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authorCarlos O'Donell <carlos@redhat.com>2020-05-21 17:50:53 -0400
committerCarlos O'Donell <carlos@redhat.com>2020-06-01 12:26:32 -0400
commit61af4bbb2ae5a4eefc4c4243135747bbdb0f0684 (patch)
tree38a02e433414e331b7bf484b5a540f81864a8be0 /stdlib
parent9e2dc874e62b0950891b319c000b009ea12ac8c2 (diff)
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mbstowcs: Document, test, and fix null pointer dst semantics (Bug 25219)
The function mbstowcs, by an XSI extension to POSIX, accepts a null
pointer for the destination wchar_t array.  This API behaviour allows
you to use the function to compute the length of the required wchar_t
array i.e. does the conversion without storing it and returns the
number of wide characters required.

We remove the __write_only__ markup for the first argument because it
is not true since the destination may be a null pointer, and so the
length argument may not apply.  We remove the markup otherwise the new
test case cannot be compiled with -Werror=nonnull.

We add a new test case for mbstowcs which exercises the destination is
a null pointer behaviour which we have now explicitly documented.

The mbsrtowcs and mbsnrtowcs behave similarly, and mbsrtowcs is
documented as doing this in C11, even if the standard doesn't come out
and call out this specific use case.  We add one note to each of
mbsrtowcs and mbsnrtowcs to call out that they support a null pointer
for the destination.

The wcsrtombs function behaves similarly but in the other way around
and allows you to use a null destination pointer to compute how many
bytes you would need to convert the wide character input.  We document
this particular case also, but leave wcsnrtombs as a references to
wcsrtombs, so the reader must still read the details of the semantics
for wcsrtombs.
Diffstat (limited to 'stdlib')
-rw-r--r--stdlib/stdlib.h2
1 files changed, 1 insertions, 1 deletions
diff --git a/stdlib/stdlib.h b/stdlib/stdlib.h
index dd779bd740..f971df4247 100644
--- a/stdlib/stdlib.h
+++ b/stdlib/stdlib.h
@@ -932,7 +932,7 @@ extern int wctomb (char *__s, wchar_t __wchar) __THROW;
 /* Convert a multibyte string to a wide char string.  */
 extern size_t mbstowcs (wchar_t *__restrict  __pwcs,
 			const char *__restrict __s, size_t __n) __THROW
-    __attr_access ((__write_only__, 1, 3)) __attr_access ((__read_only__, 2));
+    __attr_access ((__read_only__, 2));
 /* Convert a wide char string to multibyte string.  */
 extern size_t wcstombs (char *__restrict __s,
 			const wchar_t *__restrict __pwcs, size_t __n)