NAME
printf
, fprintf
,
dprintf
, sprintf
,
snprintf
, snprintf_ss
,
asprintf
, vprintf
,
vfprintf
, vsprintf
,
vdprintf
, vsnprintf
,
vsnprintf_ss
, vasprintf
— formatted output
conversion
LIBRARY
library “libc”
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
* restrict str, size_t
size, const char *
restrict format,
...);
int
snprintf_ss
(char
* restrict str, size_t
size, const char *
restrict format,
...);
int
asprintf
(char
** restrict ret, const
char * restrict format,
...);
#include
<stdarg.h>
int
vprintf
(const
char * restrict format,
va_list ap);
int
vfprintf
(FILE
* restrict stream, const
char * restrict format,
va_list ap);
int
vsprintf
(char
* restrict str, const
char * restrict format,
va_list ap);
int
vdprintf
(int
fd, const char * restrict
format, va_list
ap);
int
vsnprintf
(char
* restrict str, size_t
size, const char *
restrict format, va_list
ap);
int
vsnprintf_ss
(char
* restrict str, size_t
size, const char *
restrict format, va_list
ap);
int
vasprintf
(char
** restrict ret, const
char * restrict format,
va_list ap);
DESCRIPTION
Theprintf
()
family of functions produces output according to a
format as described below. The
printf
() and
vprintf
()
functions write output to
stdout,
the standard output stream;
fprintf
()
and
vfprintf
()
write output to the given output stream;
dprintf
()
and
vdprintf
()
write output to the given file descriptor fd;
sprintf
(), snprintf
(),
snprintf_ss
(), vsprintf
(),
vsnprintf
(), and
vsnprintf_ss
() write to the character string
str; and 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_ss
()
and
vsnprintf_ss
()
are signal-safe standalone versions that do not handle floating point
formats, positional arguments, and wide characters.
asprintf
()
and
vasprintf
()
return a pointer to a buffer sufficiently large to hold the string in the
ret argument. This pointer 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 and
set ret to be a NULL
pointer.
Please note that these functions are not standardized, and not all
implementations can be assumed to set the ret argument
to NULL
on error. It is more portable to check for a
return value of -1 instead.
snprintf
(),
vsnprintf
(),
and vsnprintf_ss
() 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, nothing is written and
str may be a NULL
pointer.
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
. If unaccessed arguments in the format string are interspersed with ones that are accessed the results will be indeterminate. - Zero or more of the following flags:
- ‘
#
’ - The value should be converted to an “alternate form”.
For
c
,d
,i
,n
,p
,s
, andu
conversions, this option has no effect. 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. - ‘
0
’ (zero) - Zero padding. For all conversions except
n
, 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
), the0
flag is ignored. - ‘
-
’ - A negative field width flag; the converted value is to be left
adjusted on the field boundary. 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. - ‘ ’ (space)
- A blank should be left before a positive number produced by a signed
conversion (
a
,A
d
,e
,E
,f
,F
,g
,G
, ori
). - ‘
+
’ - A sign must always be placed before a number produced by a signed
conversion. A ‘
+
’ overrides a space if both are used. - ‘
'
’ - Decimal conversions (
d
,u
, ori
) or the integral portion of a floating point conversion (f
orF
) should be grouped and separated by thousands using the non-monetary separator returned by localeconv(3).
- ‘
- 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
conversions: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 ao
,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.Note: if the standard integer types described in stdint(3) are used, it is recommended that the predefined format string specifier macros are used when possible. These are further described in inttypes(3).
The following length modifiers are valid for the
a
,A
,e
,E
,f
,F
,g
, orG
conversions:Modifier a
,A
,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
conversions: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 letters “abcdef
” are used forx
conversions; the letters “ABCDEF
” 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 been
ld
,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 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.For
a
,A
,e
,E
,f
,F
,g
, andG
conversions, positive and negative infinity are represented asinf
and-inf
respectively when using the lowercase conversion character, andINF
and-INF
respectively when using the uppercase conversion character. Similarly, NaN is represented asnan
when using the lowercase conversion, andNAN
when using the uppercase conversion. 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. gG
- The double argument is converted in style
f
ore
(or in styleF
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. aA
- The double argument is rounded and converted to
hexadecimal notation in the style
[-]
0x
h.
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
. C
- Treated as
c
with thel
(ell) modifier. c
- The int argument is converted to an
unsigned char, and the resulting character is
written.
