GPGME and Debian

Marcus Brinkmann marcus.brinkmann at
Mon Apr 11 20:18:30 CEST 2005

>   2. I compiled t-encrypt-sign.c manually and linked it against my
>      systems library running the command
>      'gcc -lgpgme  t-encrypt-sign.c -o t-encrypt-sign'

You need -D_FILE_OFFSET_BITS=64.  I past the relevant section of the
manual below.

This is a FQ, we probably should have a FAQ.


Largefile Support (LFS)

GPGME is compiled with largefile support by default, if it is available
on the system.  This means that GPGME supports files larger than two
gigabyte in size, if the underlying operating system can.  On some
systems, largefile support is already the default.  On such systems,
nothing special is required.  However, some systems provide only
support for files up to two gigabyte in size by default.  Support for
larger file sizes has to be specifically enabled.

   To make a difficult situation even more complex, such systems provide
two different types of largefile support.  You can either get all
relevant functions replaced with alternatives that are largefile
capable, or you can get new functions and data types for largefile
support added.  Those new functions have the same name as their
smallfile counterparts, but with a suffix of 64.

   An example: The data type `off_t' is 32 bit wide on GNU/Linux PC
systems.  To address offsets in large files, you can either enable
largefile support add-on.  Then a new data type `off64_t' is provided,
which is 64 bit wide.  Or you can replace the existing `off_t' data
type with its 64 bit wide counterpart.  All occurences of `off_t' are
then automagically replaced.

   As if matters were not complex enough, there are also two different
types of file descriptors in such systems.  This is important because
if file descriptors are exchanged between programs that use a different
maximum file size, certain errors must be produced on some file
descriptors to prevent subtle overflow bugs from occuring.

   As you can see, supporting two different maximum file sizes at the
same time is not at all an easy task.  However, the maximum file size
does matter for GPGME, because some data types it uses in its
interfaces are affected by that.  For example, the `off_t' data type is
used in the `gpgme_data_seek' function, to match its POSIX counterpart.
This affects the call-frame of the function, and thus the ABI of the
library.  Furthermore, file descriptors can be exchanged between GPGME
and the application.

   For you as the user of the library, this means that your program must
be compiled in the same file size mode as the library.  Luckily, there
is absolutely no valid reason for new programs to not enable largefile
support by default and just use that.  The compatibility modes (small
file sizes or dual mode) can be considered an historic artefact, only
useful to allow for a transitional period.

   GPGME is compiled using largefile support by default.  This means
that your application must do the same, at least as far as it is
relevant for using the `gpgme.h' header file.  All types in this header
files refer to their largefile counterparts, if they are different from
any default types on the system.

   You can enable largefile support, if it is different from the default
on the system the application is compiled on, by using the Autoconf
macro `AC_SYS_LARGEFILE'.  If you do this, then you don't need to worry
about anything else: It will just work.  In this case you might also
want to use `AC_FUNC_FSEEKO' to take advantage of some new interfaces,
and `AC_TYPE_OFF_T' (just in case).

   If you do not use Autoconf, you can define the preprocessor symbol
`_FILE_OFFSET_BITS' to 64 _before_ including any header files, for
example by specifying the option `-D_FILE_OFFSET_BITS=64' on the
compiler command line.  You will also want to define the preprocessor
symbol `LARGEFILE_SOURCE' to 1 in this case, to take advantage of some
new interfaces.

   If you do not want to do either of the above, you probably know
enough about the issue to invent your own solution.  Just keep in mind
that the GPGME header file expects that largefile support is enabled,
if it is available.  In particular, we do not support dual mode

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