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PRINTF(3) Linux Programmer's Manual PRINTF(3)
printf, fprintf, sprintf, snprintf, vprintf, vfprintf, vsprintf,
vsnprintf - formatted output conversion
#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
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 functions snprintf() and vsnprintf() write at most size bytes
(including the terminating null byte ('\0')) to 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. Because they invoke the va_arg macro, the
value of ap is undefined 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
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.
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.
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 '*' 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. 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 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.) Except for n
conversions, 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 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(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.
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.
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
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.
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.
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
argument.
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).
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.
(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 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) 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, 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.
m (Glibc extension.) Print output of strerror(errno). No
argument is required.
% A '%' is written. No argument is converted. The complete
conversion specification is '%%'.
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
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)).
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 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):
If truncation occurs in glibc versions prior to 2.0.6, this is
treated as an error instead of being handled gracefully.
#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;
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);
/* Check error code */
if (n < 0)
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 */
if ((np = realloc (p, size)) == NULL) {
free(p);
return NULL;
} else {
p = np;
}
}
}
printf(1), asprintf(3), dprintf(3), scanf(3), setlocale(3),
wcrtomb(3), wprintf(3), locale(5)
This page is part of release 3.51 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can
be found at http://www.kernel.org/doc/man-pages/.
GNU 2013-03-05 PRINTF(3)
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