correct FLEXPTR_* example in comment
PATCH 1/1 correct FLEXPTR_* example in comment
This section is about "The FLEXPTR_* variants", so use FLEXPTR_ALLOC_STR in the example.
--- git-compat-util.h | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
git-compat-util.h +1 -1
@@ -815,7 +815,7 @@ extern FILE *fopen_for_writing(const char *path);* you can do: * * struct foo *f; -* FLEX_ALLOC_STR(f, name, src); +* FLEXPTR_ALLOC_STR(f, name, src); * * and "name" will point to a block of memory after the struct, which will be * freed along with the struct (but the pointer can be repointed anywhere).
Signature · 1 line
2.9.3
Quoted 2 lines · René Scharfe
This section is about "The FLEXPTR_* variants", so use
FLEXPTR_ALLOC_STRin the example.
Oops, yeah. Your patch is clearly an improvement.
Since this is obviously an easy mistake to make (using one form rather than the other), I wondered if the compiler would catch it.
I think it would catch an accidental use of FLEXPTR instead of FLEX, because it involves an attempted assignment of an array. But I don't think we would catch the reverse; we'd just write the data directly on top of the pointer. That would probably crash immediately at runtime, so if you exercise the code at all in tests, it is OK. But something to be aware of.
I suppose it could assert(sizeof((x)->flexname) == FLEX_ALLOC) or something, but I'm not sure if it is worth worrying about.
-Peff
Am 13.08.2016 um 11:09 schrieb Jeff King:
Quoted 17 lines · René Scharfe
Quoted 3 lines
This section is about "The FLEXPTR_* variants", so use
FLEXPTR_ALLOC_STRin the example.Oops, yeah. Your patch is clearly an improvement.
Since this is obviously an easy mistake to make (using one form rather than the other), I wondered if the compiler would catch it.
I think it would catch an accidental use of FLEXPTR instead of FLEX, because it involves an attempted assignment of an array. But I don't think we would catch the reverse; we'd just write the data directly on top of the pointer. That would probably crash immediately at runtime, so if you exercise the code at all in tests, it is OK. But something to be aware of.
Quoted 2 lines
I suppose it could
assert(sizeof((x)->flexname) == FLEX_ALLOC)or something, but I'm not sure if it is worth worrying about.
A compilation error or warning would be nice. I scratched my head quite a bit before figuring out that the example was wrong. Copy&paste from a reputable source must be correct, right? :)
René
Am 13.08.2016 um 11:09 schrieb Jeff King:
Quoted 5 lines · René Scharfe
Quoted 3 lines
This section is about "The FLEXPTR_* variants", so use
FLEXPTR_ALLOC_STRin the example.
Quoted 7 lines
Oops, yeah. Your patch is clearly an improvement.
Since this is obviously an easy mistake to make (using one form rather than the other), I wondered if the compiler would catch it.
I think it would catch an accidental use of FLEXPTR instead of FLEX, because it involves an attempted assignment of an array.
Gcc 5.4 reports "invalid use of flexible array member".
Quoted 5 lines
But I don't
think we would catch the reverse; we'd just write the data directly on top of the pointer. That would probably crash immediately at runtime, so if you exercise the code at all in tests, it is OK. But something to be aware of.
No hint at compile time, segfaults at runtime.
Quoted 2 lines
I suppose it could
assert(sizeof((x)->flexname) == FLEX_ALLOC)or something, but I'm not sure if it is worth worrying about.
You can't use sizeof with an actual flexible array. It only works if FLEX_ARRAY is defined as 1 (for platforms without native support), and perhaps also if it's 0.
offsetof(struct x, arr) == sizeof(struct x) won't work either because of padding.
I have no idea what you can do with a flexible array that would throw a compile error when done with a pointer.
René
Quoted 7 lines · René Scharfe
Quoted 2 lines
I suppose it could
assert(sizeof((x)->flexname) == FLEX_ALLOC)or something, but I'm not sure if it is worth worrying about.You can't use sizeof with an actual flexible array. It only works if
FLEX_ARRAYis defined as 1 (for platforms without native support), and perhaps also if it's 0.
Quoted 5 lines
offsetof(struct x, arr)== sizeof(struct x) won't work either because of padding.I have no idea what you can do with a flexible array that would throw a compile error when done with a pointer.
Thanks for the input. I'd say we should not worry about it. The reason this particular "bug" persisted is because it was in a comment. People tend to notice code that cannot possibly do anything but segfault before they even send in patches.
-Peff