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PRINTF(3) FreeBSD 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()
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.
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:
o 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.
o 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 b and B conversions, a non-zero
result has the string `0b' (or `0B' for B conversions)
prepended to it. 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 (b, B, 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
`+' 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).
o 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.
o 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 b, B, 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.
o An optional length modifier, that specifies the size of the argument.
The following length modifiers are valid for the b, B, d, i, n, o, u,
x, or X conversion:
Modifier d, i b, B, 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 *
wN intN_t uintN_t intN_t *
wfN int_fastN_t uint_fastN_t int_fastN_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 b, B, 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 *
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:
bBdiouxX The int (or appropriate variant) argument is converted to
unsigned binary (b and B), 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
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. FreeBSD 8.0 and later always prints finite non-
zero numbers using `1' as the digit before the hexadecimal
point. 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.
m Print the string representation of the error code stored in the
errno variable at the beginning of the call, as returned by
strerror(3). No argument is taken.
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 conversion format %m is also not
standard and provides the popular extension from the GNU C library.
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.
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. The %m format
extension first appeared in the GNU C library, and was implemented in
FreeBSD 12.0.
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
snprintf(buffer, sizeof(buffer), "%s", string);
FreeBSD 14.0-RELEASE-p11 September 5, 2023 FreeBSD 14.0-RELEASE-p11