NAME
printf
, fprintf
,
sprintf
, snprintf
,
asprintf
, dprintf
,
vprintf
, vfprintf
,
vsprintf
, vsnprintf
,
vasprintf
, vdprintf
—
formatted output conversion
SYNOPSIS
#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,
...);
int
asprintf
(char
**ret, const char
*format, ...);
int
dprintf
(int
fd, const char * restrict
format, ...);
#include
<stdarg.h>
#include <stdio.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);
int
vasprintf
(char
**ret, const char
*format, va_list
ap);
int
vdprintf
(int
fd, const char * restrict
format, va_list
ap);
DESCRIPTION
The
printf
()
family of functions produce output according to the given
format as described below. This format may contain
“conversion specifiers”; the results of such conversions, if
any, depend on the arguments following the format
string.
The
printf
()
and
vprintf
()
functions write output to the standard output stream,
stdout;
fprintf
()
and
vfprintf
()
write output to the supplied stream pointer stream;
dprintf
()
and
vdprintf
()
write output to the given file descriptor;
sprintf
(), snprintf
(),
vsprintf
(), and vsnprintf
()
write to the character string str;
asprintf
()
and
vasprintf
()
write to a dynamically allocated string that is stored in
ret.
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.
snprintf
()
and
vsnprintf
()
will write at most size-1 of the characters printed
into the output string (the size'th character then
gets the terminating ‘\0
’); if the
return value is greater than or equal to the size
argument, the string was too short and some of the printed characters were
discarded. If size is zero, str
may be a null pointer and no characters will be written; the number of bytes
that would have been written excluding the terminating
‘\0
’ byte, or -1 on error, will be
returned.
sprintf
()
and
vsprintf
()
effectively assume an infinite size.
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 %
. The arguments must
correspond properly (after type promotion) with the conversion specifier.
After the %
, the following appear in sequence:
- An optional field, consisting of a decimal digit string followed by a
$
specifying the next argument to access. If this field is not provided, the argument following the last argument accessed will be used. Arguments are numbered starting at1
. - Zero or more of the following flags:
- A hash ‘
#
’ character specifying that the value should be converted to an “alternate form”. Foro
conversions, the precision of the number is increased to force the first character of the output string to a zero (except if a zero value is printed with an explicit precision of zero). Forx
andX
conversions, a non-zero result has the string ‘0x
’ (or ‘0X
’ forX
conversions) prepended to it. Fora
,A
,e
,E
,f
,F
,g
, andG
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). Forg
andG
conversions, trailing zeros are not removed from the result as they would otherwise be. For all other formats, behaviour is undefined. - A zero ‘
0
’ character specifying zero padding. For all conversions exceptn
, the converted value is padded on the left with zeros rather than blanks. If a precision is given with a numeric conversion (d
,i
,o
,u
,x
, andX
), the ‘0
’ flag is ignored. - A negative field width flag ‘
-
’ indicates the converted value is to be left adjusted on the field boundary. Except forn
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, specifying that a blank should be left before a positive
number produced by a signed conversion (
d
,a
,A
,e
,E
,f
,F
,g
,G
, ori
). - A ‘
+
’ character specifying that a sign always be placed before a number produced by a signed conversion. A ‘+
’ overrides a space if both are used.
- A hash ‘
- An optional decimal digit string 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) to fill out the field width.
- An optional precision, in the form of a period
‘
.
’ followed by an optional digit string. If the digit string is omitted, the precision is taken as zero. This gives the minimum number of digits to appear ford
,i
,o
,u
,x
, andX
conversions, the number of digits to appear after the decimal-point fora
,A
,e
,E
,f
, andF
conversions, the maximum number of significant digits forg
andG
conversions, or the maximum number of characters to be printed from a string fors
conversions. - An optional length modifier, that specifies the size of the argument. The
following length modifiers are valid for the
d
,i
,n
,o
,u
,x
, orX
conversion:Modifier d, i o, u, x, X n hh signed char unsigned char signed char * h short unsigned short short * l (ell) long unsigned long long * ll (ell ell) long long unsigned long long long long * j intmax_t uintmax_t intmax_t * t ptrdiff_t (see note) ptrdiff_t * z (see note) size_t (see note) q (deprecated) quad_t u_quad_t quad_t * Note: the
t
modifier, when applied to ano
,u
,x
, orX
conversion, indicates that the argument is of an unsigned type equivalent in size to a ptrdiff_t. Thez
modifier, when applied to ad
ori
conversion, indicates that the argument is of a signed type equivalent in size to a size_t. Similarly, when applied to ann
conversion, it indicates that the argument is a pointer to a signed type equivalent in size to a size_t.The following length modifier is valid for the
a
,A
,e
,E
,f
,F
,g
, orG
conversion:Modifier e, E, f, F, g, G l (ell) double (ignored: same behavior as without it) L long double The following length modifier is valid for the
c
ors
conversion:Modifier c s l (ell) wint_t wchar_t * - A character that specifies the type of conversion to be applied.
