Large file support

Question

Could someone please explain, what exactly this O_LARGEFILE option does to support opening of large files. And can there be any side effects of compiling with -D_FILE_OFFSET_BITS=64 flag. In other words, when compiled with this option do we have to make sure something.

possible duplicate of [Is O_LARGEFILE needed just to write a large file?](http://stackoverflow.com/questions/2888425/is-o-largefile-needed-just-to-write-a-large-file) — Cédric Julien, Jan 02 '12 at 14:09

score 1 · Answer 1 · edited May 08 '17 at 15:52

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From man 2 open:

   O_LARGEFILE
          (LFS)  Allow  files  whose  sizes  cannot  be  represented  in  an  off_t (but can be represented in an off64_t) to be opened.  The _LARGE‐
          FILE64_SOURCE macro must be defined in order to obtain this definition.  Setting the _FILE_OFFSET_BITS feature test  macro  to  64  (rather
          than   using  O_LARGEFILE)  is  the  preferred  method  of  obtaining  method  of  accessing  large  files  on  32-bit  systems  (see  fea‐
          ture_test_macros(7)).

Edit: (ie. RTM :P)

edited May 08 '17 at 15:52

Funk Forty Niner

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answered Jan 02 '12 at 14:10

Ricardo Cárdenes

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The manpage usually doesn't point out the reason directly, so sometimes people like OP and me may want some confirmation from others, ensuring that we get it right. – Rick Oct 26 '21 at 06:53

score 1 · Accepted Answer · answered Jan 02 '12 at 14:11

Use _FILE_OFFSET_BITS in preference to O_LARGEFILE. These are used on 32 bit systems to allow opening files so large that they exceed the range of a 32bit file pointer.

No, you don't have to do anything special. If you are on 64bit Linux it makes no difference anyway.

Large file support

2 Answers2