suse man page for syscall_get_nr

SYSCALL_GET_NR(9)						   Machine State						 SYSCALL_GET_NR(9)

NAME
       syscall_get_nr - find what system call a task is executing

SYNOPSIS
       int syscall_get_nr(struct task_struct * task, struct pt_regs * regs);

ARGUMENTS
       task
	   task of interest, must be blocked

       regs

	   task_pt_regs of task

DESCRIPTION
       If task is executing a system call or is at system call tracing about to attempt one, returns the system call number. If task is not
       executing a system call, i.e. it's blocked inside the kernel for a fault or signal, returns -1.

       Note this returns int even on 64-bit machines. Only 32 bits of system call number can be meaningful. If the actual arch value is 64 bits,
       this truncates to 32 bits so 0xffffffff means -1.

       It's only valid to call this when task is known to be blocked.

Kernel Hackers Manual 2.6.					     July 2010							 SYSCALL_GET_NR(9)