lib.cpp

#include <stdlib.h>
#include <stdio.h>

struct Global {
  Global() {
    printf("a %s %s\n", getenv("HOME"), getenv("BLAH"));
    setenv("BLAH", "blah", 1);
    printf("b %s %s\n", getenv("HOME"), getenv("BLAH"));
  }
} global;

extern "C" int close(int) {
  printf("close %s %s\n", getenv("HOME"), getenv("BLAH"));
  return 0;
}

output

$ g++ lib.cpp -shared -fPIC -o libtest.so
$ LD_PRELOAD=$(pwd)/libtest.so bash -c "echo hi"
a /home/baldurk (null)
b (null) blah
close (null) blah
hi

$ LD_PRELOAD=$(pwd)/libtest.so python -c "print('hi')"
a /home/baldurk (null)
b /home/baldurk blah
close /home/baldurk blah
# more identical close prints
close /home/baldurk blah
hi