- Fix for possible unset uid/gid in toproto - Fix for default mtree style - Update libelf - Importing libexecinfo - Resynchronize GCC, mpc, gmp, mpfr - build.sh: Replace params with show-params. This has been done as the make target has been renamed in the same way, while a new target named params has been added. This new target generates a file containing all the parameters, instead of printing it on the console. - Update test48 with new etc/services (Fix by Ben Gras <ben@minix3.org) get getservbyport() out of the inner loop Change-Id: Ie6ad5226fa2621ff9f0dee8782ea48f9443d2091
1008 lines
24 KiB
Groff
1008 lines
24 KiB
Groff
.\" $NetBSD: printf.3,v 1.63 2013/05/04 19:17:38 wiz Exp $
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.\"
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.\" Copyright (c) 1990, 1991, 1993
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.\" The Regents of the University of California. All rights reserved.
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.\"
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.\" This code is derived from software contributed to Berkeley by
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.\" Chris Torek and the American National Standards Committee X3,
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.\" on Information Processing Systems.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\" 3. Neither the name of the University nor the names of its contributors
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.\" may be used to endorse or promote products derived from this software
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.\" without specific prior written permission.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" @(#)printf.3 8.1 (Berkeley) 6/4/93
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.\"
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.Dd May 4, 2013
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.Dt PRINTF 3
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.Os
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.Sh NAME
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.Nm printf ,
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.Nm fprintf ,
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.Nm dprintf
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.Nm sprintf ,
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.Nm snprintf ,
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.Nm snprintf_ss ,
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.Nm asprintf ,
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.Nm vprintf ,
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.Nm vfprintf ,
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.Nm vsprintf ,
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.Nm vdprintf ,
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.Nm vsnprintf ,
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.Nm vsnprintf_ss ,
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.Nm vasprintf
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.Nd formatted output conversion
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.Sh LIBRARY
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.Lb libc
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.Sh SYNOPSIS
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.In stdio.h
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.Ft int
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.Fn printf "const char * restrict format" ...
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.Ft int
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.Fn fprintf "FILE * restrict stream" "const char * restrict format" ...
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.Ft int
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.Fn dprintf "int fd" "const char * restrict format" ...
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.Ft int
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.Fn sprintf "char * restrict str" "const char * restrict format" ...
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.Ft int
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.Fn snprintf "char * restrict str" "size_t size" "const char * restrict format" ...
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.Ft int
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.Fn snprintf_ss "char * restrict str" "size_t size" "const char * restrict format" ...
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.Ft int
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.Fn asprintf "char ** restrict ret" "const char * restrict format" ...
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.In stdarg.h
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.Ft int
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.Fn vprintf "const char * restrict format" "va_list ap"
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.Ft int
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.Fn vfprintf "FILE * restrict stream" "const char * restrict format" "va_list ap"
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.Ft int
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.Fn vsprintf "char * restrict str" "const char * restrict format" "va_list ap"
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.Ft int
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.Fn vdprintf "int fd" "const char * restrict format" "va_list ap"
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.Ft int
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.Fn vsnprintf "char * restrict str" "size_t size" "const char * restrict format" "va_list ap"
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.Ft int
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.Fn vsnprintf_ss "char * restrict str" "size_t size" "const char * restrict format" "va_list ap"
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.Ft int
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.Fn vasprintf "char ** restrict ret" "const char * restrict format" "va_list ap"
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.Sh DESCRIPTION
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The
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.Fn printf
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family of functions produces output according to a
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.Fa format
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as described below.
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The
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.Fn printf
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and
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.Fn vprintf
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functions
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write output to
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.Em stdout ,
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the standard output stream;
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.Fn fprintf
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and
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.Fn vfprintf
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write output to the given output
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.Fa stream ;
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.Fn dprintf
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and
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.Fn vdprintf
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write output to the given file descriptor
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.Fa fd ;
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.Fn sprintf ,
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.Fn snprintf ,
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.Fn snprintf_ss ,
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.Fn vsprintf ,
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.Fn vsnprintf ,
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and
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.Fn vsnprintf_ss
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write to the character string
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.Fa str ;
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and
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.Fn asprintf
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and
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.Fn vasprintf
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write to a dynamically allocated string that is stored in
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.Fa ret .
