PRINTF(3) BSD Programmer's Manual PRINTF(3)NAME
printf, fprintf, sprintf, snprintf, asprintf, vprintf, vfprintf,
vsprintf, vsnprintf, vasprintf - formatted output conversion
SYNOPSIS
#include <stdio.h>
int
printf(const char *format, ...);
int
fprintf(FILE *stream, const char *format, ...);
int
sprintf(char *str, const char *format, ...);
int
snprintf(char *str, size_t size, const char *format, ...);
int
asprintf(char **ret, const char *format, ...);
#include <stdarg.h>
int
vprintf(const char *format, va_list ap);
int
vfprintf(FILE *stream, const char *format, va_list ap);
int
vsprintf(char *str, const char *format, va_list ap);
int
vsnprintf(char *str, size_t size, const char *format, va_list ap);
int
vasprintf(char **ret, const char *format, va_list ap);
DESCRIPTION
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 argu-
ments 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 out-
put.
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
were discarded. If size is zero, str may be a null pointer and no charac-
ters will be written; the number of bytes that would have been written
excluding the terminating '\0' byte, or -1 on error, will be returned.
sprintf() and vsprintf() effectively assume an infinite size.
The format string is composed of zero or more directives: ordinary multi-
byte 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 pro-
motion) 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.
+ Zero or more of the following flags:
- A hash '#' character specifying that the value should be convert-
ed to an "alternate form". For c, d, i, n, p, s, and u conver-
sions, this option has no effect. For o conversions, the preci-
sion 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 e, E, 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 oth-
erwise 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 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-
adjustment 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
number 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 sequence hh, specifying that a following d, i,
o, u, x, or X conversion corresponds to a signed char or unsigned
char argument, or that a following n conversion corresponds to a
pointer to a signed char argument.
+ The optional character j, specifying that a following d, i, o, u, x,
or X conversion corresponds to a intmax_t or uintmax_t argument, or
that a following n conversion corresponds to a pointer to a intmax_t
argument. (In the current implementation, this is a quad integer.)
+ 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, or that a following c, or s conversion
corresponds to a wide character argument.
+ The optional character sequence ll, specifying that a following d, i,
o, u, x, or X conversion corresponds to a quad int or unsigned quad
int argument, or that a following n conversion corresponds to a
pointer to a quad int argument. The use of q has been deprecated as
conversion character.
+ The optional character t, specifying that a following d, i, o, u, x,
or X conversion corresponds to a ptrdiff_t or intptr_t argument, or
that a following n conversion corresponds to a pointer to a ptrdiff_t
argument.
+ The optional character z, specifying that a following d, i, o, u, x,
or X conversion corresponds to a size_t or ssize_t argument, or that
a following n conversion corresponds to a pointer to a ssize_t argu-
ment.
+ The character L specifying that a following e, E, f, g, or G conver-
sion corresponds to a long double argument.
+ 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 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
conversion uses the letter E (rather than e) to introduce the ex-
ponent. 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 de-
cimal point appears, at least one digit appears before it.
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 ex-
ponent from its conversion is less than -4 or greater than or
equal to the precision. Trailing zeros are removed from the frac-
tional part of the result; a decimal point appears only if it is
followed by at least one digit.
C Deprecated alias for lc. Do not use.
c If the l specifier is not given, the int argument is converted to
an unsigned char, and the resulting character is written. Other-
wise, the wint_t argument is converted to an wchar_t, and the
resulting wide character is written as multibyte string according
to the rules of the ls conversion.
S Deprecated alias for ls. Do not use.
s If the l specifier is not given, 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 in-
cluding) a terminating NUL character; if a precision is speci-
fied, no more than the number specified are written. If a preci-
sion 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 NUL character.
If the l specifier is given, the wchar_t * argument is expected
to be a pointer to an array of wide character type (pointer to a
wide string). The wide character string must be NUL terminated.
The wide string is then converted to a multibyte string; if the
conversion fails due to an invalid wide character, the function
fails with EILSEQ. Otherwise, the output is truncated to at most
the number of octets (bytes) corresponding to the field preci-
sion; however, no incomplete multibyte characters are written.
Then, it is padded to the field width if specified.
p The void * pointer argument is printed in hexadecimal (as if by
'%#x' or '%#lx').
m The equivalent of strerror(errno) is written. This is a non-
portable extension, don't use.
n The number of characters written so far is stored into the in-
teger 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.
RETURN VALUES
The printf(), fprintf(), sprintf(), vprintf(), vfprintf(), vsprintf(),
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'). If an output or encoding error oc-
curs, a negative value is returned instead.
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 re-
turn -1. The value of ret in this situation is implementation-dependent
(on MirOS, ret will be set to the null pointer, but this behavior should
not be relied upon).
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 pi 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 <stdarg.h>
#include <stdio.h>
#include <stdlib.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);
}
SEE ALSOprintf(1), scanf(3)STANDARDS
The fprintf(), printf(), sprintf(), vprintf(), vfprintf(), and vsprintf()
functions conform to ANSI X3.159-1989 ("ANSI C").
Some of the ANSI/ISO/IEC 9899-1999 ("ANSI C99") extensions have been im-
plemented.
HISTORY
The functions snprintf() and vsnprintf() first appeared in 4.4BSD.
The %m conversion first appeared in the GNU/Linux libc5 library. This im-
plementation, which processes width and precision too, first appeared in
MirOS #10.
The functions asprintf() and vasprintf() first appeared in the GNU C li-
brary. This implementation first appeared in OpenBSD 2.3.
The %C and %S conversions appear in Plan 9 as well as GNU libc 2.0, and
were first implemented for MirOS #10.
CAVEATS
The conversion formats %C, %D, %O, %S, 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 conver-
sions, 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, call-
ers must be careful not to overflow the actual space; this is often im-
possible 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 for-
mat 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.
On MirOS, wide characters are always in UCS-2 Unicode, and multibyte
characters are always in OPTU-8 (CESU-8, UTF-8), for both the source and
execution character sets; however, this is not portable. Other operating
systems might not implement locale at all (even some of the other BSDs)
or on a non-Unicode basis (e.g. the Citrus I18N extension for BSD) or re-
quire calls to set an appropriate locale.
MirOS BSD #10-current April 11, 2007 5