If the
l
(ell) modifier is used, the wint_t argument shall be converted to a wchar_t, and the (potentially multi-byte) sequence representing the single wide character is written, including any shift sequences. If a shift sequence is used, the shift state is also restored to the original state after the character. S
- Treated as
s
with thel
(ell) modifier. 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 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 terminatingNUL
character.If the
l
(ell) modifier is used, the wchar_t * argument is expected to be a pointer to an array of wide characters (pointer to a wide string). For each wide character in the string, the (potentially multi-byte) sequence representing the wide character is written, including any shift sequences. If any shift sequence is used, the shift state is also restored to the original state after the string. Wide characters from the array are written up to (but not including) a terminating wideNUL
character; if a precision is specified, no more than the number of bytes specified are written (including shift sequences). Partial characters are never written. If a precision is given, no null character need be present; if the precision is not specified, or is greater than the number of bytes required to render the multibyte representation of the string, the array must contain a terminating wideNUL
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 ‘%%
’.
The decimal point character is defined in the program's locale
(category LC_NUMERIC
).
In no case does a non-existent or small field width cause truncation of a numeric field; if the result of a conversion is wider than the field width, the field is expanded to contain the conversion result.
RETURN VALUES
These functions return the number of characters printed, or that
would be printed if there was adequate space in case of
snprintf
(), vsnprintf
(), and
vsnprintf_ss
() (not including the trailing
‘\0
’ used to end output to strings).
If an output error was encountered, these functions shall return a negative
value.
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 <stdio.h> #include <stdlib.h> #include <stdarg.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 size argument exceeds
INT_MAX
, or the return value would be too large to be represented by an int.
SEE ALSO
printf(1), fmtcheck(3), scanf(3), setlocale(3), wprintf(3), printf(9)
STANDARDS
Subject to the caveats noted in the
BUGS section below, the
fprintf
(), printf
(),
sprintf
(), vprintf
(),
vfprintf
(), and vsprintf
()
functions conform to ANSI X3.159-1989
(“ANSI C89”) and ISO/IEC
9899:1999 (“ISO C99”). With the same
reservation, the snprintf
() and
vsnprintf
() functions conform to
ISO/IEC 9899:1999
(“ISO C99”).
HISTORY
The functions snprintf
() and
vsnprintf
() first appeared in
4.4BSD. The functions
asprintf
() and vasprintf
()
are modeled on the ones that first appeared in the GNU C library. The
function vsnprintf_ss
() is non-standard and appeared
in NetBSD 4.0. The functions
dprintf
() and vdprintf
() are
parts of IEEE Std 1003.1-2008
(“POSIX.1”) and appeared in NetBSD
6.0.
CAVEATS
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
snprintf
() interfaces are not available on older
systems and the asprintf
() interfaces are not yet
portable.
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 your stack,
leading to a possible security hole. This holds true even if you have built
the string “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:
snprintf(buffer, sizeof(buffer), "%s", string);
There is no way for printf
() to know the
size of each argument passed. If you use positional arguments you must
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 your code is
non-portable and liable to fail.
In this implementation, passing a
NULL
char * argument to the
%s
format specifier will output
(null) instead of
crashing. Programs that depend on this behavior are non-portable and may
crash on other systems or in the future.
BUGS
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.
The printf
() family of functions do not
correctly handle multibyte characters in the format
argument.
SECURITY CONSIDERATIONS
The sprintf
() and
vsprintf
() functions are easily misused in a manner
which enables malicious users to arbitrarily change a running program's
functionality through a buffer overflow attack. Because
sprintf
() and vsprintf
()
assume an infinitely long string, callers must be careful not to overflow
the actual space; this is often hard to assure. For safety, programmers
should use the snprintf
() interface instead. For
example:
void foo(const char *arbitrary_string, const char *and_another) { char onstack[8]; #ifdef BAD /* * This first sprintf is bad behavior. Do not use sprintf! */ sprintf(onstack, "%s, %s", arbitrary_string, and_another); #else /* * The following two lines demonstrate better use of * snprintf(). */ snprintf(onstack, sizeof(onstack), "%s, %s", arbitrary_string, and_another); #endif }
The printf
() and
sprintf
() family of functions are also easily
misused in a manner allowing malicious users to arbitrarily change a running
program's functionality by either causing the program to print potentially
sensitive data “left on the stack”, or causing it to generate
a memory fault or bus error by dereferencing an invalid pointer.
%n
can be used to write arbitrary data to
potentially carefully-selected addresses. Programmers are therefore strongly
advised to never pass untrusted strings as the format
argument, as an attacker can put format specifiers in the string to mangle
your stack, leading to a possible security hole. This holds true even if the
string was built using a function like snprintf
(),
as the resulting string may still contain user-supplied conversion
specifiers for later interpolation by printf
().
Always use the proper secure idiom:
snprintf(buffer, sizeof(buffer),
"%s", string);