string is made zero
(by prefixing a 0 if it was not zero already). For x and X con-
versions, 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 ig-
nored. 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 neg-
ative 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 infor-
mation indicates any. (See setlocale(3).) Note that many ver-
sions of gcc(1) cannot parse this option and will issue a warn-
ing. (SUSv2 did not include %'F, but SUSv3 added it.) Note also
that the default locale of a C program is "C" whose locale infor-
mation 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 lo-
cale'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$" (for some decimal integer m) to spec-
ify that the field width is given in the next argument, or in the m-th
argument, respectively, which must be of type int. A negative field
width is taken as a '-' flag followed by a positive field width. In no
case does a nonexistent or small field width cause truncation of a
field; if the result of a conversion is wider than the field width, the
field is expanded to contain the conversion result.
Precision
An optional precision, in the form of a period ('.') followed by an op-
tional decimal digit string. Instead of a decimal digit string one may
write "*" or "*m$" (for some decimal integer m) to specify that the pre-
cision is given in the next argument, or in the m-th argument, respec-
tively, 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 dig-
its 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 conver-
sions, 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 un-
signed long argument, or a following n conversion corresponds to
a pointer to a long argument, or a following c conversion corre-
sponds to a wint_t argument, or a following s conversion corre-
sponds 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 argu-
ment, 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), un-
signed 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 pre-
cision 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 pre-
cision 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 rep-
resentations 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 "[-]INFIN-
ITY" 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 con-
verted to a multibyte sequence by a call to the wcrtomb(3) func-
tion, 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 ex-
pected 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 ex-
pected 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 charac-
ter), up to and including a terminating null wide character. The
resulting multibyte characters are written up to (but not includ-
ing) 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 deter-
mines the number of bytes written, not the number of wide charac-
ters 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 inte-
ger 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 con-
version specification is '%%'.
RETURN VALUE
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.
ATTRIBUTES
For an explanation of the terms used in this section, see attributes(7).
┌─────────────────────────────────────┬───────────────┬────────────────┐
│ Interface │ Attribute │ Value │
├─────────────────────────────────────┼───────────────┼────────────────┤
│ printf(), fprintf(), sprintf(), │ Thread safety │ MT-Safe locale │
│ snprintf(), vprintf(), vfprintf(), │ │ │
│ vsprintf(), vsnprintf() │ │ │
└─────────────────────────────────────┴───────────────┴────────────────┘
STANDARDS
fprintf()
printf()
sprintf()
vprintf()
vfprintf()
vsprintf()
snprintf()
vsnprintf()
C11, POSIX.1-2008.
dprintf()
vdprintf()
GNU, POSIX.1-2008.
HISTORY
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 contra-
dict each other: when snprintf() is called with size=0 then SUSv2
stipulates an unspecified return value less than 1, while C99 al-
lows 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 charac-
ters 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.
CAVEATS
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.
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 vs-
nprintf() 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.
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 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.
SEE ALSO
printf(1), asprintf(3), puts(3), scanf(3), setlocale(3), strfromd(3),
wcrtomb(3), wprintf(3), locale(5)
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