The meaning of signal types and special signals under Linux

you can see a list of supported signals from Linux:

$ kill-l1) sighup        2) sigint        3) sigquit       4) Sigill 5) sigtrap       6) SIGABRT        7) sigbus        8) SIGFPE 9) sigkill       sigusr1     ) sigsegv      12) SIGUSR213) (sigpipe     ) sigalrm     ) sigterm      ) sigstop      20 SIGCHLD18) sigcont      ) (sigtstp     ) SIGTTIN22) sigttou     ) sigurg        sigxcpu     ) SIGXFSZ26) sigvtalrm    27) sigprof      sigwinch    ) SIGIO30) SIGRTMIN  (sigpwr      ) sigsys      )    ) sigrtmin+136) (sigrtmin+2  ) sigrtmin+3  ) sigrtmin+4   39) sigrtmin+540) (sigrtmin+6  ) sigrtmin+7  ) sigrtmin+8  ) SIGRTMIN+944) SIGRTMIN+ SIGRTMIN+11) (sigrtmin+12) sigrtmin+1348) sigrtmin+14) sigrtmin+15) SIGRTMAX-14) SIGRTMAX-1352 MAX-12) (SIGRTMAX-11) SIGRTMAX-10) SIGRTMAX-956) sigrtmax-8   (sigrtmax-7  ) SIGRTMAX-6   ) SIGRTMAX-560) sigrtmax-4   (sigrtmax-3  ) sigrtmax-2   63) SIGRTMAX-164) Sigrtmax

In the list, the signals numbered 1 ~ 31 are signals that are supported by traditional UNIX, are unreliable signals (not real-time), and the signals numbered 32 to 63 are later expanded, called reliable signals (real-time signals). The difference between unreliable and reliable signals is that the former does not support queueing and may cause signal loss, while the latter does not.

Below we discuss a signal with a number less than sigrtmin.

1) SIGHUP

This signal is issued at the end of the user terminal connection (normal or abnormal), usually at the end of the control process of the terminal to notify the same session of the various jobs, when they are no longer associated with the control terminal.

When you log in to Linux, the system is assigned to a terminal (Session) of the logged-on user. All programs running at this terminal, including the foreground process group and the background process group, usually belong to this session. When the user exits the Linux login, the foreground process group and the background will receive a sighup signal to the terminal output process. The default action for this signal is to terminate the process, so the foreground process group and the process that has terminal output in the background will be aborted. However, this signal can be captured, such as wget can capture the sighup signal, and ignore it, so even if you quit the Linux login, wget can continue to download.

In addition, for daemons with terminal disengagement, this signal is used to notify it to reread the configuration file.

2) SIGINT

A program termination (interrupt) signal, issued when the user types the intr character (usually ctrl-c), to inform the foreground process group that the process is terminated.

3) Sigquit

Similar to SIGINT, but controlled by the quit character (usually ctrl-\). The process generates a core file when it receives a sigquit exit, similar to a program error signal in this sense.

4) Sigill

An illegal instruction was executed. This is usually due to an error in the executable file itself or an attempt to execute a data segment. This signal can also be generated when a stack overflows.

5) SIGTRAP

Generated by a breakpoint instruction or other trap instruction. Used by debugger.

6) SIGABRT

Invokes the signal generated by the abort function.

7) Sigbus

Illegal address, including memory address alignment (alignment) error. For example, a four-word integer is accessed, but its address is not a multiple of 4. It differs from SIGSEGV in that the latter is triggered by illegal access to a legitimate storage address (such as Access does not belong to its own storage space or read-only storage space).

8) SIGFPE

Emitted when a fatal arithmetic operation error occurs. This includes not only floating-point arithmetic errors, but also all other arithmetic errors such as overflow and divisor 0.

9) SIGKILL

Used to immediately end the run of the program. This signal cannot be blocked, processed, or ignored. If an administrator discovers that a process is not terminating, try sending this signal.

) SIGUSR1

Leave it to the user

One) SIGSEGV

An attempt was made to access memory that is not assigned to itself or to write data to a memory address that does not have write permissions.

SIGUSR2)

Leave it to the user

Sigpipe)

Pipe rupture. This signal is usually generated between interprocess communication, such as two processes with FIFO (pipeline) communication, the read pipeline is not opened or terminated unexpectedly to the pipeline to write, the writing process will receive sigpipe signal. In addition, two processes with the socket communication, the write process when the socket is written, the read process has been terminated.

SIGALRM)

A clock timing signal that calculates the actual time or clock time. The alarm function uses this signal.

SIGTERM)

The end of program (terminate) signal, unlike Sigkill, is that the signal can be blocked and processed. Typically used to require the program to exit gracefully, the shell command kill generates this signal by default. If the process does not stop, we will try to Sigkill.

SIGCHLD)

The parent process receives this signal at the end of the child process.

If the parent process does not process the signal and does not wait for the (wait) child process, although the child process terminates, it also occupies the table entry in the kernel process table, when the child process is called a zombie process. In this case we should avoid (the parent process either ignores the sigchild signal, or catches it, or wait for its derived child process, or the parent process terminates first, and the termination of the child process is automatically taken over by the Init process).

) Sigcont

Let a stop (stopped) process continue execution. This signal cannot be blocked. You can use a handler to get the program to do certain work when the stopped state changes to continue. For example, to re-display the prompt

SIGSTOP)

Stop (stopped) the execution of the process. Notice the difference between it and terminate and interrupt: The process is not over yet, just pause execution. This signal cannot be blocked, processed or ignored.

SIGTSTP)

Stops the process from running, but the signal can be processed and ignored. This signal is emitted when the user types Susp characters (usually ctrl-z)

) Sigttin

When a background job wants to read data from the user terminal, all processes in the job receive a sigttin signal. By default, these processes stop executing.

Sigttou)

Similar to Sigttin, but received when writing a terminal (or modifying terminal mode).

Sigurg)

When there is "emergency" data or Out-of-band data arrives at the socket.

) sigxcpu

CPU time resource limit exceeded. This restriction can be read/changed by Getrlimit/setrlimit.

SIGXFSZ)

When the process attempts to enlarge the file so that it exceeds the file size resource limit.

SIGVTALRM)

Virtual clock signal. is similar to SIGALRM, but calculates the CPU time that is consumed by the process.

SIGPROF)

Similar to SIGALRM/SIGVTALRM, but includes the CPU time used by the process and the time of the system call.

Sigwinch)

emitted when the window size changes.

) SIGIO

The file descriptor is ready to start the input/output operation.

SIGPWR)

Power failure

Sigsys)

An illegal system call.

In the signals listed above, the program can not capture, block or ignore the signal is: sigkill,sigstop

Signals that cannot be restored to the default action are: Sigill,sigtrap

The default causes the process to abort the signal is: Sigabrt,sigbus,sigfpe,sigill,sigiot,sigquit,sigsegv,sigtrap,sigxcpu,sigxfsz

The signal that causes the process to exit by default is: SIGALRM,SIGHUP,SIGINT,SIGKILL,SIGPIPE,SIGPOLL,SIGPROF,SIGSYS,SIGTERM,SIGUSR1,SIGUSR2,SIGVTALRM

The signal that causes the process to stop by default is: Sigstop,sigtstp,sigttin,sigttou

The default process ignores signals such as: Sigchld,sigpwr,sigurg,sigwinch

Additionally, Sigio is exited in SVR4, which is ignored in 4.3BSD; Sigcont is continued when the process hangs, otherwise it is ignored and cannot be blocked

The type of signal and the meaning of special signals under Linux