PRINTF(3) BSD 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
#define _WITH_DPRINTF
#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, 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() dynami‐
cally allocate a new string with malloc(3).
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 out‐
put.
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 size
were unlimited (again, not including the final ‘\0’).
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.
The sprintf() and vsprintf() functions effectively assume an infinite
size.
The format string is composed of zero or more directives: ordinary char‐
acters (not %), which are copied unchanged to the output stream; and con‐
version 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 promo‐
tion) 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 pro‐
vided, 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 num‐
ber 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 con‐
versions) 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 oth‐
erwise be.
‘0’ (zero) Zero padding. For all conversions except n, the con‐
verted 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.
‘'’ 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-mone‐
tary 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-adjust‐
ment 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 con‐
versions.
· 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 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 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 *
· 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 adjust‐
ment 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 pre‐
cision; if the precision is missing, it is taken as 6; if the
precision is zero, no decimal-point character appears. An E con‐
version 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 nota‐
tion 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 let‐
ter ‘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.
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 appropri‐
ate) 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 charac‐
ter; if a precision is specified, no more than the number speci‐
fied 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 speci‐
fied 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 repre‐
sentation 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 inte‐
ger indicated by the int * (or variant) pointer argument. No
argument is converted.
% A ‘%’ is written. No argument is converted. The complete con‐
version 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
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 π 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);
}
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 pro‐
gram'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);
COMPATIBILITY
Many application writers used the name dprintf before the dprintf() func‐
tion was introduced in IEEE Std 1003.1 (“POSIX.1”), so a prototype is not
provided by default in order to avoid compatibility problems. Applica‐
tions that wish to use the dprintf() function described herein should
either request a strict IEEE Std 1003.1-2008 (“POSIX.1”) environment by
defining the macro _POSIX_C_SOURCE to the value 200809 or greater, or by
defining the macro _WITH_DPRINTF, prior to the inclusion of <stdio.h>.
For compatibility with GNU libc, defining either _BSD_SOURCE or
_GNU_SOURCE prior to the inclusion of <stdio.h> will also make dprintf()
available.
The conversion formats %D, %O, and are not standard and are provided only
for backward compatibility. The effect of padding the 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 combi‐
nations 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 ALSOprintf(1), fmtcheck(3), scanf(3), setlocale(3), wprintf(3)STANDARDS
Subject to the caveats noted in the BUGS section below, the fprintf(),
printf(), sprintf(), vprintf(), vfprintf(), and vsprintf() functions con‐
form to ANSI X3.159-1989 (“ANSI C89”) and ISO/IEC 9899:1999 (“ISO C99”).
With the same reservation, the snprintf() and vsnprintf() functions con‐
form 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
Todd C. Miller ⟨Todd.Miller@courtesan.com⟩ for OpenBSD 2.3. The
dprintf() and vdprintf() functions were added in FreeBSD 8.0.
BUGS
The printf family of functions do not correctly handle multibyte charac‐
ters in the format argument.
BSD March 3, 2009 BSD
