1FMTCHECK(3) BSD Library Functions Manual FMTCHECK(3)
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4 fmtcheck — sanitizes user-supplied printf(3)-style format string
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7 Utility functions from BSD systems (libbsd, -lbsd)
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10 #include <stdio.h>
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12 const char *
13 fmtcheck(const char *fmt_suspect, const char *fmt_default);
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16 The fmtcheck() scans fmt_suspect and fmt_default to determine if
17 fmt_suspect will consume the same argument types as fmt_default and to
18 ensure that fmt_suspect is a valid format string.
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20 The printf(3) family of functions cannot verify the types of arguments
21 that they are passed at run-time. In some cases, like catgets(3), it is
22 useful or necessary to use a user-supplied format string with no guaran‐
23 tee that the format string matches the specified arguments.
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25 The fmtcheck() was designed to be used in these cases, as in:
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27 printf(fmtcheck(user_format, standard_format), arg1, arg2);
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29 In the check, field widths, fillers, precisions, etc. are ignored (unless
30 the field width or precision is an asterisk ‘*’ instead of a digit
31 string). Also, any text other than the format specifiers is completely
32 ignored.
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35 If fmt_suspect is a valid format and consumes the same argument types as
36 fmt_default, then the fmtcheck() will return fmt_suspect. Otherwise, it
37 will return fmt_default.
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40 Note that the formats may be quite different as long as they accept the
41 same arguments. For example, "%p %o %30s %#llx %-10.*e %n" is compatible
42 with "This number %lu %d%% and string %s has %qd numbers and %.*g floats
43 (%n)". However, "%o" is not equivalent to "%lx" because the first
44 requires an integer and the second requires a long.
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47 printf(3)
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50 The fmtcheck() function does not understand all of the conversions that
51 printf(3) does.
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53BSD October 16, 2002 BSD