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.Pp
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These functions write the output under the control of a
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.Fa format
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string that specifies how subsequent arguments
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(or arguments accessed via the variable-length argument facilities of
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.Xr stdarg 3 )
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are converted for output.
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.Pp
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.Fn snprintf_ss
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and
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.Fn vsnprintf_ss
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are signal-safe standalone versions that do not handle
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floating point formats, positional arguments, and wide characters.
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.Pp
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.Fn asprintf
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and
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.Fn vasprintf
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return a pointer to a buffer sufficiently large to hold the
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string in the
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.Fa ret
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argument.
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This pointer should be passed to
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.Xr free 3
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to release the allocated storage when it is no longer needed.
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If sufficient space cannot be allocated, these functions
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will return \-1 and set
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.Fa ret
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to be a
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.Dv NULL
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pointer.
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Please note that these functions are not standardized, and not all
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implementations can be assumed to set the
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.Fa ret
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argument to
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.Dv NULL
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on error.
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It is more portable to check for a return value of \-1 instead.
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.Pp
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.Fn snprintf ,
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.Fn vsnprintf ,
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and
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.Fn vsnprintf_ss
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will write at most
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.Fa size Ns \-1
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of the characters printed into the output string
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(the
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.Fa size Ns 'th
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character then gets the terminating
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.Ql \e0 ) ;
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if the return value is greater than or equal to the
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.Fa size
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argument, the string was too short
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and some of the printed characters were discarded.
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If
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.Fa size
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is zero, nothing is written and
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.Fa str
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may be a
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.Dv NULL
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pointer.
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.Pp
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.Fn sprintf
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and
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.Fn vsprintf
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effectively assume an infinite
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.Fa size .
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.Pp
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The format string is composed of zero or more directives:
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ordinary
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.\" multibyte
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characters (not
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.Cm % ) ,
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which are copied unchanged to the output stream;
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and conversion specifications, each of which results
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in fetching zero or more subsequent arguments.
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Each conversion specification is introduced by
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the character
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.Cm % .
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The arguments must correspond properly (after type promotion)
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with the conversion specifier.
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After the
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.Cm % ,
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the following appear in sequence:
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.Bl -bullet
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.It
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An optional field, consisting of a decimal digit string followed by a
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.Cm $ ,
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specifying the next argument to access.
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If this field is not provided, the argument following the last
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argument accessed will be used.
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Arguments are numbered starting at
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.Cm 1 .
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If unaccessed arguments in the format string are interspersed with ones that
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are accessed the results will be indeterminate.
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.It
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Zero or more of the following flags:
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.Bl -tag -width ".So \ Sc (space)"
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.It Sq Cm #
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The value should be converted to an
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.Dq alternate form .
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For
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.Cm c ,
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.Cm d ,
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.Cm i ,
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.Cm n ,
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.Cm p ,
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.Cm s ,
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and
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.Cm u
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conversions, this option has no effect.
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For
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.Cm o
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conversions, the precision of the number is increased to force the first
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character of the output string to a zero (except if a zero value is printed
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with an explicit precision of zero).
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For
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.Cm x
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and
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.Cm X
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conversions, a non-zero result has the string
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.Ql 0x
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(or
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.Ql 0X
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for
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.Cm X
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conversions) prepended to it.
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For
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.Cm a ,
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.Cm A ,
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.Cm e ,
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.Cm E ,
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.Cm f ,
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.Cm F ,
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.Cm g ,
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and
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.Cm G
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conversions, the result will always contain a decimal point, even if no
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digits follow it (normally, a decimal point appears in the results of
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those conversions only if a digit follows).
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For
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.Cm g
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and
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.Cm G
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conversions, trailing zeros are not removed from the result as they
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would otherwise be.
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.It So Cm 0 Sc (zero)
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Zero padding.
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For all conversions except
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.Cm n ,
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the converted value is padded on the left with zeros rather than blanks.
