memset (was: Redundant memset in AIO read_events)

Etienne Lorrain
2003-07-10T09:49:40Z

Note that using memset() is better reserved to initialise variable-size
structures or buffers. Even if memset() is extremely optimised,
it is still not as fast as not doing anything.

read_events(...) {
struct io_event ent;
memset(&ent, 0, sizeof(ent));
while (...) {
aio_read_evt(ctx, &ent);
}
...
}

Should be written (when "ent" has to be cleared):
read_events(...) {
struct io_event ent = {};
while (...) {
aio_read_evt(ctx, &ent);
}
...
}

Just compare the code generated by (using GCC):
struct io_event ent;
memset(&ent, 0, sizeof(ent));
ent.data = 0;
if (ent.obj != 0) printf ("bad");

And:
struct io_event ent = {};
ent.data = 0;
if (ent.obj != 0) printf ("bad");

and that is even without speaking of complete variable elimination
when the structure is not used, unknown pointer alignement when
memset function is not inlined, or aliasing optimisation.

Etienne.

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