A field width or precision, or both, may be indicated by an
asterisk ‘*
’ or an asterisk followed
by one or more decimal digits and a
‘$
’ instead of a digit string. In this
case, an int
argument supplies the field width or
precision. A negative field width is treated as a left adjustment flag
followed by a positive field width; a negative precision is treated as
though it were missing. If a single format directive mixes positional (nn$)
and non-positional arguments, the results are undefined.
The conversion specifiers and their meanings are:
diouxX
- The
int
(or appropriate variant) argument is converted to signed decimal (d
andi
), unsigned octal (o
), unsigned decimal (u
), or unsigned hexadecimal (x
andX
) notation. The lettersabcdef
are used forx
conversions; the lettersABCDEF
are used forX
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. DOU
- The
long int
argument is converted to signed decimal, unsigned octal, or unsigned decimal, as if the format had beenld
,lo
, orlu
respectively. These conversion characters are deprecated, and will eventually disappear. eE
- 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. AnE
conversion uses the letterE
(rather thane
) to introduce the exponent. The exponent always contains at least two digits; if the value is zero, the exponent is 00.If the argument is infinity, it will be converted to [-]inf (
e
) or [-]INF (E
), respectively. If the argument is not-a-number (NaN), it will be converted to [-]nan (e
) or [-]NAN (E
), respectively. fF
- 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.If the argument is infinity, it will be converted to [-]inf (
f
) or [-]INF (F
), respectively. If the argument is not-a-number (NaN), it will be converted to [-]nan (f
) or [-]NAN (F
), respectively. gG
- The
double
argument is converted in stylef
ore
(orE
forG
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. Stylee
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.If the argument is infinity, it will be converted to [-]inf (
g
) or [-]INF (G
), respectively. If the argument is not-a-number (NaN), it will be converted to [-]nan (g
) or [-]NAN (G
), respectively. aA
- The
double
argument is rounded and converted to hexadecimal notation in the style [-]0xh.
hhhp
[±]d where the number of digits after the hexadecimal-point character is equal to the precision specification. If the precision is missing, it is taken as enough to represent the floating-point number exactly, and no rounding occurs. If the precision is zero, no hexadecimal-point character appears. Thep
is a literal character ‘p
’, and the exponent consists of a positive or negative sign followed by a decimal number representing an exponent of 2. TheA
conversion uses the prefix “0X
” (rather than “0x
”), the letters “ABCDEF
” (rather than “abcdef
”) to represent the hex digits, and the letter ‘P
’ (rather than ‘p
’) to separate the mantissa and exponent.Note that there may be multiple valid ways to represent floating-point numbers in this hexadecimal format. For example,
0x3.24p+0
,0x6.48p-1
and0xc.9p-2
are all equivalent. The format chosen depends on the internal representation of the number, but the implementation guarantees that the length of the mantissa will be minimized. Zeroes are always represented with a mantissa of 0 (preceded by a ‘-
’ if appropriate) and an exponent of+0
.If the argument is infinity, it will be converted to [-]inf (
a
) or [-]INF (A
), respectively. If the argument is not-a-number (NaN), it will be converted to [-]nan (a
) or [-]NAN (A
), respectively. c
- The
int
argument is converted to anunsigned char
, and the resulting character is written. s
- The
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 NUL 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. 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 ‘%%
’.
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.
RETURN VALUES
For all these functions if an output or encoding error occurs, a value less than 0 is returned.