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If a precision is given with a numeric conversion
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.Pf ( Cm d ,
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.Cm i ,
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.Cm o ,
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.Cm u ,
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.Cm i ,
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.Cm x ,
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and
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.Cm X ) ,
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the
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.Cm 0
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flag is ignored.
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.It Sq Cm \-
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A negative field width flag;
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the converted value is to be left adjusted on the field boundary.
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Except for
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.Cm n
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conversions, the converted value is padded on the right with blanks,
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rather than on the left with blanks or zeros.
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A
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.Sq Cm \-
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overrides a
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.Sq Cm \&0
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if both are given.
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.It So "\ " Sc (space)
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A blank should be left before a positive number
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produced by a signed conversion
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.Pf ( Cm a ,
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.Cm A
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.Cm d ,
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.Cm e ,
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.Cm E ,
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.Cm f ,
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.Cm F ,
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.Cm g ,
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.Cm G ,
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or
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.Cm i ) .
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.It Sq Cm +
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A sign must always be placed before a
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number produced by a signed conversion.
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A
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.Sq Cm +
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overrides a space if both are used.
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.It Sq Cm '
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Decimal conversions
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.Cm ( d , u ,
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or
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.Cm i )
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|
or the integral portion of a floating point conversion
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|
.Cm ( f
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or
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.Cm F )
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|
should be grouped and separated by thousands using
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the non-monetary separator returned by
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.Xr localeconv 3 .
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.El
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.It
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An optional decimal digit string specifying a minimum field width.
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If the converted value has fewer characters than the field width, it will
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be padded with spaces on the left (or right, if the left-adjustment
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flag has been given) to fill out the field width.
|
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.It
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|
An optional precision, in the form of a period
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.Sq Cm \&.
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followed by an optional digit string.
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If the digit string is omitted, the precision is taken as zero.
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This gives the minimum number of digits to appear for
|
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.Cm d ,
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|
.Cm i ,
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.Cm o ,
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.Cm u ,
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.Cm x ,
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|
and
|
|
.Cm X
|
|
conversions, the number of digits to appear after the decimal-point for
|
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.Cm a ,
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|
.Cm A ,
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.Cm e ,
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|
.Cm E ,
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|
.Cm f ,
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|
and
|
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.Cm F
|
|
conversions, the maximum number of significant digits for
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.Cm g
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and
|
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.Cm G
|
|
conversions, or the maximum number of characters to be printed from a
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string for
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.Cm s
|
|
conversions.
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.It
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An optional length modifier, that specifies the size of the argument.
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The following length modifiers are valid for the
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.Cm d , i , n , o , u , x ,
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or
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.Cm X
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conversion:
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.Bl -column ".Cm q Em (deprecated)" ".Vt signed char" ".Vt unsigned long long" ".Vt long long *"
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.It Sy Modifier Ta Cm d , i Ta Cm o , u , x , X Ta Cm n
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.It Cm hh Ta Vt "signed char" Ta Vt "unsigned char" Ta Vt "signed char *"
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.It Cm h Ta Vt short Ta Vt "unsigned short" Ta Vt "short *"
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.It Cm l No (ell) Ta Vt long Ta Vt "unsigned long" Ta Vt "long *"
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.It Cm ll No (ell ell) Ta Vt "long long" Ta Vt "unsigned long long" Ta Vt "long long *"
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.It Cm j Ta Vt intmax_t Ta Vt uintmax_t Ta Vt "intmax_t *"
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.It Cm t Ta Vt ptrdiff_t Ta (see note) Ta Vt "ptrdiff_t *"
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.It Cm z Ta (see note) Ta Vt size_t Ta (see note)
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.It Cm q Em (deprecated) Ta Vt quad_t Ta Vt u_quad_t Ta Vt "quad_t *"
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.El
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.Pp
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|
Note:
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the
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.Cm t
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modifier, when applied to a
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.Cm o , u , x ,
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or
|
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.Cm X
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|
conversion, indicates that the argument is of an unsigned type
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|
equivalent in size to a
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.Vt ptrdiff_t .
