C++ :: Stubbing Standard Function - Ambiguous Call

I need to make a stub for some standard library function, but the compiler gets confused about which function I want to call in my code. I thought that putting the stubbed function together with its caller into an anonymous namespace will give higher precedence to the stub, but it didn't.

Here is an example code:

#include <cstdlib>
#include <cstdio>
#include <cstring>
#include <signal.h>
namespace{
int sigaction(int signum, const struct sigaction * act, struct sigaction * oldact)

[Code] ....

Result:
main.cpp: In function ‘void {anonymous}::test()’:
main.cpp:23:47: error: call of overloaded ‘sigaction(int, sigaction*, NULL)’ is ambiguous
main.cpp:23:47: note: candidates are:
main.cpp:8:5: note: int {anonymous}::sigaction(int, const sigaction*, sigaction*)
In file included from main.cpp:4:0:
/usr/include/signal.h:266:12: note: int sigaction(int, const sigaction*, sigaction*)

It possible to force the compiler to use my stubbed version of sigaction() and if yes, how to do that? I am not allowed to modify test() function.