PRINTF(3) OpenBSD Programmer's Manual PRINTF(3)
printf, fprintf, sprintf, snprintf, asprintf, vprintf, vfprintf,
vsprintf, vsnprintf, vasprintf, vansprintf - 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 produces output according to a format as
described below. printf() and vprintf() write output to stdout, the
standard output stream; fprintf() and vfprintf() write output to the giv-
en output stream; sprintf(), snprintf(), vsprintf() and vsnprintf() write
to the character string str; asprintf() and vasprintf() write to a dynam-
ically 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-
These functions return the number of characters printed (not including
the trailing `\0' used to end output to strings).
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.
If asprintf() or vasprintf() are given a size of 0, they set ret to be a
snprintf() and vsnprintf() will write at most size-1 of the characters
printed into the output string (the size'th character then gets the ter-
minating `\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
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
- 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.
- Zero or more of the following flags:
- A # 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 out-
put 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 e, E, f, g, and G, conversions, the result will al-
ways contain a decimal point, even if no digits follow it (nor-
mally, a decimal point appears in the results of those conver-
sions only if a digit follows). For g and G conversions, trail-
ing zeros are not removed from the result as they would otherwise
- A zero `0' character specifying zero padding. For all conver-
sions 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 `-' indicates the converted value is
to be left adjusted on the field boundary. Except for n conver-
sions, 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, e, E, 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.
- 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 op-
tional 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 e, E, and f conversions, the maximum num-
ber of significant digits for g and G conversions, or the maximum
number of characters to be printed from a string for s conversions.
- The optional character h, specifying that a following d, i, o, u, x,
or X conversion corresponds to a short int or unsigned short int ar-
gument, or that a following n conversion corresponds to a pointer to
a short int argument.
- The optional character l (ell) specifying that a following d, i, o,
u, x, or X conversion applies to a pointer to a long int or unsigned
long int argument, or that a following n conversion corresponds to a
pointer to a long int argument.
- The optional character q, specifying that a following d, i, o, u, x,
or X conversion corresponds to a quad int or unsigned quad int argu-
ment, or that a following n conversion corresponds to a pointer to a
quad int argument.
- The character L specifying that a following e, E, f, g, or G conver-
sion corresponds to a long double argument (but note that long double
values are not currently supported by the VAX and Tahoe compilers).
- 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 conver-
sions. 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 expo-
nent. The exponent always contains at least two digits; if the
value is zero, the exponent is 00.
f 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 E for G con-
versions). 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.
c The int argument is converted to an unsigned char, and the re-
sulting 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 ar-
ray are written up to (but not including) a terminating NUL char-
acter; if a precision is specified, no more than the number spec-
ified 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
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 conver-
sion 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.
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);
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 li-
brary. 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.
Never print a user-supplied string directly as a format without using %s,
as an attacker can put format specifiers in that string to mangle your
stack. 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
OpenBSD 2.6 June 4, 1993 5
Source: OpenBSD 2.6 man pages. Copyright: Portions are copyrighted by BERKELEY
SOFTWARE DESIGN, INC., The Regents of the University of California, Massachusetts
Institute of Technology, Free Software Foundation, FreeBSD Inc., and others.