libc/stdio/printf.3

.\"	$OpenBSD: printf.3,v 1.94 2024/08/07 05:15:28 guenther Exp $
.\"
.\" Copyright (c) 1990, 1991, 1993
.\"	The Regents of the University of California.  All rights reserved.
.\"
.\" This code is derived from software contributed to Berkeley by
.\" Chris Torek and the American National Standards Committee X3,
.\" on Information Processing Systems.
.\"
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.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\"    notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\"    notice, this list of conditions and the following disclaimer in the
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.\" 3. Neither the name of the University nor the names of its contributors
.\"    may be used to endorse or promote products derived from this software
.\"    without specific prior written permission.
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.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
.\" ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\"     @(#)printf.3	8.1 (Berkeley) 6/4/93
.\"
.Dd $Mdocdate: August 7 2024 $
.Dt PRINTF 3
.Os
.Sh NAME
.Nm printf ,
.Nm fprintf ,
.Nm sprintf ,
.Nm snprintf ,
.Nm asprintf ,
.Nm dprintf ,
.Nm vprintf ,
.Nm vfprintf ,
.Nm vsprintf ,
.Nm vsnprintf ,
.Nm vasprintf ,
.Nm vdprintf
.Nd formatted output conversion
.Sh SYNOPSIS
.In stdio.h
.Ft int
.Fn printf "const char * restrict format" ...
.Ft int
.Fn fprintf "FILE *stream" "const char * restrict format" ...
.Ft int
.Fn sprintf "char * restrict str" "const char * restrict format" ...
.Ft int
.Fn snprintf "char * restrict str" "size_t size" "const char * restrict format" ...
.Ft int
.Fn asprintf "char ** restrict ret" "const char * restrict format" ...
.Ft int
.Fn dprintf "int fd" "const char * restrict format" ...
.In stdarg.h
.In stdio.h
.Ft int
.Fn vprintf "const char * restrict format" "va_list ap"
.Ft int
.Fn vfprintf "FILE *stream" "const char * restrict format" "va_list ap"
.Ft int
.Fn vsprintf "char * restrict str" "const char * restrict format" "va_list ap"
.Ft int
.Fn vsnprintf "char * restrict str" "size_t size" "const char * restrict format" "va_list ap"
.Ft int
.Fn vasprintf "char ** restrict ret" "const char * restrict format" "va_list ap"
.Ft int
.Fn vdprintf "int fd" "const char * restrict format" "va_list ap"
.Sh DESCRIPTION
The
.Fn printf
family of functions produce output according to the given
.Fa format
as described below.
This format may contain
.Dq conversion specifiers ;
the results of such conversions, if any, depend on the arguments
following the
.Fa format
string.
.Pp
The
.Fn printf
and
.Fn vprintf
functions write output to the standard output stream,
.Em stdout ;
.Fn fprintf
and
.Fn vfprintf
write output to the supplied stream pointer
.Fa stream ;
.Fn dprintf
and
.Fn vdprintf
write output to the given file descriptor;
.Fn sprintf ,
.Fn snprintf ,
.Fn vsprintf ,
and
.Fn vsnprintf
write to the character string
.Fa str ;
.Fn asprintf
and
.Fn vasprintf
write to a dynamically allocated string that is stored in
.Fa ret .
.Pp
These functions write the output under the control of a
.Fa format
string that specifies how subsequent arguments
(or arguments accessed via the variable-length argument facilities of
.Xr va_start 3 )
are converted for output.
.Pp
.Fn snprintf
and
.Fn vsnprintf
write at most
.Fa size Ns \-1
characters to
.Fa str ,
followed by a terminating
.Ql \e0 .
If
.Fa size
is zero,
no characters are written and
.Fa str
may be a
.Dv NULL
pointer.
.Pp
.Fn sprintf
and
.Fn vsprintf
effectively assume an infinite
.Fa size ;
their use is not recommended.
.Pp
The format string is composed of zero or more directives:
ordinary
.\" multibyte
characters (not
.Cm % ) ,
which are copied unchanged to the output stream,
and conversion specifications, each of which results
in fetching zero or more subsequent arguments.
