[pacman-dev] macros needed when using libalpm

[Dan McGee]

On Wed, Feb 15, 2012 at 7:30 AM, jjacky <i.am.jack.mail at gmail.com> wrote:
> Hi,
>
> Just a quick question, it might be pretty obvious but I'm still
> relatively new at this, so excuse my "noobness."
>
> While working on a little app using libalpm, I wanted to use the
> download callback. But I had a problem, because the total size would
> always be zero. I didn't understand why it didn't work, especially since
> it did for pacman, which seemed to do exactly the same as I did.
>
> After investigation, I figured out that this was because I needed to use
> a macro when compiling, namely:
> #define _FILE_OFFSET_BITS 64
>
> Apparently one of the things this does is turn off_t (type of both size
> parameters of the dl callback, among other things) into off64_t. I'm
> guessing that means one more byte to store the data, and therefore
> explains why with my "regular" off_t I didn't get the correct values.
>
> Thing is, I'm not sure how I was supposed to know that using this macro
> was required of me in order to get things to work properly, so my
> questions are:
>
> - how was I supposed to know? :) As in, is there any indication or
> something about this written somewhere that I missed? I did look at the
> README and such, but couldn't find anything. (and if not, shouldn't
> there be, maybe?)
> Or is this just usual/common knowledge or something?
This is
1) only necessary on 32-bit platforms
2) an unfortunate hack
3) widely known, for the most part- 99% of apps are compiled with this
because 32-bits is not enough to have anything > 2GiB signed, or >4
GiB unsigned.

>
> - any chance there is/are other such things I need to use for things to
> work properly? I'm guessing not, but just in case...
Don't think so. Your best bet is to look at the pacman side of things
as a reference, in addition to the configure.ac file and the generated
config.h.

-Dan