PRINTF(3) BSD Library Functions Manual PRINTF(3)NAME
printf, fprintf, dprintf sprintf, snprintf, asprintf, vprintf, vfprintf,
vsprintf, vdprintf, vsnprintf, vsnprintf_ss, vasprintf — 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
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
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
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
give file descriptor fd; sprintf(), snprintf(), 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 out‐
put.
vsnprintf_ss() is a signal-safe standalone version that does not handle
floating point formats.
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 func‐
tions 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 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 (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.
‘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.
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 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 in style 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,
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 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.
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 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);
}
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), printf(9)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”).
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-stan‐
dard 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, call‐
ers 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 for‐
mat 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 charac‐
ters 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 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);
BSD December 26, 2010 BSD
