Hello,

        I would like to get advices on using Shared Library to achieve Shared
Memory between processes. For example, the sharing of data structures
inside the shared library by processes.

        Thanks a lot for your advice.

        Fred Xia

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Thanks.

Fred

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And yes, gcc can be used to compile position independent code that can
be built into a shared library.  Use gcc -fpic to do that.  I think
the way to actually construct the shared library from position
independent object files is system specific.  On SunOS it can be done
like this:

ld -o libfoo.so.X.Y -assert pure-text <pic .o files>

X is the major and Y the minor version number for the shared library.
The version numbers are checked by the dynamic loader before running a
program.  When you upgrade a shared library and only the minor version
changes, your programs still run, but you get a warning message.  When
the major version number of your library changes, it means an
incompatible change has occured and the loader refuses to run your
program.  See man ld.so for details.
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Only the program text of shared libraries is actually shared. Each process
gets it's own copy of data areas (usually as copy-on-write pages). To share
data look at the mmap calls (man 2 mmap) or shared memory (man 2 shmop should
get you started on the manual pages).

Actually constructing the shared library is normally the job of the linker and
is somewhat system dependant so look at the man pages for ld, however on a Sun
running SunOS 4 you just say

   ld -o foo.so.x.y o1.o o2.o o3.o ....

where x and y are the major and minor version numbers of the libraries. The
individual objects must be compiled as "position independant" code. Later
versions of gcc can generate PIC (use the -fpic or -fPIC options) so it should
be possible.

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: Only the program text of shared libraries is actually shared. Each process
: gets it's own copy of data areas (usually as copy-on-write pages). To share
: data look at the mmap calls (man 2 mmap) or shared memory (man 2 shmop should
: get you started on the manual pages).

This I understand. However, I was under the impression that you didn't
necessarily need to build your exe with shareable libraries.  If for example
you created a file xyz.exe and multiple users were using this, is the text
area shared?  In some cases, (ie copy on write) would therefore the data
areas also be shared.

Thanks in advance?

Jon

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