Linux

I am developing a simple client-server (TCP) application using QT. The server side is to run on CentOS. I am developing both the server and client programs using Ubuntu.

I am now attempting to deploy an experimental version of the server application on the CentOS server, but I am not yet familiar with all the vagaries of Linux and have run into some difficulties.

I started by copying the executable to the server, attempting to run it, and then copying the Qt libraries it reported as missing, one at a time. So far so good.

I then received a series of messages of the form:

#.../lib/libc.so.6: version `GLIBC_2.9' not found (required by /usr/lib/libQtGui.so.4)

On investigating I found that libc.so.6 was linked to libc-2.5.so on the server, but to libc-2.12.1.so on the development sysem. I duly copied libc-2.12.1.so to the server, updated libc.so.6 to point to it and attempted to run the executable again.

This resulted in the error message:

#...relocation error: /lib/libglib-2.0.so.0: symbol strncmp, version GLIBC_2.0 not defined in file libc.so.6 with link time reference

I quickly set the link back to what it was, in case anything else was broken by the change. No harm seems to have been done!

I would very much appreciate some advice on the correct way to do this!

Many thanks.

Your development system has much newer software than your server, too new to still be compatible it seems. Replacing your current libc is a very bad idea -- I'm glad you didn't lock yourself out of your server, we get a new threads about "I messed up my libc now nothing works what do I do" with fair regularity.

I'd make the executable demand very specific versions and paths -- i.e. link in /usr/local/lib/libc-2.12.1.so instead of libc.so.6. Specific versions of any other libraries it complains about, too. Then you can copy them over to your server without overwriting existing libraries. I suggest /usr/local/lib instead of /lib/ or /usr/lib/ because that lets you track very carefully what things you've copied over by hand instead of dumping them in with the rest of the pile.

---------- Post updated at 11:51 AM ---------- Previous update was at 10:14 AM ----------

If you can add -Xlinker -R /usr/local/lib to your code when you compile it, that will force it to look in /usr/local/lib/ first, and fall back on /lib/ etc. if it doesn't find things available in there.

Code:

$ echo "int main() { return 42; }" > 42.c
$ gcc 42.c -o 42
$ ldd 42
        linux-gate.so.1 =>  (0xb7888000)
        libc.so.6 =>  /lib/libc.so.6 (0xb762b000)
        /lib/ld-linux.so.2 (0xb7889000)
$ cp /lib/libc.so.6 ./
$ gcc -Xlinker -rpath=$PWD 42.c -o 42
$ ldd ./42
        linux-gate.so.1 =>  (0xb78bb000)
        libc.so.6 => /home/tyler/code/c/1shot/libc.so.6 (0xb7771000)
        /lib/ld-linux.so.2 (0xb78bc000)
$ rm libc.so.6
$ ldd ./42
        linux-gate.so.1 =>  (0xb777a000)
        libc.so.6 => /lib/libc.so.6 (0xb7627000)
        /lib/ld-linux.so.2 (0xb777b000)

Many thanks for a full and helpful reply! I must admit I was a bit nervous about changing the libc link at the time - and relieved when the system survived in good order.