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The
|
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.Cm z
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modifier, when applied to a
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.Cm d
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or
|
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.Cm i
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conversion, indicates that the argument is of a signed type equivalent in
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|
size to a
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.Vt size_t .
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Similarly, when applied to an
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.Cm n
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conversion, it indicates that the argument is a pointer to a signed type
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equivalent in size to a
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.Vt size_t .
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.Pp
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|
Note:
|
|
if the standard integer types described in
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.Xr stdint 3
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|
are used, it is recommended that the predefined format string specifier
|
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macros are used when possible.
|
|
These are further described in
|
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.Xr inttypes 3 .
|
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.Pp
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The following length modifier is valid for the
|
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.Cm a ,
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|
.Cm A ,
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.Cm e ,
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|
.Cm E ,
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|
.Cm f ,
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|
.Cm F ,
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|
.Cm g ,
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|
or
|
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.Cm G
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|
conversion:
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|
.Bl -column ".Sy Modifier" ".Cm a , A , e , E , f , F , g , G"
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|
.It Sy Modifier Ta Cm a , A , e , E , f , F , g , G
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.It Cm l No (ell) Ta Vt double
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(ignored, same behavior as without it)
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.It Cm L Ta Vt "long double"
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.El
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.Pp
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The following length modifier is valid for the
|
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.Cm c
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|
or
|
|
.Cm s
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|
conversion:
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|
.Bl -column ".Sy Modifier" ".Vt wint_t" ".Vt wchar_t *"
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|
.It Sy Modifier Ta Cm c Ta Cm s
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|
.It Cm l No (ell) Ta Vt wint_t Ta Vt "wchar_t *"
|
|
.El
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|
.It
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|
A character that specifies the type of conversion to be applied.
|
|
.El
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|
.Pp
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|
A field width or precision, or both, may be indicated by
|
|
an asterisk
|
|
.Ql *
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|
or an asterisk followed by one or more decimal digits and a
|
|
.Ql $
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|
instead of a
|
|
digit string.
|
|
In this case, an
|
|
.Vt int
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|
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
|
|
.Pq Li nn$
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|
and non-positional arguments, the results are undefined.
|
|
.Pp
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|
The conversion specifiers and their meanings are:
|
|
.Bl -tag -width ".Cm diouxX"
|
|
.It Cm diouxX
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|
The
|
|
.Vt int
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|
(or appropriate variant) argument is converted to signed decimal
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|
.Pf ( Cm d
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|
and
|
|
.Cm i ) ,
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|
unsigned octal
|
|
.Pq Cm o ,
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|
unsigned decimal
|
|
.Pq Cm u ,
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|
or unsigned hexadecimal
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|
.Pf ( Cm x
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|
and
|
|
.Cm X )
|
|
notation.
|
|
The letters
|
|
.Dq Li abcdef
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|
are used for
|
|
.Cm x
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|
conversions; the letters
|
|
.Dq Li ABCDEF
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|
are used for
|
|
.Cm X
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|
conversions.
|
|
The precision, if any, gives the minimum number of digits that must
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|
appear; if the converted value requires fewer digits, it is padded on
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|
the left with zeros.
|
|
.It Cm DOU
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|
The
|
|
.Vt long int
|
|
argument is converted to signed decimal, unsigned octal, or unsigned
|
|
decimal, as if the format had been
|
|
.Cm ld ,
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|
.Cm lo ,
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|
or
|
|
.Cm lu
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|
respectively.
|
|
These conversion characters are deprecated, and will eventually disappear.
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|
.It Cm eE
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|
The
|
|
.Vt double
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|
argument is rounded and converted in the style
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|
.Sm off
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|
.Oo \- Oc Ar d Li \&. Ar ddd Li e \*[Pm] Ar dd
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|
.Sm on
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|
where there is one digit before the
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|
decimal-point character
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|
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.
