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This is PURE_LIBC: an overlay library for glibc that allows system call capturing.

(C) 2006 Renzo Davoli University of Bologna (ITALY)
(C) 2006 Andrea Gasparini University of Bologna (ITALY)
 
This is FREE software: this work has been released under the GPLv2
license (see the file COPYING and the header note in the source files).

Pure_libc converts glibc from a libc+system interfacing library into a libc-only
library.
Pure_libc is not complete yet. Stdio has been implemented onto the
fopencookie call. 
Due to current limitations of fopencookie, freopen cannot be "purified".

The process can (re)define the variable
extern sfun _pure_syscall;

being sfun as follows:
typedef long int (*sfun)(long int __sysno, ...);

All the system call of the programs are converted into calls of the
_pure_syscall function.
_pure_syscall is defined by default to be the glibc syscall function, thus
_pure_libc has no effects (provided there are not bugs ;-) before _pure_syscall gets
assigned.

WARNING: 'syscall' call itself gets diverted to the _pure_syscall function, too.
The library defines the _pure_native_syscall function to access the
native syscalls (through the original glibc function).

The following test program prints the number of the system call before actually calling it (it is a 'cat' like stdin to stdout copy, when EOF is sent it prints
"hello world"):
--------------
#include <stdio.h>
#include <string.h>
#include <stdarg.h>
#include <sys/syscall.h>
#include <unistd.h>
#include <purelibc.h>

static char buf[128];
static long int mysc(long int sysno, ...){
	va_list ap;
	long int a1,a2,a3,a4,a5,a6;
	va_start (ap, sysno);
	snprintf(buf,128,"SC=%d\n",sysno);
	_pure_native_syscall(__NR_write,2,buf,strlen(buf));
	a1=va_arg(ap,long int);
	a2=va_arg(ap,long int);
	a3=va_arg(ap,long int);
	a4=va_arg(ap,long int);
	a5=va_arg(ap,long int);
	a6=va_arg(ap,long int);
	va_end(ap);
	_pure_native_syscall(sysno,a1,a2,a3,a4,a5,a6);
}

main() {
	int c;
	_pure_syscall=mysc;
	while ((c=getchar()) != EOF)
		putchar(c);
	printf("hello world\n");
}
--------------


There is also a var to redefine the socket calls:
extern sfun _pure_socketcall;

when not defined all the socket calls are managed by the __NR_socketcall
system call (diverted or not depending on _pure_syscall).

when defined all the socket calls are into _pure_socketcall function calls
(no more __NR_socketcall system calls are subsequently invoked).