The arguments must correspond properly (after type promotion)
with the conversion specifiers.
.Pp
The overall syntax of a conversion specification is:
.Bd -filled -offset indent
.Sm off
.Cm %
.Op Ar argno Cm $
.Op Ar flags
.Op Ar width
.Op . Ar precision
.Op Ar size
.Ar conversion
.Sm on
.Ed
.Pp
Not all combinations of these parts are meaningful;
see the description of the individual
.Ar conversion
specifiers for details.
.Pp
The parts of a conversion specification are as follows:
.Bl -tag -width Ds
.It Cm %
A literal percent character begins a conversion specification.
.It Ar argno Ns Cm $
An unsigned decimal digit string followed by a dollar character
specifies the index of the next argument to access.
By default, the argument following the last argument accessed is used.
Arguments are numbered starting at 1.
.It Ar flags
Zero or more of the following flag characters can be given:
.Bl -tag -width 11n
.It Cm # Pq hash
Use an alternate form for the output.
The effect differs depending on the conversion specifier.
.It So \~ Sc Pq space
For signed conversions, print a space character before a positive number.
.It Cm + Pq plus
For signed conversions, always print a sign before the number,
even if it is positive.
This overrides the space flag if both are specified.
.It Cm 0 Pq zero
Pad numbers with leading zeros instead of space characters
to fill the field
.Ar width .
This flag is ignored if the
.Ar precision
modifier is also given, which in this case specifies
.Ar mindigits .
.It Cm \- Pq minus
Left adjust: pad to the field
.Ar width
with space characters on the right rather than on the left.
This overrides the
.Sq Cm 0
flag if both are specified.
.El
.It Ar width
An unsigned decimal digit string specifies a minimum field width in bytes.
Unless the
.Sq Cm 0
or
.Sq Cm \-
flag is given, the value is right adjusted in the field and
padded with space characters on the left.
By default, no padding is added.
In no case does a non-existent or small field
.Ar 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.
.It Pf . Ar precision
The meaning of an unsigned decimal digit string prefixed with a
period character depends on the conversion specifier:
it provides the minimum number of digits for integer conversions,
of decimals for some floating point conversions and of significant
digits for others, or the maximum number of bytes to print for
string conversions.
.Pp
A field
.Ar width
or
.Ar precision ,
or both, may alternatively be indicated as
.Cm * Ns Op Ar argno Ns Cm $ ,
i.e. as an asterisk optionally followed
by an unsigned decimal digit string and a dollar sign.
In this case, an additional
.Vt int
argument supplies the field width or precision.
If a single conversion specification tries to use arguments
both with and without
.Ar argno Ns Cm $
modifiers, the result is undefined.
.It Ar size
An argument size modifier.
The syntax, the precise meaning, and the default size of the argument
depend on the following
.Ar conversion
character.
.It Ar conversion
Each conversion specification ends with a conversion specifier,
which is a single letter determining which argument type is expected
and how it is formatted.
.El
.Pp
The conversion specifiers are:
.Bl -tag -width Ds
.It Cm %a
.Sm off
.Cm %
.Op Ar argno Cm $
.Op Cm #
.Op Cm \~ | +
.Op Cm \- | 0
.Op Ar width
.Op . Ar hexadecimals
.Op Cm L | l
.Cm a
.Sm on
.Pp
The
.Vt double
argument is converted to the hexadecimal notation
.Sm off
.Oo \- Oc Sy 0x No h.hhh Sy p No \(+-d
.Sm on
with one digit before the hexadecimal point.
If specified, the number is rounded to
.Ar hexadecimals
after the hexadecimal point; otherwise,
enough digits are printed to represent it exactly.
The hexadecimal point is only printed if at least one digit follows it
or if the
.Sq Cm #
flag is given.
.Pp
The exponent is expressed in base 2, not in base 16.
Consequently, there are multiple ways to represent a number in this 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 is minimized.
Zeroes are always represented with a mantissa of
.Ql 0
(preceded by a sign if appropriate) and an exponent of
.Ql +0 .
.Pp
If the argument is infinity, it is converted to
.Ql [-]inf .
