"offsetof" directive to obtain offset within struct to struct member

[ian-ross]

Bug imported from C2HS Trac

Trac ticket created: 2009-08-16T10:45:26-0700

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I find it useful to be able to obtain the offset to a struct member within a struct, e.g. for constructing pointers to members. get and set implicitly do this when constructing their code.

I've implemented a working version of this in my local repository, and I was hoping it could be put in the upstream. The version I've implemented has similar syntax to get and set.

Consider:

typedef struct {
    int field;
} substruct_t;

typedef struct {
    char somefield[32];
    substruct_t substruct;
    substruct_t* substruct_p;
} struct_t; 

with my patch you can write:

ptrToMember :: Ptr Struct -> IO (Ptr SubStruct)
ptrToMember p = p `plusPtr` {#offsetof struct_t->substruct#}

compared to get/set this emits the byte offset as an int instead of emitting peekByteOff/pokeByteOff code, e.g.:

ptrToMember :: Ptr Struct -> IO (Ptr SubStruct)
ptrToMember p = p `plusPtr` (32)

The results are confusing (and so my patch errors) if you try to get the offset of an indirect member, e.g.

{#offsetof struct_t->substruct_ptr->field#}

is computable (it's 0), but probably not what you want, so my patch forces you to instead use:

{#offsetof substruct_t->field#}

Note this patch is against 0.16, but there is no 0.16 in the version dropdown for trac, so I set no version.

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Original Trac ticket had attachments:

• offsetof.patch (Download)

[ian-ross]

Original comment date: 2009-08-16T10:47:55-0700

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