Originální popis anglicky:
printf, fprintf, sprintf, snprintf, vprintf, vfprintf, vsprintf, vsnprintf -
formatted output conversion
Návod, kniha: Linux Programmer's Manual
#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);
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
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. Consequently, the value of
ap is undefined after
the call. The application should call
va_end(ap) itself afterwards.
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.
Upon successful return, these functions return the number of characters printed
(not including the trailing '\0' used to end output to strings). The functions
snprintf and
vsnprintf do not write more than
size bytes
(including the trailing '\0'). If the output was truncated due to this limit
then the return value is the number of characters (not including the trailing
'\0') 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 non-zero 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 -) 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 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 non-existent 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'. BSD 4.4 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 only knows about 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*(Pmdd 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*(Pmd; 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 non-normalized 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 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 NUL character; if a precision is
specified, no more than the number specified are written. If a precision
is given, no null character need be present; if the precision is not
specified, or is greater than the size of the array, the array must
contain a terminating NUL character.
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 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.
- %
- A `%' is written. No argument is converted. The complete
conversion specification is `%%'.
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, ...) {
/* Guess we need no more than 100 bytes. */
int n, size = 100;
char *p;
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);
/* If that worked, return the string. */
if (n > -1 && n < size)
return p;
/* Else try again with more space. */
if (n > -1) /* glibc 2.1 */
size = n+1; /* precisely what is needed */
else /* glibc 2.0 */
size *= 2; /* twice the old size */
if ((p = realloc (p, size)) == NULL)
return NULL;
}
}
The glibc implementation of the functions
snprintf and
vsnprintf
conforms to the C99 standard, i.e., behaves as described above, since glibc
version 2.1. Until glibc 2.0.6 they would return -1 when the output was
truncated.
The
fprintf,
printf,
sprintf,
vprintf,
vfprintf, and
vsprintf functions conform to ANSI X3.159-1989
(``ANSI C'') and ISO/IEC 9899:1999 (``ISO C99''). The
snprintf and
vsnprintf functions conform to ISO/IEC 9899:1999.
Concerning the return value of
snprintf, the SUSv2 and the C99 standard
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 cdeEfFgGinopsuxX, where F is a synonym
for f. Additionally, it accepts D,O,U as synonyms for ld,lo,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,q, but accepts L and q
both for long doubles and for long long integers (this is a bug). It no longer
recognizes FDOU, but adds a new 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,z and conversion characters a,A.
glibc 2.2 adds the conversion character F with C99 semantics, and the flag
character I.
Unix V7 defines the three routines
printf,
fprintf,
sprintf, and has the flag -, the width or precision *, the length
modifier l, and the conversions doxfegcsu, and also D,O,U,X as synonyms for
ld,lo,lu,lx. This is still true for BSD 2.9.1, but BSD 2.10 has the flags #, +
and <space> and no longer mentions D,O,U,X. BSD 2.11 has
vprintf,
vfprintf,
vsprintf, and warns not to use D,O,U,X. BSD 4.3 Reno
has the flag 0, the length modifiers h and L, and the conversions n, p, E, G,
X (with current meaning) and deprecates D,O,U. BSD 4.4 introduces the
functions
snprintf and
vsnprintf, and the length modifier q.
FreeBSD also has functions
asprintf and
vasprintf, that allocate
a buffer large enough for
sprintf. In glibc there are functions
dprintf and
vdprintf that print to a file descriptor instead of
a stream.
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 and
vasprintf).
Linux libc4.[45] does not have a
snprintf, but provides a libbsd that
contains an
snprintf equivalent to
sprintf, i.e., 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.
printf(1),
asprintf(3),
dprintf(3),
scanf(3),
wcrtomb(3),
wprintf(3),
locale(5)