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|
An
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.Cm E
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|
conversion uses the letter
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.Ql E
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|
(rather than
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|
.Ql e )
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|
to introduce the exponent.
|
|
The exponent always contains at least two digits; if the value is zero,
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|
the exponent is 00.
|
|
.Pp
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|
For
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.Cm a ,
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.Cm A ,
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.Cm e ,
|
|
.Cm E ,
|
|
.Cm f ,
|
|
.Cm F ,
|
|
.Cm g ,
|
|
and
|
|
.Cm G
|
|
conversions, positive and negative infinity are represented as
|
|
.Li inf
|
|
and
|
|
.Li -inf
|
|
respectively when using the lowercase conversion character, and
|
|
.Li INF
|
|
and
|
|
.Li -INF
|
|
respectively when using the uppercase conversion character.
|
|
Similarly, NaN is represented as
|
|
.Li nan
|
|
when using the lowercase conversion, and
|
|
.Li NAN
|
|
when using the uppercase conversion.
|
|
.It Cm fF
|
|
The
|
|
.Vt double
|
|
argument is rounded and converted to decimal notation in the style
|
|
.Sm off
|
|
.Oo \- Oc Ar ddd Li \&. Ar ddd ,
|
|
.Sm on
|
|
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.
|
|
.It Cm gG
|
|
The
|
|
.Vt double
|
|
argument is converted in style
|
|
.Cm f
|
|
or
|
|
.Cm e
|
|
(or in style
|
|
.Cm F
|
|
or
|
|
.Cm E
|
|
for
|
|
.Cm 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
|
|
.Cm 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.
|
|
.It Cm aA
|
|
The
|
|
.Vt double
|
|
argument is rounded and converted to hexadecimal notation in the style
|
|
.Sm off
|
|
.Oo \- Oc Li 0x Ar h Li \&. Ar hhhp Oo \*[Pm] Oc Ar d ,
|
|
.Sm on
|
|
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
|
|
.Cm p
|
|
is a literal character
|
|
.Ql p ,
|
|
and the exponent consists of a positive or negative sign
|
|
followed by a decimal number representing an exponent of 2.
|
|
The
|
|
.Cm A
|
|
conversion uses the prefix
|
|
.Dq Li 0X
|
|
(rather than
|
|
.Dq Li 0x ) ,
|
|
the letters
|
|
.Dq Li ABCDEF
|
|
(rather than
|
|
.Dq Li abcdef )
|
|
to represent the hex digits, and the letter
|
|
.Ql P
|
|
(rather than
|
|
.Ql p )
|
|
to separate the mantissa and exponent.
|
|
.Pp
|
|
Note that there may be multiple valid ways to represent floating-point
|
|
numbers in this hexadecimal format.
|
|
For example,
|
|
.Li 0x3.24p+0 , 0x6.48p-1
|
|
and
|
|
.Li 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
|
|
.Ql -
|
|
if appropriate) and an exponent of
|
|
.Li +0 .
|
|
.It Cm C
|
|
Treated as
|
|
.Cm c
|
|
with the
|
|
.Cm l
|
|
(ell) modifier.
|
|
.It Cm c
|
|
The
|
|
.Vt int
|
|
argument is converted to an
|
|
.Vt "unsigned char" ,
|
|
and the resulting character is written.
|
|
.Pp
|
|
If the
|
|
.Cm l
|
|
(ell) modifier is used, the
|
|
.Vt wint_t
|
|
argument shall be converted to a
|
|
.Vt wchar_t ,
|
|
and the (potentially multi-byte) sequence representing the
|
|
single wide character is written, including any shift sequences.
|
|
If a shift sequence is used, the shift state is also restored
|
|
to the original state after the character.
|
|
.It Cm S
|
|
Treated as
|
|
.Cm s
|
|
with the
|
|
.Cm l
|
|
(ell) modifier.
|
|
.It Cm s
|
|
The
|
|
.Vt "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
|
|
.Dv NUL
|
|
character;
|
|
if a precision is specified, no more than the number specified 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
|
|
.Dv NUL
|
|
character.
|
|
.Pp
|
|
If the
|
|
.Cm l
|
|
(ell) modifier is used, the
|
|
.Vt "wchar_t *"
|
|
argument is expected to be a pointer to an array of wide characters
|
|
(pointer to a wide string).
|
|
For each wide character in the string, the (potentially multi-byte)
|
|
sequence representing the
|
|
wide character is written, including any shift sequences.
