PRINTF(3) OpenBSD Programmer's Manual PRINTF(3)NAME
printf, fprintf, sprintf, snprintf, asprintf, vprintf, vfprintf,
vsprintf, vsnprintf, vasprintf - formatted output conversion
printf(const char *format, ...);
fprintf(FILE *stream, const char *format, ...);
sprintf(char *str, const char *format, ...);
snprintf(char *str, size_t size, const char *format, ...);
asprintf(char **ret, const char *format, ...);
vprintf(const char *format, va_list ap);
vfprintf(FILE *stream, const char *format, va_list ap);
vsprintf(char *str, const char *format, va_list ap);
vsnprintf(char *str, size_t size, const char *format, va_list ap);
vasprintf(char **ret, const char *format, va_list ap);
The printf() family of functions produce output according to the given
format as described below. This format may contain ``conversion
specifiers''; the results of such conversions, if any, depend on the
arguments following the format string.
The printf() and vprintf() functions write output to the standard output
stream, stdout; fprintf() and vfprintf() write output to the supplied
stream pointer stream; sprintf(), snprintf(), vsprintf(), and vsnprintf()
write to the character string str; 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
snprintf() and vsnprintf() 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, str may be a null pointer
and no characters will be written; the number of bytes that would have
been written excluding the terminating `\0' byte, or -1 on error, will be
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
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.
o Zero or more of the following flags:
- A hash `#' character specifying that 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
conversions, trailing zeros are not removed from the result as
they would otherwise be.
- A zero `0' character specifying 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, x, and X), the `0' flag is
- A negative field width flag `-' indicates 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.
- A space, specifying that a blank should be left before a positive
number produced by a signed conversion (d, a, A, e, E, f, F, g,
G, or i).
- A `+' character specifying that a sign always be placed before a
number produced by a signed conversion. A `+' overrides a space
if both are used.
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
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
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 an 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
The following length modifier is valid for the a, A, e, E, f, F, g,
or G conversion:
Modifier 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 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.
If the argument is infinity, it will be converted to [-]inf (e)
or [-]INF (E), respectively. If the argument is not-a-number
(NaN), it will be converted to [-]nan (e) or [-]NAN (E),
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.
If the argument is infinity, it will be converted to [-]inf (f)
or [-]INF (F), respectively. If the argument is not-a-number
(NaN), it will be converted to [-]nan (f) or [-]NAN (F),
gG The double argument is converted in style f or e (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.
If the argument is infinity, it will be converted to [-]inf (g)
or [-]INF (G), respectively. If the argument is not-a-number
(NaN), it will be converted to [-]nan (g) or [-]NAN (G),
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
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.
If the argument is infinity, it will be converted to [-]inf (a)
or [-]INF (A), respectively. If the argument is not-a-number
(NaN), it will be converted to [-]nan (a) or [-]NAN (A),
c The int argument is converted to an unsigned char, and the
resulting character is written.
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 NUL 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
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 is `%%'.
In no case does a non-existent or small field width cause truncation of a
field; if the result of a conversion is wider than the field width, the
field is expanded to contain the conversion result.
For all these functions if an output or encoding error occurs, a value of
-1 is returned.
The printf(), fprintf(), sprintf(), vprintf(), vfprintf(), vsprintf(),
asprintf(), and vasprintf() functions return the number of characters
printed (not including the trailing `\0' used to end output to strings).
The snprintf() and vsnprintf() functions return the number of characters
that would have been output if the size were unlimited (again, not
including the final `\0'.).
The asprintf() and vasprintf() functions return the number of characters
that were output to the newly allocated string (excluding the final
`\0'). A pointer to the newly allocated string is returned in ret; it
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. The value of ret in this situation is
implementation-dependent (on OpenBSD, ret will be set to the null
pointer, but this behavior should not be relied upon).
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:
newfmt(const char *fmt, ...)
if ((p = malloc(128)) == NULL)
(void) vsnprintf(p, 128, fmt, ap);
SEE ALSOprintf(1), scanf(3)STANDARDS
The fprintf(), printf(), sprintf(), vprintf(), vfprintf(), and vsprintf()
functions conform to ANSI X3.159-1989 (``ANSI C'').
The functions snprintf() and vsnprintf() first appeared in 4.4BSD.
The functions asprintf() and vasprintf() first appeared in the GNU C
library. This implementation first appeared in OpenBSD 2.3.
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.
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, programmers should use the snprintf()
and asprintf() family of interfaces instead. Unfortunately, the
snprintf() interface is not available on older systems and the asprintf()
interface is not 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 the stack, leading to a possible security hole. This
holds true even if the string has been built ``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 positional arguments are used, care must be taken to 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 the code is non-portable
and liable to fail.
OpenBSD 4.9 March 26, 2010 OpenBSD 4.9