The printf
(),
dprintf
(), fprintf
(),
sprintf
(), vprintf
(),
vdprintf
(), vfprintf
(),
vsprintf
(), asprintf
(), and
vasprintf
() functions return the number of
characters printed (not including the trailing
‘\0
’ used to end output to
strings).
The snprintf
() and
vsnprintf
() functions return the number of
characters that would have been output if the size
were unlimited (again, not including the final
‘\0
’.).
The asprintf
() and
vasprintf
() functions return the number of
characters that were output to the newly allocated string (excluding the
final ‘\0
’). A pointer to the newly
allocated string is returned in ret; it should be
passed to free(3) to release the allocated storage when it is no longer needed.
If sufficient space cannot be allocated, these functions will return -1. The
value of ret in this situation is
implementation-dependent (on OpenBSD,
ret will be set to the null pointer, but this behavior
should not be relied upon).
EXAMPLES
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);
To print pi to five decimal places:
#include <math.h> #include <stdio.h> fprintf(stdout, "pi = %.5f\n", 4 * atan(1.0));
To allocate a 128-byte string and print into it:
#include <stdarg.h> #include <stdio.h> #include <stdlib.h> char * newfmt(const char *fmt, ...) { char *p; va_list ap; if ((p = malloc(128)) == NULL) return (NULL); va_start(ap, fmt); (void) vsnprintf(p, 128, fmt, ap); va_end(ap); return (p); }
ERRORS
In addition to the errors documented for the
write(2) system call, the printf
() family of
functions may fail if:
- [
EILSEQ
] - An invalid wide character code was encountered.
- [
ENOMEM
] - Insufficient storage space is available.
- [
EOVERFLOW
] - The return value would be too large to be represented by an int.
SEE ALSO
STANDARDS
The fprintf
(),
printf
(), snprintf
(),
sprintf
(), vfprintf
(),
vprintf
(), vsnprintf
(), and
vsprintf
() functions conform to
ISO/IEC 9899:1999 (“ISO C99”).
The dprintf
() and vdprintf
()
functions conform to IEEE Std 1003.1-2008
(“POSIX.1”).
HISTORY
The predecessors ftoa
() and
itoa
() first appeared in
Version 1 AT&T UNIX. The function
printf
() first appeared in
Version 2 AT&T UNIX, and
fprintf
() and sprintf
() in
Version 7 AT&T UNIX.
The functions snprintf
() and
vsnprintf
() first appeared in
4.4BSD.
The functions asprintf
() and
vasprintf
() first appeared in the GNU C library.
This implementation first appeared in OpenBSD
2.3.
The functions dprintf
() and
vdprintf
() first appeared in
OpenBSD 5.3.
CAVEATS
The conversion formats %D
,
%O
, and %U
are not standard
and are provided only for backward compatibility. The effect of padding the
%p
format with zeros (either by the
‘0
’ flag or by specifying a
precision), and the benign effect (i.e., none) of the
‘#
’ flag on %n
and %p
conversions, as well as other nonsensical
combinations such as %Ld
, are not standard; such
combinations should be avoided.
Because sprintf
() and
vsprintf
() assume an infinitely long string, callers
must be careful not to overflow the actual space; this is often impossible
to assure. For safety, programmers should use the
snprintf
() and asprintf
()
family of interfaces instead. Unfortunately, the
asprintf
() interface is not available on all systems
as it is not part of ISO/IEC 9899:1999
(“ISO C99”).
It is important never to pass a string with user-supplied data as
a format without using ‘%s
’. An
attacker can put format specifiers in the string to mangle the stack,
leading to a possible security hole. This holds true even if the string has
been built “by hand” using a function like
snprintf
(), as the resulting string may still
contain user-supplied conversion specifiers for later interpolation by
printf
().
Be sure to use the proper secure idiom:
int ret = snprintf(buffer, sizeof(buffer), "%s", string); if (ret == -1 || ret >= sizeof(buffer)) goto toolong;
There is no way for printf
() to know the
size of each argument passed. If positional arguments are used, care must be
taken to ensure that all parameters, up to the last positionally specified
parameter, are used in the format string. This allows for the format string
to be parsed for this information. Failure to do this will mean the code is
non-portable and liable to fail.