gen/FreeBSD/fmtcheck.3

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  34 .\" $FreeBSD: src/lib/libc/gen/fmtcheck.3,v 1.8 2002/12/19 09:40:21 ru Exp $
  35 .Dd October 16, 2002
  36 .Os
  37 .Dt FMTCHECK 3
  38 .Sh NAME
  39 .Nm fmtcheck
  40 .Nd sanitizes user-supplied
  41 .Xr printf 3 Ns -style
  42 format string
  43 .Sh LIBRARY
  44 .Lb libc
  45 .Sh SYNOPSIS
  46 .In stdio.h
  47 .Ft const char *
  48 .Fn fmtcheck "const char *fmt_suspect" "const char *fmt_default"
  49 .Sh DESCRIPTION
  50 The
  51 .Fn fmtcheck
  52 scans
  53 .Fa fmt_suspect
  54 and
  55 .Fa fmt_default
  56 to determine if
  57 .Fa fmt_suspect
  58 will consume the same argument types as
  59 .Fa fmt_default
  60 and to ensure that
  61 .Fa fmt_suspect
  62 is a valid format string.
  63 .Pp
  64 The
  65 .Xr printf 3
  66 family of functions cannot verify the types of arguments that they are
  67 passed at run-time.  In some cases, like
  68 .Xr catgets 3 ,
  69 it is useful or necessary to use a user-supplied format string with no
  70 guarantee that the format string matches the specified arguments.
  71 .Pp
  72 The
  73 .Fn fmtcheck
  74 was designed to be used in these cases, as in:
  75 .Bd -literal -offset indent
  76 printf(fmtcheck(user_format, standard_format), arg1, arg2);
  77 .Ed
  78 .Pp
  79 In the check, field widths, fillers, precisions, etc. are ignored (unless
  80 the field width or precision is an asterisk
  81 .Ql *
  82 instead of a digit string).  Also, any text other than the format specifiers
  83 is completely ignored.
  84 .Sh RETURN VALUES
  85 If
  86 .Fa fmt_suspect
  87 is a valid format and consumes the same argument types as
  88 .Fa fmt_default ,
  89 then the
  90 .Fn fmtcheck
  91 will return
  92 .Fa fmt_suspect .
  93 Otherwise, it will return
  94 .Fa fmt_default .
  95 .Sh SECURITY CONSIDERATIONS
  96 Note that the formats may be quite different as long as they accept the
  97 same arguments.  For example,
  98 .Qq Li "%p %o %30s %#llx %-10.*e %n"
  99 is compatible with
 100 .Qq Li "This number %lu %d%% and string %s has %qd numbers and %.*g floats (%n)" .
 101 However,
 102 .Qq Li %o
 103 is not equivalent to
 104 .Qq Li %lx
 105 because
 106 the first requires an integer and the second requires a long.
 107 .Sh SEE ALSO
 108 .Xr printf 3
 109 .Sh BUGS
 110 The
 111 .Fn fmtcheck
 112 function does not understand all of the conversions that
 113 .Xr printf 3
 114 does.