Understanding the Linux Kernel

9.2. Sending a Signal

When a signal is sent to a process, either from the kernel or from another process, the kernel delivers it by invoking the send_sig_info( ), send_sig( ), force_sig( ), or force_sig_info( ) functions. These accomplish the first phase of signal handling described earlier in Section 9.1: updating the process descriptor as needed. They do not directly perform the second phase of receiving the signal but, depending on the type of signal and the state of the process, may wake up the process and force it to receive the signal.

9.2.1. The send_sig_info( ) and send_sig( ) Functions

The send_sig_info( ) function acts on three parameters:

sig

The signal number.

info

Either the address of a siginfo_t table associated with real-time signals or one of two special values: means that the signal has been sent by a User Mode process, while 1 means that it has been sent by the kernel. The siginfo_t data structure has information that must be passed to the process receiving the real-time signal, such as the PID of the sender process and the UID of its owner.

t

A pointer to the descriptor of the destination process.

The send_sig_info( ) function starts by checking whether the parameters are correct:

if (sig < 0 || sig > 64) 
    return -EINVAL;

The function checks then if the signal is being sent by a User Mode process. This occurs when info is equal to or when the si_code field of the siginfo_t table is negative or zero (the positive values of this field are reserved to identify ...