gen/FreeBSD/fmtcheck.3

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  34 .\" $FreeBSD: src/lib/libc/gen/fmtcheck.3,v 1.10 2008/08/02 06:02:42 das 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.
  68 In some cases, like
  69 .Xr catgets 3 ,
  70 it is useful or necessary to use a user-supplied format string with no
  71 guarantee that the format string matches the specified arguments.
  72 .Pp
  73 The
  74 .Fn fmtcheck
  75 was designed to be used in these cases, as in:
  76 .Bd -literal -offset indent
  77 printf(fmtcheck(user_format, standard_format), arg1, arg2);
  78 .Ed
  79 .Pp
  80 In the check, field widths, fillers, precisions, etc.\& are ignored (unless
  81 the field width or precision is an asterisk
  82 .Ql *
  83 instead of a digit string).
  84 Also, any text other than the format specifiers
  85 is completely ignored.
  86 .Sh RETURN VALUES
  87 If
  88 .Fa fmt_suspect
  89 is a valid format and consumes the same argument types as
  90 .Fa fmt_default ,
  91 then the
  92 .Fn fmtcheck
  93 will return
  94 .Fa fmt_suspect .
  95 Otherwise, it will return
  96 .Fa fmt_default .
  97 .Sh SECURITY CONSIDERATIONS
  98 Note that the formats may be quite different as long as they accept the
  99 same arguments.
 100 For example,
 101 .Qq Li "%p %o %30s %#llx %-10.*e %n"
 102 is compatible with
 103 .Qq Li "This number %lu %d%% and string %s has %qd numbers and %.*g floats (%n)" .
 104 However,
 105 .Qq Li %o
 106 is not equivalent to
 107 .Qq Li %lx
 108 because
 109 the first requires an integer and the second requires a long.
 110 .Sh SEE ALSO
 111 .Xr printf 3
 112 .Sh BUGS
 113 The
 114 .Fn fmtcheck
 115 function does not recognize positional parameters.