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0

I want to implement a wrapper over the open system call, and I'm using the LD_PRELOAD trick to call my new open.

The problem is that open expects a variable number of arguments, and I cannot figure out how to call open with the same set of arguments, as open cannot take a va_list pointer as an argument(or I do not know of any such function).

How could I achieve this?

code so far:

#define _GNU_SOURCE
#include <stdio.h>
#include <dlfcn.h>

typedef int (*open_func_t)(const char*, int, ...)

int open(const char *pathname, int flags, ...)
 // some custom code

 // what args should I supply to dlsym?
 return ((open_func_t)dlsym(RTLD_NEXT, "open"))(args);

c system-calls variadic

share|improve this question

asked Mar 24 at 8:41

Tanmay SachanTanmay Sachan

714

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Then why not make your open wrapper accept va_list*?
  
  – JiaHao Xu
  Mar 24 at 8:53
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  What exactly do you mean by that? Shouldn't my wrapper expect the exact same args as the call itself?
  
  – Tanmay Sachan
  Mar 24 at 9:13
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Process flags like open() and then call your function with 2 or 3 parameters. open
  
  – chux
  Mar 24 at 9:17
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Yes, that's probably what I'll end up doing if nothing else works.
  
  – Tanmay Sachan
  Mar 24 at 9:22
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @Tanmay Sachan The code you show here that use dlsym, does jt call syscall open or your wrapper? I am confused
  
  – JiaHao Xu
  Mar 24 at 9:35
  

  
  
  
  

 | 
show 3 more comments

0

I want to implement a wrapper over the open system call, and I'm using the LD_PRELOAD trick to call my new open.

The problem is that open expects a variable number of arguments, and I cannot figure out how to call open with the same set of arguments, as open cannot take a va_list pointer as an argument(or I do not know of any such function).

How could I achieve this?

code so far:

#define _GNU_SOURCE
#include <stdio.h>
#include <dlfcn.h>

typedef int (*open_func_t)(const char*, int, ...)

int open(const char *pathname, int flags, ...)
 // some custom code

 // what args should I supply to dlsym?
 return ((open_func_t)dlsym(RTLD_NEXT, "open"))(args);

c system-calls variadic

share|improve this question

asked Mar 24 at 8:41

Tanmay SachanTanmay Sachan

714

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Then why not make your open wrapper accept va_list*?
  
  – JiaHao Xu
  Mar 24 at 8:53
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  What exactly do you mean by that? Shouldn't my wrapper expect the exact same args as the call itself?
  
  – Tanmay Sachan
  Mar 24 at 9:13
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Process flags like open() and then call your function with 2 or 3 parameters. open
  
  – chux
  Mar 24 at 9:17
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Yes, that's probably what I'll end up doing if nothing else works.
  
  – Tanmay Sachan
  Mar 24 at 9:22
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @Tanmay Sachan The code you show here that use dlsym, does jt call syscall open or your wrapper? I am confused
  
  – JiaHao Xu
  Mar 24 at 9:35
  

  
  
  
  

 | 
show 3 more comments

I want to implement a wrapper over the open system call, and I'm using the LD_PRELOAD trick to call my new open.

The problem is that open expects a variable number of arguments, and I cannot figure out how to call open with the same set of arguments, as open cannot take a va_list pointer as an argument(or I do not know of any such function).

How could I achieve this?

code so far:

#define _GNU_SOURCE
#include <stdio.h>
#include <dlfcn.h>

typedef int (*open_func_t)(const char*, int, ...)

int open(const char *pathname, int flags, ...)
 // some custom code

 // what args should I supply to dlsym?
 return ((open_func_t)dlsym(RTLD_NEXT, "open"))(args);

c system-calls variadic

share|improve this question

asked Mar 24 at 8:41

Tanmay SachanTanmay Sachan

714

#define _GNU_SOURCE
#include <stdio.h>
#include <dlfcn.h>

typedef int (*open_func_t)(const char*, int, ...)

int open(const char *pathname, int flags, ...)
 // some custom code

 // what args should I supply to dlsym?
 return ((open_func_t)dlsym(RTLD_NEXT, "open"))(args);

share|improve this question

asked Mar 24 at 8:41

Tanmay SachanTanmay Sachan

714

share|improve this question

asked Mar 24 at 8:41

Tanmay SachanTanmay Sachan

714

asked Mar 24 at 8:41

Tanmay SachanTanmay Sachan

714

asked Mar 24 at 8:41

Tanmay SachanTanmay Sachan

714