Understanding the Linux Kernel, Second Edition

Generating a Signal

When a signal is sent to a process, either from the kernel or from another process, the kernel generates 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 10.1, updating the process descriptor as needed. They do not directly perform the second phase of delivering 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.

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 or one of two special values. 0 means that the signal has been sent by a User Mode process, while 1 means that it has been sent by the kernel.

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 then checks if the signal is being sent by a User Mode process. This occurs when info is equal to 0 or when the si_code field of the siginfo_t table is negative or 0 (positive values of this field mean that the signal was sent by some kernel function):

if ((!info || ((unsigned long)info != 1 && (info->si_code <=0))) && ((sig != SIGCONT) || (current->session != t->session)) && (current->euid ^ t->suid) && (current->euid ^ t->uid) && (current->uid ...