LD_PRELOAD with boost fileystem library - c++

Question

I am trying to intercept open system call in Linux. It works fine with other libraries but doesn't wotk with boost libboost_fileystem. Here is my code (stripped down for readability).
#include <boost/filesystem/fstream.hpp>
#include <sys/stat.h>
#include <unistd.h>
#include <fcntl.h>
#include <dlfcn.h>
#include <stdarg.h>
#include <stdio.h>
using namespace std;
using namespace boost::filesystem;
typedef int (*open_func_type)(const char * pathname, int flags, ...);
int open(const char *path, int flags, ...)
{
va_list arg;
mode_t mode = 0;
if (flags & O_CREAT)
{
va_start(arg, flags);
mode = va_arg(arg, mode_t);
va_end(arg);
}
//some stuff here
open_func_type open_func = (open_func_type) dlsym(RTLD_NEXT, "open");
return open_func(path, flags, mode);
}
int main()
{
boost::filesystem::fstream build_path;
build_path.open("/tmp/test.txt", ios::in);
//other stuff
return 0;
}
I stepped though the code using gdb, my open implementation doesn't get called. But doing strace shows the open system call being called. If I call other library functions that call open, I see my implementation getting called. Anything that I am doing wrong here? I am dynamically linking with boost libraries.

Answer 1

I gave sometime to figure it out and found that internally boost calls std::basic_filebuf open which in turn calls fopen. fopen seems to call open system call in the kernel without calling open library call (it would be nice if somebody could point it out why). I intercepted fopen call and this works fine now. If large file support is used, fopen64 also needs to be implemented.