Fork#39224: GCC-7.3 and later apply wrong -Wformat attributes for snprintf() and vsnprintf() functions
Open Date: 2019-05-06 15:51
Last Update: 2019-07-03 19:18
URL for this Ticket:
https://osdn.net//projects/mingw/ticket/39224
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Last Changes/Comment on this Ticket:
2019-07-03 19:18 Updated by: keith
Comment:
There is, in fact, a defect in the MinGW -Wformat handler; it affects the
handling of the "%n" conversion specification, when specified with the "hh"
length modifier, (i.e. as "%hhn"). Although this combination is fully supported
by the underlying pformat.c implementation, the associated -Wformat handler
does not recognize this, and consequently emits an entirely bogus, and
unhelpful warning, when this is parsed.
Fixing this isn't difficult, but it does require a (tedious) rebuild of GCC, to
deploy it.
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Ticket Status:
Reporter: keith
Owner: keith
Type: Issues
Status: Open [Owner assigned]
Priority: 5 - Medium
MileStone: (None)
Component: WSL
Severity: 5 - Medium
Resolution: None
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Ticket details:
As originally reported by Eli Zaretskii, in this mingw-users mail posting, use
of ISO-C99 standard format specifications, such as "%02hhx", (which is not
understood by Microsoft's MSVCRT.DLL family of printf() functions), in calls to
snprintf() may result in unexpected compile-time warning diagnostics, when
compiled with any warning option, such as -Wall, which implicitly (or even
explicitly) enables -Wformat.
The issue appears to be caused by GCC's use of an internal prototype, for
snprintf(), which seems to arbitrarily assume Microsoft's -Wformat semantics,
(even though Microsoft have never provided a snprintf() implementation, in any
version of the system-supplied MSVCRT.DLL runtime library, on which every
MinGW-GCC compiler build is dependent). The issue does not arise when calls to
snprintf() are replaced by its equivalent __mingw_snprintf(), (which is
explicitly declared as exhibiting MinGW's -Wformat semantics); nor does it
arise when GCC's internal prototype is overridden, (by use of the
-fno-builtin-snprintf option), and thus the <stdio.h> prototype, (which also
explicitly exhibits MinGW's -Wformat semantics), becomes applicable.
It is anticipated that the vsnprintf() implementation will be similarly
affected.
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