|
|
If any shift sequence is used, the shift state is also restored
|
|
to the original state after the string.
|
|
Wide characters from the array are written up to (but not including)
|
|
a terminating wide
|
|
.Dv NUL
|
|
character;
|
|
if a precision is specified, no more than the number of bytes specified are
|
|
written (including shift sequences).
|
|
Partial characters are never written.
|
|
If a precision is given, no null character
|
|
need be present; if the precision is not specified, or is greater than
|
|
the number of bytes required to render the multibyte representation of
|
|
the string, the array must contain a terminating wide
|
|
.Dv NUL
|
|
character.
|
|
.It Cm p
|
|
The
|
|
.Vt "void *"
|
|
pointer argument is printed in hexadecimal (as if by
|
|
.Ql %#x
|
|
or
|
|
.Ql %#lx ) .
|
|
.It Cm n
|
|
The number of characters written so far is stored into the
|
|
integer indicated by the
|
|
.Vt "int *"
|
|
(or variant) pointer argument.
|
|
No argument is converted.
|
|
.It Cm %
|
|
A
|
|
.Ql %
|
|
is written.
|
|
No argument is converted.
|
|
The complete conversion specification is
|
|
.Ql %% .
|
|
.El
|
|
.Pp
|
|
The decimal point
|
|
character is defined in the program's locale (category
|
|
.Dv LC_NUMERIC ) .
|
|
.Pp
|
|
In no case does a non-existent or small field width cause truncation of
|
|
a numeric field; if the result of a conversion is wider than the field
|
|
width, the
|
|
field is expanded to contain the conversion result.
|
|
.Sh RETURN VALUES
|
|
These functions return
|
|
the number of characters printed, or that would be printed if there
|
|
was adequate space in case of
|
|
.Fn snprintf ,
|
|
.Fn vsnprintf ,
|
|
and
|
|
.Fn vsnprintf_ss
|
|
(not including the trailing
|
|
.Ql \e0
|
|
used to end output to strings).
|
|
If an output error was encountered, these functions shall return a
|
|
negative value.
|
|
.Sh EXAMPLES
|
|
To print a date and time in the form
|
|
.Dq Li "Sunday, July 3, 10:02" ,
|
|
where
|
|
.Fa weekday
|
|
and
|
|
.Fa month
|
|
are pointers to strings:
|
|
.Bd -literal -offset indent
|
|
#include \*[Lt]stdio.h\*[Gt]
|
|
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 \*[Lt]math.h\*[Gt]
|
|
#include \*[Lt]stdio.h\*[Gt]
|
|
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 \*[Lt]stdio.h\*[Gt]
|
|
#include \*[Lt]stdlib.h\*[Gt]
|
|
#include \*[Lt]stdarg.h\*[Gt]
|
|
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.
|
|
.El
|
|
.Sh SEE ALSO
|
|
.Xr printf 1 ,
|
|
.Xr fmtcheck 3 ,
|
|
.Xr scanf 3 ,
|
|
.Xr setlocale 3 ,
|
|
.Xr wprintf 3 ,
|
|
.Xr printf 9
|
|
.Sh STANDARDS
|
|
Subject to the caveats noted in the
|
|
.Sx BUGS
|
|
section below, the
|
|
.Fn fprintf ,
|
|
.Fn printf ,
|
|
.Fn sprintf ,
|
|
.Fn vprintf ,
|
|
.Fn vfprintf ,
|
|
and
|
|
.Fn vsprintf
|
|
functions
|
|
conform to
|
|
.St -ansiC
|
|
and
|
|
.St -isoC-99 .
|
|
With the same reservation, the
|
|
.Fn snprintf
|
|
and
|
|
.Fn vsnprintf
|
|
functions conform to
|
|
.St -isoC-99 .
|
|
.Sh HISTORY
|
|
The functions
|
|
.Fn snprintf
|
|
and
|
|
.Fn vsnprintf
|
|
first appeared in
|
|
.Bx 4.4 .
