PRINTF(3) BSD Library Functions Manual PRINTF(3)
printf, fprintf, dprintf sprintf, snprintf, asprintf, vprintf, vfprintf, vsprintf, vdprintf,
vsnprintf, vsnprintf_ss, vasprintf -- formatted output conversion
Standard C Library (libc, -lc)
printf(const char * restrict format, ...);
fprintf(FILE * restrict stream, const char * restrict format, ...);
dprintf(int fd, const char * restrict format, ...);
sprintf(char * restrict str, const char * restrict format, ...);
snprintf(char * restrict str, size_t size, const char * restrict format, ...);
asprintf(char ** restrict ret, const char * restrict format, ...);
vprintf(const char * restrict format, va_list ap);
vfprintf(FILE * restrict stream, const char * restrict format, va_list ap);
vsprintf(char * restrict str, const char * restrict format, va_list ap);
vdprintf(int fd, const char * restrict format, va_list ap);
vsnprintf(char * restrict str, size_t size, const char * restrict format, va_list ap);
vsnprintf_ss(char * restrict str, size_t size, const char * restrict format, va_list ap);
vasprintf(char ** restrict ret, const char * restrict format, va_list ap);
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 output.
vsnprintf_ss() is a signal-safe standalone version that does not handle floating point for-
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 allo-
cated 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 functions
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 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 promo-
tion) 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 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 conver-
sions, trailing zeros are not removed from the result as they would other-
'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 con-
version (a, A d, e, E, f, F, g, G, or i).
'+' A sign must always be placed before a number produced by a signed conver-
sion. 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).
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 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 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 equiva-
lent 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 *
o 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 fol-
lowed 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-posi-
tional 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) nota-
tion. 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 preci-
sion 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 rep-
resented as inf and -inf respectively when using the lowercase conversion character,
and INF and -INF respectively when using the uppercase conversion character. Simi-
larly, 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 conver-
sions). 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 lit-
eral character 'p', and the exponent consists of a positive or negative sign fol-
lowed 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 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
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 includ-
ing) a terminating NUL character; if a precision is specified, no more than the num-
ber 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.
% A '%' is written. No argument is converted. The complete conversion specification
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.
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.
To print a date and time in the form ``Sunday, July 3, 10:02'', where weekday and month are
pointers to strings:
fprintf(stdout, "%s, %s %d, %.2d:%.2d\n",
weekday, month, day, hour, min);
To print pi to five decimal places:
fprintf(stdout, "pi = %.5f\n", 4 * atan(1.0));
To allocate a 128 byte string and print into it:
char *newfmt(const char *fmt, ...)
if ((p = malloc(128)) == NULL)
(void) vsnprintf(p, 128, fmt, ap);
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.
printf(1), fmtcheck(3), scanf(3), setlocale(3), wprintf(3), printf(9)
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'').
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 func-
tion vsnprintf_ss() is non-standard 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.
Because sprintf() and vsprintf() assume an infinitely long string, callers must be careful
not to overflow the actual space; this is often impossible to assure. For safety, program-
mers 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 format 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 parame-
ter, 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 out-
put (null) instead of crashing. Programs that depend on this behavior are non-portable and
may crash on other systems or in the future.
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 con-
versions, as well as other nonsensical combinations such as %Ld, are not standard; such com-
binations should be avoided.
The printf family of functions do not correctly handle multibyte characters in the format
The sprintf() and vsprintf() functions are easily misused in a manner which enables mali-
cious users to arbitrarily change a running program's functionality through a buffer over-
flow 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:
foo(const char *arbitrary_string, const char *and_another)
* This first sprintf is bad behavior. Do not use sprintf!
sprintf(onstack, "%s, %s", arbitrary_string, and_another);
* The following two lines demonstrate better use of
snprintf(onstack, sizeof(onstack), "%s, %s", arbitrary_string,
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 gen-
erate a memory fault or bus error by dereferencing an invalid pointer.
%n can be used to write arbitrary data to potentially carefully-selected addresses. Pro-
grammers are therefore strongly advised to never pass untrusted strings as the format argu-
ment, 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 speci-
fiers for later interpolation by printf().
Always use the proper secure idiom:
snprintf(buffer, sizeof(buffer), "%s", string);
BSD December 26, 2010 BSD