If the argument is not-a-number (NaN), it is converted to
.Ql [-]nan .
.Pp
.Cm %La
is similar to
.Cm %a
except that it takes an argument of
.Vt long double .
.Cm %la Pq ell a
is an alias for
.Cm %a .
.It Cm \&%A
Identical to
.Cm %a
except that upper case is used, i.e.\&
.Ql 0X
for the prefix,
.Ql 0123456789ABCDEF
for the digits,
.Ql P
to introduce the exponent,
and
.Ql [-]INF
and
.Ql [-]NAN
for infinity and not-a-number, respectively.
.It Cm %c
.Sm off
.Cm %
.Op Ar argno Cm $
.Op Cm \-
.Op Ar width
.Cm c
.Sm on
.Pp
The
.Vt int
argument is converted to an
.Vt unsigned char ,
and the resulting single-byte character is written, with optional padding.
.It Cm %lc
.Sm off
.Cm %
.Op Ar argno Cm $
.Op Cm \-
.Op Ar width
.Cm lc
.Sm on
.Pp
The
.Vt wint_t
argument is converted to a multibyte character according to the current
.Dv LC_CTYPE
.Xr locale 1 ,
and that character is written.
For example, under a UTF-8 locale on
.Ox ,
.Ql printf("%lc", 0x03c0)
writes the greek letter pi, whereas the same call fails
under the default POSIX locale.
Padding assures at least
.Ar width
bytes are printed; the number of characters printed may be smaller,
and the number of display columns occupied may be smaller or larger.
.It Cm %d
.Sm off
.Cm %
.Op Ar argno Cm $
.Op Cm \~ | +
.Op Cm \- | 0
.Op Ar width
.Op . Ar mindigits
.Op Ar size
.Cm d
.Sm on
.Pp
The
.Vt int
argument is converted to signed decimal notation.
If specified, at least
.Ar mindigits
are printed, padding with leading zeros if needed.
The following are similar to
.Cm %d
except that they take an argument of a different size:
.Bl -column %hhd
.It Cm %hhd Ta Vt signed char
.It Cm %hd  Ta Vt signed short
.It Cm %d   Ta Vt signed int
.It Cm %ld  Ta Vt signed long Pq percent ell dee
.It Cm %lld Ta Vt signed long long Pq percent ell ell dee
.It Cm %jd  Ta Vt intmax_t
.It Cm %td  Ta Vt ptrdiff_t
.It Cm %zd  Ta Vt ssize_t
.It Cm %qd  Ta Vt quad_t Pq deprecated
.El
.It Cm \&%D
A deprecated alias for
.Cm %ld .
.It Cm %e
.Sm off
.Cm %
.Op Ar argno Cm $
.Op Cm #
.Op Cm \~ | +
.Op Cm \- | 0
.Op Ar width
.Op . Ar decimals
.Op Cm L | l
.Cm e
.Sm on
.Pp
The
.Vt double
argument is rounded and converted to the scientific notation
.Pf [\-]d.dddddd Sy e Ns \(+-dd
with one digit before the decimal point and
.Ar decimals ,
or six digits by default, after it.
If
.Ar decimals
is zero and the
.Sq Cm #
flag is not given, the decimal point is omitted.
The exponent always contains at least two digits; if the value is zero,
the exponent is
.Ql +00 .
If the argument is infinity, it is converted to
.Ql [-]inf .
If the argument is not-a-number (NaN), it is converted to
.Ql [-]nan .
.Pp
.Cm %Le
is similar to
.Cm %e
except that it takes an argument of
.Vt long double .
.Cm %le Pq ell e
is an alias for
.Cm %e .
.It Cm \&%E
Identical to
.Cm %e
except that upper case is used, i.e.\&
.Ql E
instead of
.Ql e
to introduce the exponent and
.Ql [-]INF
and
.Ql [-]NAN
for infinity and not-a-number, respectively.
.It Cm %f
.Sm off
.Cm %
.Op Ar argno Cm $
.Op Cm #
.Op Cm \~ | +
.Op Cm \- | 0
.Op Ar width
.Op . Ar decimals
.Op Cm L | l
.Cm f
.Sm on
.Pp
The
.Vt double
argument is rounded and converted to the decimal notation [\-]ddd.dddddd with
.Ar decimals ,
or six digits by default, after the decimal point.
