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.Dd June 14, 2014 .Dt FMTCHECK 3 .Os .Sh NAME .Nm fmtcheck .Nd sanitizes user-supplied printf(3)-style format string .Sh LIBRARY .Lb libc .Sh SYNOPSIS n stdio.h .Ft const char * .Fn fmtcheck "const char *fmt_suspect" "const char *fmt_default" .Sh DESCRIPTION The .Nm function scans .Fa fmt_suspect and .Fa fmt_default to determine if .Fa fmt_suspect will consume the same argument types as .Fa fmt_default and to ensure that .Fa fmt_suspect is a valid format string.
p The .Xr printf 3 family of functions can not verify the types of arguments that they are passed at run-time. In some cases, like .Xr catgets 3 , it is useful or necessary to use a user-supplied format string with no guarantee that the format string matches the specified parameters.
p The .Nm function was designed to be used in these cases, as in: d -literal -offset indent printf(fmtcheck(user_format, standard_format), arg1, arg2); .Ed
p In the check, field widths, fillers, precisions, etc. are ignored (unless the field width or precision is an asterisk .Ql * instead of a digit string). Also, any text other than the format specifiers is completely ignored.
p Note that the formats may be quite different as long as they accept the same parameters. For example, "%ld %o %30s %#llx %-10.*e %n" is compatible with "This number %lu %d%% and string %s has %qd numbers and %.*g floats (%n)." However, "%o" is not equivalent to "%lx" because the first requires an integer and the second requires a long, and "%p" is not equivalent to "%lu" because the first requires a pointer and the second requires a long. .Sh RETURN VALUES If .Fa fmt_suspect is a valid format and consumes the same argument types as .Fa fmt_default , then the .Nm function will return .Fa fmt_suspect . Otherwise, it will return .Fa fmt_default . .Sh SEE ALSO .Xr printf 3