|
|
The functions
|
|
.Fn asprintf
|
|
and
|
|
.Fn vasprintf
|
|
are modeled on the ones that first appeared in the GNU C library.
|
|
The function
|
|
.Fn vsnprintf_ss
|
|
is non-standard and appeared in
|
|
.Nx 4.0 .
|
|
The functions
|
|
.Fn dprintf
|
|
and
|
|
.Fn vdprintf
|
|
are parts of
|
|
.St -p1003.1-2008
|
|
and appeared in
|
|
.Nx 6.0 .
|
|
.Sh CAVEATS
|
|
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 snprintf
|
|
interfaces are not available on older
|
|
systems and the
|
|
.Fn asprintf
|
|
interfaces are not yet portable.
|
|
.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 your stack,
|
|
leading to a possible security hole.
|
|
This holds true even if you have built the string
|
|
.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
|
|
snprintf(buffer, sizeof(buffer), "%s", string);
|
|
.Ed
|
|
.Pp
|
|
There is no way for
|
|
.Fn 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 fail.
|
|
.Pp
|
|
In this implementation, passing a
|
|
.Dv NULL
|
|
.Vt char *
|
|
argument to the
|
|
.Cm %s
|
|
format specifier will output
|
|
.Em "(null)"
|
|
instead of crashing.
|
|
Programs that depend on this behavior are non-portable and may crash
|
|
on other systems or in the future.
|
|
.Sh BUGS
|
|
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
|
|
The
|
|
.Nm
|
|
family of functions do not correctly handle multibyte characters in the
|
|
.Fa format
|
|
argument.
|
|
.Sh SECURITY CONSIDERATIONS
|
|
The
|
|
.Fn sprintf
|
|
and
|
|
.Fn vsprintf
|
|
functions are easily misused in a manner which enables malicious users
|
|
to arbitrarily change a running program's functionality through
|
|
a buffer overflow attack.
|
|
Because
|
|
.Fn sprintf
|
|
and
|
|
.Fn vsprintf
|
|
assume an infinitely long string,
|
|
callers must be careful not to overflow the actual space;
|
|
this is often hard to assure.
|
|
For safety, programmers should use the
|
|
.Fn snprintf
|
|
interface instead.
|
|
For example:
|
|
.Bd -literal
|
|
void
|
|
foo(const char *arbitrary_string, const char *and_another)
|
|
{
|
|
char onstack[8];
|
|
|
|
#ifdef BAD
|
|
/*
|
|
* This first sprintf is bad behavior. Do not use sprintf!
|
|
*/
|
|
sprintf(onstack, "%s, %s", arbitrary_string, and_another);
|
|
#else
|
|
/*
|
|
* The following two lines demonstrate better use of
|
|
* snprintf().
|
|
*/
|
|
snprintf(onstack, sizeof(onstack), "%s, %s", arbitrary_string,
|
|
and_another);
|
|
#endif
|
|
}
|
|
.Ed
|
|
.Pp
|
|
The
|
|
.Fn printf
|
|
and
|
|
.Fn sprintf
|
|
family of functions are also easily misused in a manner
|
|
allowing malicious users to arbitrarily change a running program's
|
|
functionality by either causing the program
|
|
to print potentially sensitive data
|
|
.Dq "left on the stack" ,
|
|
or causing it to generate a memory fault or bus error
|
|
by dereferencing an invalid pointer.
|
|
.Pp
|
|
.Cm %n
|
|
can be used to write arbitrary data to potentially carefully-selected
|
|
addresses.
|
|
Programmers are therefore strongly advised to never pass untrusted strings
|
|
as the
|
|
.Fa format
|
|
argument, as an attacker can put format specifiers in the string
|
|
to mangle your stack,
|
|
leading to a possible security hole.
|
|
This holds true even if the string was built using a function like
|
|
.Fn snprintf ,
|
|
as the resulting string may still contain user-supplied conversion specifiers
|
|
for later interpolation by
|
|
.Fn printf .
|
|
.Pp
|
|
Always use the proper secure idiom:
|
|
.Pp
|
|
.Dl "snprintf(buffer, sizeof(buffer), \*q%s\*q, string);"
|