[lug] OFF: sizeof structs in gcc?

[Tkil]

>>>>> "Tkil" == Tkil  <tkil at scrye.com> writes:

Tkil> it usually prefers the fastest allocation for the given
Tkil> platform, which is usually at least 4 bytes (e.g. doubles might
Tkil> be faster on 8-byte boundaries)

i apologize for that paragraph.  i was smoking something bad,
obviously.  there are some grains of truth, but i managed to confuse
things...

for some processors, memory values must be aligned on boundaries that
are a multiple of the data size -- so a 4-byte int would be on a
4-byte boundary, an 8-byte double or long long on an 8-byte boundary,
etc.  so, doing any work with a non-aligned value implies doing two
reads, reconstructing the value in a register, manipulating it, then
doing two writes.

so, when i said "fastest" in my original paragraph, i was referring to
execution speed.  using the "packed" attribute (or pragma, however you
get there) is usually a space/time tradeoff, although another common
use is to make c structures map directly onto a hardware or media data
layout.

for some discussion on this topic, please see the C FAQ, questions
2.12 through 2.15 inclusive:

   http://www.eskimo.com/~scs/C-faq/q2.12.html
   http://www.eskimo.com/~scs/C-faq/q2.13.html
   http://www.eskimo.com/~scs/C-faq/q2.14.html
   http://www.eskimo.com/~scs/C-faq/q2.15.html

t.