#include <stdio.h> int
printf (const char * restrict format ...); int
fprintf (FILE * restrict stream 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
asprintf (char **ret const char *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
vsnprintf (char * restrict str size_t size const char * restrict format va_list ap); int
vasprintf (char **ret const char *format va_list ap);
DESCRIPTION
The
printf ();
family of functions produces output according to a
Fa 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
Fa stream ;
sprintf (,);
snprintf (,);
vsprintf (,);
and
vsnprintf ();
write to the character string
Fa str ;
and
asprintf ();
and
vasprintf ();
dynamically allocate a new string with
malloc(3).
These functions write the output under the control of a
Fa format
string that specifies how subsequent arguments
(or arguments accessed via the variable-length argument facilities of
stdarg(3))
are converted for output.
These functions return the number of characters printed
(not including the trailing
`\0'
used to end output to strings) or a negative value if an output error occurs,
except for
snprintf ();
and
vsnprintf (,);
which return the number of characters that would have been printed if the
Fa size
were unlimited
(again, not including the final
`\0'
) .
The
asprintf ();
and
vasprintf ();
functions
set
Fa *ret
to be a pointer to a buffer sufficiently large to hold the formatted string.
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,
asprintf ();
and
vasprintf ();
will return -1 and set
Fa ret
to be a
NULL
pointer.
The
snprintf ();
and
vsnprintf ();
functions
will write at most
Fa size Ns -1
of the characters printed into the output string
(the
Fa size Ns 'th
character then gets the terminating
`\0'
) ;
if the return value is greater than or equal to the
Fa size
argument, the string was too short
and some of the printed characters were discarded.
The output is always null-terminated.
The
sprintf ();
and
vsprintf ();
functions
effectively assume an infinite
Fa 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 at
1
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
and
u
conversions, this option has no effect.
For
o
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).
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.
So 0 Sc (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 , i , x
and
X )
the
0
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.
So Sc (space)
A blank should be left before a positive number
produced by a signed conversion
( a , A , d , e , E , f , F , g , G
or
i )
`+
'
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
or
i
or the integral portion of a floating point conversion
( f
or
F
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 for
d , i , o , u , x
and
X
conversions, the number of digits to appear after the decimal-point 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
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
or
X
conversion:
Modifier Ta d , i Ta o , u , x , X Ta n
hh Ta Vt signed char Ta Vt unsigned char Ta Vt signed char *
h Ta Vt short Ta Vt unsigned short Ta Vt short *
l (ell) Ta Vt long Ta Vt unsigned long Ta Vt long *
ll (ell ell) Ta Vt long long Ta Vt unsigned long long Ta Vt long long *
j Ta Vt intmax_t Ta Vt uintmax_t Ta Vt intmax_t *
t Ta Vt ptrdiff_t Ta (see note) Ta Vt ptrdiff_t *
z Ta (see note) Ta Vt size_t Ta (see note)
q (deprecated) Ta Vt quad_t Ta Vt u_quad_t Ta Vt quad_t *
Note:
the
t
modifier, when applied to a
o , u , x
or
X
conversion, indicates that the argument is of an unsigned type
equivalent in size to a
Vt ptrdiff_t .
The
z
modifier, when applied to a
d
or
i
conversion, indicates that the argument is of a signed type equivalent in
size to a
Vt size_t .
Similarly, when applied to an
n
conversion, it indicates that the argument is a pointer to a signed type
equivalent in size to a
Vt size_t .
The following length modifier is valid for the
a , A , e , E , f , F , g
or
G
conversion:
Modifier Ta a , A , e , E , f , F , g , G
l (ell) Ta Vt double
(ignored, same behavior as without it)
L Ta Vt long double
The following length modifier is valid for the
c
or
s
conversion:
Modifier Ta c Ta s
l (ell) Ta Vt wint_t Ta Vt 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
`$'
instead of a
digit string.
In this case, an
Vt int
argument supplies the field width or precision.
A negative field width is treated as a left adjustment flag followed by a
positive field width; a negative precision is treated as though it were
missing.
If a single format directive mixes positional
(nn$
)
and non-positional arguments, the results are undefined.
The conversion specifiers and their meanings are:
diouxX
The
Vt int
(or appropriate variant) argument is converted to signed decimal
( d
and
i )
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.
DOU
The
Vt long int
argument is converted to signed decimal, unsigned octal, or unsigned
decimal, as if the format had been
ld , lo
or
lu
respectively.
These conversion characters are deprecated, and will eventually disappear.
eE
The
Vt double
argument is rounded and converted in the style
[- d . ddd e \*[Pm] 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.
For
a , A , e , E , f , F , g
and
G
conversions, positive and negative infinity are represented as
inf
and
-inf
respectively when using the lowercase conversion character, and
INF
and
-INF
respectively when using the uppercase conversion character.
Similarly, NaN is represented as
nan
when using the lowercase conversion, and
NAN
when using the uppercase conversion.
fF
The
Vt 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
Vt 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.
aA
The
Vt double
argument is rounded and converted to hexadecimal notation in the style
[- 0x h . hhhp [\*[Pm] 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.
The
p
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.
The
A
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
and
0xc.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 the
l
(ell) modifier.
c
The
Vt int
argument is converted to an
Vt unsigned char ,
and the resulting character is written.
If the
l
(ell) modifier is used, the
Vt wint_t
argument shall be converted to a
Vt 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 the
l
(ell) modifier.
s
The
Vt 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 the
l
(ell) modifier is used, the
Vt 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 wide
NUL
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 wide
NUL
character.
p
The
Vt 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
Vt 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.
EXAMPLES
To print a date and time in the form
``Sunday, July 3, 10:02
''
where
Fa weekday
and
Fa month
are pointers to strings:
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
Fa 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);"
ERRORS
In addition to the errors documented for the
write(2)
system call, the
printf ();
family of functions may fail if:
Subject to the caveats noted in the
Sx BUGS
section below, the
fprintf (,);
printf (,);
sprintf (,);
vprintf (,);
vfprintf (,);
and
vsprintf ();
functions
conform to
St -ansiC
and
St -isoC-99 .
With the same reservation, the
snprintf ();
and
vsnprintf ();
functions conform to
St -isoC-99 .
HISTORY
The functions
asprintf ();
and
vasprintf ();
first appeared in the
GNU C
library.
These were implemented by
An Peter Wemm Aq [email protected]
in
Fx 2.2 ,
but were later replaced with a different implementation
from
An Todd C. Miller Aq [email protected]
for
Ox 2.3 .
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
family of functions do not correctly handle multibyte characters in the
Fa format
argument.
