Manuel López-Ibáñez <lopeziba...@gmail.com>: > On 14 October 2014 01:12, Martin Uecker <uec...@eecs.berkeley.edu> wrote: > > Converting a pointer to an array to a pointer to a constant array > > is safe. Converting a pointer to a pointer to a pointer to a pointer > > to a constant is not (as the CFAQ points out). > > You are probably right that it is safe. Unfortunately, C considers > invalid cases that are safe and that are allowed by C++ (as mentioned > in that C FAQ). I updated the FAQ with comments by Joseph Myers taken > from https://gcc.gnu.org/bugzilla/show_bug.cgi?id=47143 and > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=33076.
Thank you. I am not sure about the full C++ semantics, but the case of passing pointers to const arrays seems fairly straightforward. I also raised the issue at comp.std.c and proposed a change the following extension to the pointer conversion rule (6.3.2.3): "A pointer to an array with non-q-qualified element type may be converted to a pointer to an array with the q-qualified version of the type as element type." Maybe somebody from the committee is reading there... > I'm not sure how much effort would be to make the C FE of GCC follow > the rules of C++ or whether this would conflict with other parts of > the standard (that is, rejecting valid C programs). I played bit at gcc/c/c-typeck.c and getting rid of the warning seems simple (see below). But so far I don't fully understand the code or what I did... I also haven't figured out how to add a then-required warning about the case where a pointer-to-constant-array is converted to a pointer-to-array. > Perhaps the option could be separated into two, one only enabled by > -Wpedantic that controls cases that are known to be safe > (-Wpedantic-incompatible-pointer-types? -Wincompatible-but-safe?). My preferred solution would be to simple allow the case where it is safe and warn only when enabled by -Wpedantic. > Still, someone would need to step up and do the work: > https://gcc.gnu.org/wiki/GettingStarted#Basics:_Contributing_to_GCC_in_10_easy_steps I will talk a look at this. Martin > Cheers, > > Manuel. Index: gcc/c/c-typeck.c =================================================================== --- gcc/c/c-typeck.c (Revision 216134) +++ gcc/c/c-typeck.c (Arbeitskopie) @@ -1217,6 +1217,7 @@ comp_target_types (location_t location, tree ttl, tree ttr) { int val; + int val2; tree mvl = TREE_TYPE (ttl); tree mvr = TREE_TYPE (ttr); addr_space_t asl = TYPE_ADDR_SPACE (mvl); @@ -1228,13 +1229,20 @@ if (!addr_space_superset (asl, asr, &as_common)) return 0; + val2 = 1; + + if ((TREE_CODE (mvl) == ARRAY_TYPE) && (TREE_CODE (mvr) == ARRAY_TYPE)) + val2 = comptypes (mvl, mvr); + /* Do not lose qualifiers on element types of array types that are pointer targets by taking their TYPE_MAIN_VARIANT. */ - if (TREE_CODE (mvl) != ARRAY_TYPE) + +// if (TREE_CODE (mvl) != ARRAY_TYPE) mvl = (TYPE_ATOMIC (mvl) ? c_build_qualified_type (TYPE_MAIN_VARIANT (mvl), TYPE_QUAL_ATOMIC) : TYPE_MAIN_VARIANT (mvl)); - if (TREE_CODE (mvr) != ARRAY_TYPE) + +// if (TREE_CODE (mvr) != ARRAY_TYPE) mvr = (TYPE_ATOMIC (mvr) ? c_build_qualified_type (TYPE_MAIN_VARIANT (mvr), TYPE_QUAL_ATOMIC) : TYPE_MAIN_VARIANT (mvr)); @@ -1241,6 +1249,10 @@ enum_and_int_p = false; val = comptypes_check_enum_int (mvl, mvr, &enum_and_int_p); + if ((val == 1) && (val2 != 1)) + pedwarn (location, OPT_Wpedantic, "pointers to array have incompatible qualifiers"); + if (val == 2) pedwarn (location, OPT_Wpedantic, "types are not quite compatible");
