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PRINTF(3) Linux Programmer's Manual PRINTF(3)
printf, fprintf, dprintf, sprintf, snprintf, vprintf, vfprintf,
vdprintf, vsprintf, vsnprintf - formatted output conversion
#include <stdio.h>
int printf(const char *format, ...);
int fprintf(FILE *stream, const char *format, ...);
int dprintf(int fd, 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 vdprintf(int fd, 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():
_XOPEN_SOURCE >= 500 || _ISOC99_SOURCE ||
|| /* Glibc versions <= 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 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 NOTES.
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 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 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.) 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.)
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 int or
unsigned short int argument, or a following n conversion
corresponds 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
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.
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.
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 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±; 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.
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). 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
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.
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():
POSIX.1-2001, POSIX.1-2008, C89, C99.
snprintf(), vsnprintf(): POSIX.1-2001, POSIX.1-2008, C99.
The dprintf() and vdprintf() functions were originally GNU extensions
that were later standardized in POSIX.1-2008.
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.
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.
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.
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 size = 0;
char *p = NULL;
va_list ap;
/* Determine required size */
va_start(ap, fmt);
size = vsnprintf(p, size, fmt, ap);
va_end(ap);
if (size < 0)
return NULL;
size++; /* For '\0' */
p = malloc(size);
if (p == NULL)
return NULL;
va_start(ap, fmt);
size = vsnprintf(p, size, fmt, ap);
va_end(ap);
if (size < 0) {
free(p);
return NULL;
}
return p;
}
If truncation occurs in glibc versions prior to 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)
This page is part of release 4.12 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
https://www.kernel.org/doc/man-pages/.
GNU 2016-12-12 PRINTF(3)
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