If
.Ar decimals
is zero and the
.Sq Cm #
flag is not given, the decimal point is omitted.
If a decimal point appears, at least one digit appears before it.
If the argument is infinity, it is converted to
.Ql [-]inf .
If the argument is not-a-number (NaN), it is converted to
.Ql [-]nan .
.Pp
.Cm %Lf
is similar to
.Cm %f
except that it takes an argument of
.Vt long double .
.Cm %lf Pq ell eff
is an alias for
.Cm %f .
.It Cm \&%F
Identical to
.Cm %f
except that upper case is used, i.e.\&
.Ql [-]INF
and
.Ql [-]NAN
for infinity and not-a-number, respectively.
.It Cm %g
.Sm off
.Cm %
.Op Ar argno Cm $
.Op Cm #
.Op Cm \~ | +
.Op Cm \- | 0
.Op Ar width
.Op . Ar significant
.Op Cm L | l
.Cm g
.Sm on
.Pp
The
.Vt double
argument is converted in style
.Cm %f
or
.Cm %e
.Pq general floating point notation
with
.Ar significant
digits, or six significant digits by default.
If
.Ar significant
is zero, one is used instead.
Style
.Cm %e
is used if the exponent from its conversion is less than \-4
or greater than or equal to
.Ar significant .
Unless the
.Sq Cm #
flag is given, trailing zeros are removed from the fractional
part of the result, and the decimal point only appears if it is
followed by at least one digit.
.Pp
.Cm %Lg
is similar to
.Cm %g
except that it takes an argument of
.Vt long double .
.Cm %lg Pq ell gee
is an alias for
.Cm %g .
.It Cm \&%G
Identical to
.Cm %g
except that upper case is used, i.e.\&
.Ql E
instead of
.Ql e
to introduce the exponent and
.Ql [-]INF
and
.Ql [-]NAN
for infinity and not-a-number, respectively.
.It Cm %i
An alias for
.Cm %d ,
supporting the same modifiers.
.It Cm %n
.Sm off
.Cm %
.Op Ar argno Cm $
.Op Ar size
.Cm n
.Sm on
.Pp
The
.Cm %n
conversion specifier has serious security implications, so it was changed to
no longer store the number of bytes written so far into the variable indicated
by the pointer argument.
Instead a
.Xr syslog 3
message will be generated, after which the program is aborted with
.Dv SIGABRT .
.It Cm %o
.Sm off
.Cm %
.Op Ar argno Cm $
.Op Cm #
.Op Cm \- | 0
.Op Ar width
.Op . Ar mindigits
.Op Ar size
.Cm o
.Sm on
.Pp
Similar to
.Cm %u
except that the
.Vt unsigned int
argument is converted to unsigned octal notation.
If the
.Sq Cm #
flag is given,
.Ar mindigits
is increased such that the first digit printed is a zero,
except if a zero value is printed with an explicit
.Ar mindigits
of zero.
.It Cm \&%O
A deprecated alias for
.Cm %lo .
.It Cm %p
The
.Vt void *
pointer argument is printed in hexadecimal, similar to
.Cm %#x
or
.Cm %#lx
depending on the size of pointers.
.It Cm %s
.Sm off
.Cm %
.Op Ar argno Cm $
.Op Cm \-
.Op Ar width
.Op . Ar maxbytes
.Cm s
.Sm on
.Pp
Characters from the
.Vt char * Pq string
argument are written up to (but not including) a terminating NUL character.
If
.Ar maxbytes
is specified, at most
.Ar maxbytes
bytes are written; in that case, no NUL character needs to be present.
.It Cm %ls
.Sm off
.Cm %
.Op Ar argno Cm $
.Op Cm \-
.Op Ar width
.Op . Ar maxbytes
.Cm ls
.Sm on
.Pp
The
.Vt wchar_t * Pq wide character string
argument is converted to a multibyte character string
according to the current
.Dv LC_CTYPE
.Xr locale 1
up to (but not including) a terminating NUL character,
and that multibyte character string is written.
