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    SubjectRe: [RFC] Documentation about unaligned memory access
    On Sat, 24 Nov 2007 15:50:52 +0000
    Luciano Rocha <> wrote:

    > Dumb memcpy (while (len--) { *d++ = *s++ }) will have alignment problems
    > in any case. Intelligent ones, like the one provided in glibc, first copy
    > bytes till output is aligned (C file) *or* size is a multiple (i686 asm file)
    > of word size, and then it copies word-by-word.
    > Linux's x86_64 memcpy does the opposite, copies 64bit words, and then
    > copies the last bytes.
    > So, in effect, as long as no packed structures are used, memcpy should
    > be safer on *int, etc., than *char, as the compiler ensures
    > word-alignment.

    It most certainly does not. gcc will assume that an int* has int alignment. memcpy() is a builtin, which gcc can translate to pretty much anything. And C specifies that a pointer to foo, will point to a real object of type foo, so gcc can't be blamed for the unsafe typecasts. I have tested this the hard way, so this is not just speculation.

    E.g., we have the following struct:

    struct foo
    u8 a[4];
    u32 b;

    This struct will have a size of 8 bytes and an alignment of 4 bytes (caused by the member b). Now take the following code:

    void copy_foo(struct foo *dst, struct foo *src)
    *dst = *src;

    On a platform that supports 64-bit loads and stores (e.g. AVR32, where I got hit by this), this will generate:

    LD r1, (src)
    ST r1, (dst)

    Now if I replace that with:

    void copy_foo(struct foo *dst, struct foo *src)
    memcpy(dst, src, sizeof(struct foo));

    then it will generate the same code. So I cannot use copy_foo() to transfer a struct foo either out of, or into a packet buffer.

    In other words, memcpy() does _not_ save you from alignment issues. If you cast from char* or void* to something else, you better be damn sure the alignment is correct because gcc will assume it is.

    -- Pierre Ossman

    Linux kernel, MMC maintainer
    PulseAudio, core developer
    rdesktop, core developer
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      Last update: 2007-11-24 17:21    [W:0.023 / U:4.080 seconds]
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