Re: Need clarity on structure alignment

Keith Thompson wrote:
"Tony" <tony@xxxxxx> writes:
"Keith Thompson" <kst-u@xxxxxxx> wrote in message news:lnk58g7d68.fsf@xxxxxxxxxxxxxxxxxx
[...]
Alignment requirements for different types vary depending on the
underlying hardware, and on what the compiler chooses to enforce.
I remember seeing something that left me with the impression that C (C++?) guarantees that a struct will be aligned on a pointer-size boundary (using the std mem mgr). (?) Something about 4-byte struct alignment (on a 32-bit machine no doubt) sticks in my mind.

You remember incorrectly. There's no rule in C (or C++) about the
relationship between the alignment requirements for structs and
those for pointers. You may have seen something about a specific
implementation.

The phrase "using the std mem mgr" suggests he may have
been thinking of malloc(), in which case he's sort of right:
If he uses malloc() to obtain memory for a struct, the memory
will in fact be properly aligned for storing a pointer. (Also
for storing a double, a union, a va_list, a time_t, and a
partridge in a pear tree, of course: malloc() memory is properly
aligned for *any* kind of data.)

--
Eric Sosman
esosman@xxxxxxxxxxxxxxxxxxxx
.

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