If
.Ar maxbytes
is specified, at most
.Ar maxbytes
bytes are written; in that case, no NUL character needs to be present.
If a multibyte character does not fit into the rest of
.Ar maxbytes ,
it is omitted together with the rest of the argument string;
partial characters are not written.
Locale dependency and padding work in the same way as for
.Cm %lc .
.It Cm %u
.Sm off
.Cm %
.Op Ar argno Cm $
.Op Cm \- | 0
.Op Ar width
.Op . Ar mindigits
.Op Ar size
.Cm u
.Sm on
.Pp
The
.Vt unsigned int
argument is converted to unsigned decimal notation.
If specified, at least
.Ar mindigits
are printed, padding with leading zeros if needed.
The following are similar to
.Cm %u
except that they take an argument of a different size:
.Bl -column %hhu
.It Cm %hhu Ta Vt unsigned char
.It Cm %hu  Ta Vt unsigned short
.It Cm %u   Ta Vt unsigned int
.It Cm %lu  Ta Vt unsigned long Pq percent ell u
.It Cm %llu Ta Vt unsigned long long Pq percent ell ell u
.It Cm %ju  Ta Vt uintmax_t
.It Cm %tu  Ta unsigned type of same size as Vt ptrdiff_t
.It Cm %zu  Ta Vt size_t
.It Cm %qu  Ta Vt u_quad_t Pq deprecated
.El
.It Cm \&%U
A deprecated alias for
.Cm %lu .
.It Cm %x
.Sm off
.Cm %
.Op Ar argno Cm $
.Op Cm #
.Op Cm \- | 0
.Op Ar width
.Op . Ar mindigits
.Op Ar size
.Cm x
.Sm on
.Pp
Similar to
.Cm %u
except that the
.Vt unsigned int
argument is converted to unsigned hexadecimal notation using the digits
.Ql 0123456789abcdef .
If the
.Sq Cm #
flag is given, the string
.Ql 0x
is prepended unless the value is zero.
.It Cm \&%X
Identical to
.Cm %x
except that upper case is used, i.e.\&
.Ql 0X
for the optional prefix and
.Ql 0123456789ABCDEF
for the digits.
.It Cm %%
A single percent sign
.Pq Ql %
is written.
No argument is converted.
The complete conversion specification is
.Ql %% ;
no modifiers can be inserted between the two percent signs.
.El
.Sh RETURN VALUES
For all these functions if an output or encoding error occurs, a value
less than 0 is returned.
.Pp
The
.Fn printf ,
.Fn dprintf ,
.Fn fprintf ,
.Fn sprintf ,
.Fn vprintf ,
.Fn vdprintf ,
.Fn vfprintf ,
.Fn vsprintf ,
.Fn asprintf ,
and
.Fn vasprintf
functions
return the number of bytes printed
(not including the trailing
.Ql \e0
used to end output to strings).
.Pp
The
.Fn snprintf
and
.Fn vsnprintf
functions return the number of bytes that would have
been output if the
.Fa size
were unlimited
.Po
again, not including the final
.Ql \e0
.Pc .
A return value greater than or equal to the
.Fa size
argument indicates that the string was too small and some characters
were discarded.
.Pp
The
.Fn asprintf
and
.Fn vasprintf
functions return the number of bytes that were output
to the newly allocated string
(excluding the final
.Ql \e0 ) .
A pointer to the newly allocated string is returned in
.Fa ret ;
it should be passed to
.Xr free 3
to release the allocated storage
when it is no longer needed.
If sufficient space cannot be allocated or some other error occurs,
these functions return \-1.
The value of
.Fa ret
in this situation is implementation-dependent.
On
.Ox ,
.Fa ret
is set to the
.Dv NULL
pointer, but other implementations may leave
.Fa ret
unchanged.
.Sh ENVIRONMENT
.Bl -tag -width LC_CTYPE
.It Ev LC_CTYPE
The character encoding
.Xr locale 1 .
It decides which
.Vt wchar_t
values represent valid wide characters for the
.Cm %lc
and
.Cm %ls
conversion specifiers and how they are encoded into multibyte characters.
