The question of the Linux kernel does not understand the place

1. In the Linux 0.11\linux\kernel\exit.c file, either the Send_sig () function or the kill_session () function, the place where the signal is sent is direct (*p)->signal |= 1 <& Lt    (SIGHUP-1); Sends a hang-up process signal.

What I do not understand is: why the long signal directly in the TASK_STRUCT structure, change the variable bitmap, even if the signal is sent? After sending the signal, there must be a response, how to respond?

2.Linux system call is originally called in the user space, how to enter into the kernel space to execute system calls?

Answer: http://www.ibm.com/developerworks/cn/linux/l-system-calls/

The question of the Linux kernel does not understand the place