printf, fprintf, sprintf, snprintf, asprintf, vprintf,
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,
vasprintf(char **ret, const char *format, va_list ap);
The printf() family of functions produce output according to
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
stream, stdout; fprintf() and vfprintf() write output to the
stream pointer stream; sprintf(), snprintf(), vsprintf(),
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
that specifies how subsequent arguments (or arguments accessed via the
variable-length argument facilities of stdarg(3)) are converted for output.
These functions return the number of characters printed (not
the trailing ` ' used to end output to strings), except for
and vsnprintf(), which return the number of characters that
been printed if the size were unlimited (again, not including the final
` '). If an output or encoding error occurs, a value of -1
asprintf() and vasprintf() return a pointer to a buffer sufficiently
large to hold the string in the ret argument. This pointer
passed to free(3) to release the allocated storage when it
is no longer
needed. If sufficient space cannot be allocated, these
return -1. The value of ret in this situation is implementation-dependent
(on OpenBSD, ret will be set to the null pointer, but
should not be relied upon).
snprintf() and vsnprintf() will write at most size-1 of the
printed into the output string (the size'th character then
gets the terminating
` '); if the return value is greater than or equal
to the size
argument, the string was too short and some of the printed
sprintf() and vsprintf() effectively assume an infinite
The format string is composed of zero or more directives:
(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
with the conversion specifier. After the %, the following appear
+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
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
the output string to a zero (except if a zero value
with an explicit precision of zero). For x and X
non-zero result has the string `0x' (or `0X' for X
prepended to it. For e, E, f, g, and G conversions,
will always contain a decimal point, even if no digits follow it
(normally, a decimal point appears in the results of
only if a digit follows). For g and G conversions,
trailing zeros are not removed from the result as
they would otherwise
- A zero `0' character specifying zero padding. For
except n, the converted value is padded on the
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
rather than on the left with blanks or zeros. 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.
+o An optional decimal digit string specifying a minimum
If the converted value has fewer characters than the
field width, it
will be padded with spaces on the left (or right, if the
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,
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
of significant digits for g and G conversions, or
number of characters to be printed from a string for s
+o 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 argument,
or that a following n conversion corresponds to
a pointer to
a short int argument.
+o The optional character l (ell) specifying that a following d, i, o,
u, x, or X conversion corresponds to a long int or unsigned long int
argument, or that a following n conversion corresponds
to a pointer
to a long int argument.
+o The optional character sequence ll, specifying that a
following d, i,
o, u, x, or X conversion corresponds to a quad int or
int argument, or that a following n conversion corresponds to a
pointer to a quad int argument. The use of q has been
+o The character L specifying that a following e, E, f, g,
or G conversion
corresponds to a long double argument (but note
that long double
values are not currently supported by the VAX compiler).
+o A character that specifies the type of conversion to be
A field width or precision, or both, may be indicated by an
or an asterisk followed by one or more decimal digits and a
of a digit string. In this case, an int argument supplies
width or precision. A negative field width is treated as a
flag followed by a positive field width; a negative
treated as though it were missing. If a single format directive mixes
positional (nn$) and non-positional arguments, the results
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
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
will eventually disappear.
eE The double argument is rounded and converted in the
[-]d.ddde+-dd where there is one digit before the
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.
f The double argument is rounded and converted to decimal notation
in the style [-]ddd.ddd, where the number of digits
decimal-point character is equal to the precision
If the precision is missing, it is taken as 6; if
is explicitly zero, no decimal-point character appears. If a
decimal point appears, at least one digit appears
gG The double argument is converted in style f or e (or
E for G conversions).
The precision specifies the number of
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
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 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
if a precision is specified, no more than the
are written. If a precision is given, no null
need be present; if the precision is not specified,
or is greater
than the size of the array, the array must contain a
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
% A `%' is written. No argument is converted. The
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
field is expanded to contain the conversion result.
To print a date and time in the form `Sunday, July 3,
weekday and month are pointers to strings:
fprintf(stdout, "%s, %s %d, %.2d:%.2d0,
weekday, month, day, hour, min);
To print pi to five decimal places:
fprintf(stdout, "pi = %.5f0, 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);
The fprintf(), printf(), sprintf(), vprintf(), vfprintf(),
functions conform to ANSI X3.159-1989 (``ANSI C'').
The functions snprintf() and vsnprintf() first appeared in
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
only for backward compatibility. The effect of padding the
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
such combinations should be avoided.
Because sprintf() and vsprintf() assume an infinitely long
callers must be careful not to overflow the actual space;
this is often
impossible to assure. For safety, programmers should use
and asprintf() family of interfaces instead. Unfortunately,
snprintf() interface is not available on older systems and
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 your stack, leading to a possible security
holds true even if you have built the string ``by hand'' using a function
like snprintf(), as the resulting string may still contain
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
you use positional arguments you must ensure that all parameters, up to
the last positionally specified parameter, are used in the
This allows for the format string to be parsed for this information.
Failure to do this will mean your code is non-portable and
OpenBSD 3.6 June 4, 1993
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