If unset or set to
.Qq C ,
.Qq POSIX ,
or an unsupported value,
.Cm %lc
and
.Cm %ls
only work correctly for ASCII characters
and fail for arguments greater than 255.
.El
.Sh EXAMPLES
To print a date and time in the form `Sunday, July 3, 10:02',
where
.Va weekday
and
.Va month
are pointers to strings:
.Bd -literal -offset indent
#include <stdio.h>

fprintf(stdout, "%s, %s %d, %.2d:%.2d\en",
    weekday, month, day, hour, min);
.Ed
.Pp
To print \*(Pi
to five decimal places:
.Bd -literal -offset indent
#include <math.h>
#include <stdio.h>

fprintf(stdout, "pi = %.5f\en", 4 * atan(1.0));
.Ed
.Pp
To allocate a 128-byte string and print into it:
.Bd -literal -offset indent
#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);
}
.Ed
.Sh ERRORS
In addition to the errors documented for the
.Xr write 2
system call, the
.Fn printf
family of functions may fail if:
.Bl -tag -width Er
.It Bq Er EILSEQ
An invalid wide character code was encountered.
.It Bq Er ENOMEM
Insufficient storage space is available.
.It Bq Er EOVERFLOW
The return value would be too large to be represented by an
.Vt int .
.El
.Sh SEE ALSO
.Xr printf 1 ,
.Xr scanf 3 ,
.Xr wprintf 3
.Sh STANDARDS
The
.Fn fprintf ,
.Fn printf ,
.Fn snprintf ,
.Fn sprintf ,
.Fn vfprintf ,
.Fn vprintf ,
.Fn vsnprintf ,
and
.Fn vsprintf
functions conform to
.St -isoC-99 .
The
.Fn dprintf ,
.Fn vdprintf ,
.Fn asprintf ,
and
.Fn vasprintf
functions conform to
.St -p1003.1-2024 .
.Sh HISTORY
The predecessors
.Fn ftoa
and
.Fn itoa
first appeared in
.At v1 .
The function
.Fn printf
first appeared in
.At v2 ,
and
.Fn fprintf
and
.Fn sprintf
in
.At v7 .
.Pp
The functions
.Fn snprintf
and
.Fn vsnprintf
first appeared in
.Bx 4.3 Net/2 .
.Pp
The functions
.Fn asprintf
and
.Fn vasprintf
first appeared in the GNU C library.
This implementation first appeared in
.Ox 2.3 .
.Pp
The functions
.Fn dprintf
and
.Fn vdprintf
first appeared in
.Ox 5.3 .
.Sh CAVEATS
The conversion formats
.Cm \&%D ,
.Cm \&%O ,
and
.Cm \&%U
are not standard and
are provided only for backward compatibility.
The effect of padding the
.Cm %p
format with zeros (either by the
.Sq Cm 0
flag or by specifying a precision), and the benign effect (i.e., none)
of the
.Sq Cm #
flag on
.Cm %n
and
.Cm %p
conversions, as well as other
nonsensical combinations such as
.Cm %Ld ,
are not standard; such combinations
should be avoided.
.Pp
Because
.Fn sprintf
and
.Fn 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
.Fn snprintf
and
.Fn asprintf
family of interfaces instead.
Unfortunately, the
.Fn asprintf
interface is not available on all systems as it is not part of
.St -isoC-99 .
.Pp
It is important never to pass a string with user-supplied data as a
format without using
.Ql %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
.Dq by hand
using a function like
.Fn snprintf ,
as the resulting string may still contain user-supplied conversion specifiers
for later interpolation by
.Fn printf .
.Pp
Be sure to use the proper secure idiom:
.Bd -literal -offset indent
int ret = snprintf(buffer, sizeof(buffer), "%s", string);
if (ret < 0 || (size_t)ret >= sizeof(buffer))
	goto toolong;
.Ed
.Pp
There is no way for
.Fn 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.
.Pp
On systems other than
.Ox ,
the
.Dv LC_NUMERIC
.Xr locale 1
category can cause erratic output; see CAVEATS in
.Xr setlocale 3
for details.