[TriLUG] shared libraries aren't shared

Joseph Mack NA3T jmack at wm7d.net
Wed Nov 11 14:55:27 EST 2009

I'm noodling around with shared libraries. AFAIK, if two 
executables are running simultaneously on a machine and they 
both use the same shared library, then there should be only 
one copy of the shared library in memory. In that case I 
should be able to show a race condition with a global 
variable in the shared library.

Here's my main()


void print_int(int);

int main() {
         int i = 10;
         sleep (5);
         return (0);

Here's the source code for the shared library


#include <stdio.h>

int j = 0;	//global variable

void print_int(int x) {
         printf ("%d \n", x);
         ++j;	//increment global variable
         printf ("j=%d \n", j);

Each time print_int() is called, it will increment j.

Here's my compile

gcc -c -Wall -fPIC  print_int.c -o print_int.o
gcc -shared -Wl,-soname,libmy_function.so.2.0 -o libmy_function.so.2.0.1 print_int.o
cp -pauv libmy_function.so.2.0.1 /usr/lib

gcc -c -Wall -fPIC  main.c -o main.o
gcc -Wall -o my_function.dynamic main.o libmy_function.so.2.0.1
cp -pauv my_function.dynamic /usr/bin/my_function

ldd shows that the executable is dynamically linked to the 
shared library.

I would have thought that I'd at least get a warning about 
the global variable in a shared library, but gcc is fine 
with it.

Here's a single copy of the code running, showing the global 
variable being incremented

my_function# my_function

Now I run two copies of the executable, in two windows. With 
10secs of sleep in main(), there will be time to start both 
processes and have them running at the same time. I would 
expect one of the outputs to show j=3, but both processes 
behave as if they have their own copy of the shared library.

Any ideas? Why doesn't the shared library behave as if it's 
shared? How can I tell if there is only one copy of the 
shared library in memory?

Thanks Joe

Joseph Mack NA3T EME(B,D), FM05lw North Carolina
jmack (at) wm7d (dot) net - azimuthal equidistant map
generator at http://www.wm7d.net/azproj.shtml
Homepage http://www.austintek.com/ It's GNU/Linux!

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