Signals
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Signals provide an asynchronous communication mechanism between processes in Linux. They are specified by numbers and can be sent and handled via functions like kill, signal, and sigaction. Common signals originate from the kernel on illegal operations or from other processes. Signal handlers are registered to handle signals and should perform minimal work. Upon exit, processes return an exit code indicating normal or abnormal termination.
Signals
- 1. Signals © 2010 Anil Kumar Pugalia <email@sarika-pugs.com> All Rights Reserved.
- 2. What to Expect? W's of a Signal Signals in Linux Signal Handlers Signal related Functions Program Exit Codes © 2010 Anil Kumar Pugalia <email@sarika-pugs.com> 2 All Rights Reserved.
- 3. Asynchronous Communication In Kernel Space Interrupts (Hardware to Software) Soft Irqs (Software to Software) What about User Space? Moreover, between User Space & Kernel Space Though only software to software requirement Do System Calls take care of that? Communication: Yes, but they are Synchronous Asynchronous Solution is Signals © 2010 Anil Kumar Pugalia <email@sarika-pugs.com> 3 All Rights Reserved.
- 4. What is a Signal? Asynchronous Trigger from Kernel Space or a Process to another Process Mechanisms for communicating with and manipulating processes in Linux Handled in User Space like Interrupts in Kernel Space Remains Pending on Masking. Once handled, Immediate processing without finishing the current function or even the current line of code. Possible options Ignore Execute the Default Handler Execute the User specified Handler © 2010 Anil Kumar Pugalia <email@sarika-pugs.com> 4 All Rights Reserved.
- 5. Signals in Linux Specified by a Number (currently 1-31) Also, referred by Human readable Names On Shell Type 'kill -l' Signal Operations: Using kill In Programming / C Header: /usr/include/bits/signum.h Signal Operations: Shall discuss further © 2010 Anil Kumar Pugalia <email@sarika-pugs.com> 5 All Rights Reserved.
- 6. Signals Originate from Kernel Space Typically on an illegal operation. Namely, SIGBUS (bus error), SIGSEGV (segmentation violation) SIGFPE (floating point exception) Others like SIGCHLD A Process Synchronously using kill system call © 2010 Anil Kumar Pugalia <email@sarika-pugs.com> 6 All Rights Reserved.
- 7. Signal Handler Registration Using signal(sig_no, handler) Non-portable except for SIG_DFL & SIG_IGN as handlers Using sigaction(sig_no, action, old_action) action & new_action are pointers to sigaction structures action contains the desired disposition old_action receives the previous disposition Their sa_handler field takes one of the three values SIG_DFL SIG_IGN A pointer to a signal handler function Signal Hanlder Prototype void signal_handler(int sig_no); Exceptions to Registration: SIGKILL, SIGSTOP © 2010 Anil Kumar Pugalia <email@sarika-pugs.com> 7 All Rights Reserved.
- 8. Sensitivity of Signal Handlers Time Sensitive like Interrupt Handlers Perform the minimum work necessary & Return control to the main program, Or Terminate the program Mostly, they just record the signal occurrence And the main program does the processing © 2010 Anil Kumar Pugalia <email@sarika-pugs.com> 8 All Rights Reserved.
- 9. Signal in a Signal Signal interrupting a signal handler Rare Occurrence but could be a Problem Difficult to diagnose & debug Assigning to globals can be dangerous Solutions Self is restricted by default Unless SA_NODEFER flag is used Disable other signals in signal handlers By setting sa_mask using sigsetops during registration Protect the global variables by using sig_atomic_t type © 2010 Anil Kumar Pugalia <email@sarika-pugs.com> 9 All Rights Reserved.
- 10. Signal related Functions raise Sends a signal to calling thread kill, killpg Sends a signal to a specified process or group of processes tkill, tgkill, pthread_kill Sends a signal to a specified thread sigqueue Sends a real-time signal with accompanying data to a specified process pause, sigsuspend Suspends the calling thread until delivery of a signal whose action is either to execute a signal handler or to terminate the process sigwaitinfo, sigtimedwait, sigwait, signalfd Accepts signals synchronously © 2010 Anil Kumar Pugalia <email@sarika-pugs.com> 10 All Rights Reserved.
- 11. Signal related Functions ... alarm(unsigned int secs); Schedules an alarm signal (SIGALRM) to be sent to the calling process after “secs” seconds Should not be mixed with setitimer / sleep abort(); Causes abnormal process termination by raising abort signal (SIGABRT) © 2010 Anil Kumar Pugalia <email@sarika-pugs.com> 11 All Rights Reserved.
- 12. Process Termination Revisited Normal: System Call 'exit' is invoked With zero – Success Exit With non-zero value – Error Exit Abnormal: A Signal has terminated it From Kernel Space for a Bug – Fatal Exit SIGBUS SIGSEGV SIGFPE From a Process – Kill Exit SIGINT (Ctrl + C) SIGTERM SIGABRT / abort() ... © 2010 Anil Kumar Pugalia <email@sarika-pugs.com> 12 All Rights Reserved.
- 13. Exit Codes Decoded 8-bit unsigned integer Range: 0-255 Normal Termination (using exit(value);) “value” should be always between 0 and 127 Exit code is “value” Abnormal Termination (by signal) Exit code is 128 + “signal” © 2010 Anil Kumar Pugalia <email@sarika-pugs.com> 13 All Rights Reserved.
- 14. What all have we learnt? W's of a Signal Signals in Linux Signal Handlers Signal related Functions Program Exit Codes © 2010 Anil Kumar Pugalia <email@sarika-pugs.com> 14 All Rights Reserved.
- 15. Any Queries? © 2010 Anil Kumar Pugalia <email@sarika-pugs.com> 15 All Rights Reserved.