Function Interposition in Linux


 
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# 1  
Function Interposition in Linux

Hello,

Please I try to apply the mechanism Interposition Function in Linux on the function write (write a socket) as follows:
Code:
# include <sys/types.h>
# include <sys/socket.h>
# include <netinet/in.h>
# include <arpa/inet.h>
# include <netdb.h>
# include <stdio.h>
# include <string.h>
int write (int n, char buffer [32], int sb)
{

int nb = write (so, "Ceci est un exemple de socket!!!", 32);
}

I also have the server code and client code, when the client connects to the server, the server sends a message to the client but when running I got the following error:

Erreur de segmentation
Have you an idea please ?

Thank you so much
# 2  
write is a system call, and you have write as a local function name. At best this becomes a never-ending recursive call.

Call your function mywrite() or something else, do not name your functions the same thing as syscalls, -read write open and so on.

Interposition is acheived by writing a shared library module (call it mylibrary.so as an example), compling it, then creating a special environment variable, LD_PRELOAD=mylibrary.so, and then running your code. But you still cannot do what your code shows.
# 3  
Quote:
Interposition is acheived by writing a shared library module (call it mylibrary.so as an example), compling it, then creating a special environment variable, LD_PRELOAD=mylibrary.so, and then running your code.
--> i have done all what you say , i have modified the code like that:
Quote:
size_t write(int fd, const void *buf, size_t count)
{
static size_t (*write_func)(int, const void *, size_t) = NULL;

/* get reference to original (libc provided) write */
if (!write_func)
{
write_func = (size_t(*)(int, const void *, size_t)) dlsym(RTLD_NEXT, "write");
}
return write_func(fd, buffer, sizeof (buffer));
}
I have a question please :how canget the buffer "const void *buf" because I want to change it before sending ?

Thank you so much for help
 

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