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| PRINTF(3) Library Functions Manual PRINTF(3)
NAME
printf, fprintf, sprintf, snprintf, asprintf, dprintf, vprintf,
vfprintf, vsprintf, vsnprintf, vasprintf, vdprintf – formatted
output conversion
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#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, ...);
int
dprintf(int fd, 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
vsnprintf(char * restrict str, size_t size,
const char * restrict 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 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; sprintf(),
snprintf(), vsprintf(), and vsnprintf() write to the character
string str; and asprintf() and vasprintf() dynamically allocate a
new string with malloc(3).
Extended locale versions of these functions are documented in
printf_l(3). See xlocale(3) for more information.
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.
The asprintf() and vasprintf() functions set *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 ret to
be a NULL pointer.
The snprintf() and vsnprintf() functions 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. The output is
always null-terminated, unless size is 0.
The sprintf() and vsprintf() functions effectively assume a size of
INT_MAX + 1.
For those routines that write to a user-provided character string,
that string and the format strings should not overlap, as the
behavior is undefined.
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. 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.
‘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,
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.
‘ ’ (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.
‘'’ (apostrophe) 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 separator character ( , | ; | : | _ ) used for
separating multiple values when printing an AltiVec or SSE
vector, or other multi-value unit.
NOTE: This is an extension to the printf() specification.
Behaviour of these values for printf() is only defined for
operating systems conforming to the AltiVec Technology
Programming Interface Manual. (At time of writing this includes
only Mac OS X 10.2 and later.)
• 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 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 lon
g 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 a o, u, x, or X
conversion, indicates that the argument is of an unsigned type
equivalent in size to a 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 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 size_t.
The following length modifier is valid for the a, A, e, E, f, F,
g, or G conversion:
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 or s
conversion:
Modifier c s
l (ell) wint_t wchar_t *
The AltiVec Technology Programming Interface Manual also defines
five additional length modifiers which can be used (in place of
the conventional length modifiers) for the printing of AltiVec
or SSE vectors:
v Treat the argument as a vector value, unit length will
be determined by the conversion specifier (default = 16
8-bit units for all integer conversions, 4 32-bit units
for floating point conversions).
vh, hv Treat the argument as a vector of 8 16-bit units.
vl, lv Treat the argument as a vector of 4 32-bit units.
NOTE: The vector length specifiers are extensions to the
printf() specification. Behaviour of these values for printf()
is only defined for operating systems conforming to the AltiVec
Technology Programming Interface Manual. (At time of writing
this includes only Mac OS X 10.2 and later.)
As a further extension, for SSE2 64-bit units:
vll, llv
Treat the argument as a vector of 2 64-bit units.
• 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 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 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 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.
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 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 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 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. 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, 0x1.92p+1, 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 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 the l (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 terminating NUL 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 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 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. The format
argument must be in write-protected memory if this
specifier is used; see SECURITY CONSIDERATIONS below.
% 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 (not
including the trailing ‘\0’ used to end output to strings), except
for snprintf() and vsnprintf(), which return the number of
characters that would have been printed if the size were unlimited
(again, not including the final ‘\0’). These functions return a
negative value if an error occurs.
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);
}
COMPATIBILITY
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.
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.
SEE ALSO
printf(1), printf_l(3), fmtcheck(3), scanf(3), setlocale(3),
stdarg(3), wprintf(3)
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”), while dprintf() and vdprintf() conform to IEEE Std
1003.1-2008 (“POSIX.1”).
HISTORY
The functions asprintf() and vasprintf() first appeared in the GNU C
library. These were implemented by Peter Wemm <peter@FreeBSD.org>
in FreeBSD 2.2, but were later replaced with a different
implementation from OpenBSD 2.3 by Todd C. Miller
<Todd.Miller@courtesan.com>. The dprintf() and vdprintf() functions
were added in FreeBSD 8.0.
BUGS
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(). For this reason, a format argument
containing %n is assumed to be untrustworthy if located in writable
memory (i.e. memory with protection PROT_WRITE; see mprotect(2)) and
any attempt to use such an argument is fatal. Practically, this
means that %n is permitted in literal format strings but disallowed
in format strings located in normal stack- or heap-allocated memory.
Always use the proper secure idiom:
snprintf(buffer, sizeof(buffer), "%s", string);
macOS 12.4 December 2, 2009 macOS 12.4
(END)
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