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| :PROPERTIES: | ||
| :ID: c7a34ac9-3238-4d29-b1f9-5f96acb52a27 | ||
| :END: | ||
| #+title: process | ||
| #+filetags: :linux:programming: | ||
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| * Signals | ||
| - interrupts, used to announce asynchronous events to a process | ||
| - all start with "SIG" and around 64 of them | ||
| ** how does a process handle a signal? | ||
| three cases: | ||
| - ignores it | ||
| - catch and handle the exception | ||
| - let the default action apply | ||
| ** standard signals | ||
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| | Signal # | Name | Default Action | Comment | POSIX | | ||
| |----------+-----------+----------------+--------------------------------------------------------+-------| | ||
| | 1 | SIGHUP | Terminate | Hang up detected on controlling terminal or process. | Yes | | ||
| | 2 | SIGINT | Terminate | Interrupt from keyboard (Ctrl+C). | Yes | | ||
| | 3 | SIGQUIT | Dump | Quit from keyboard (Ctrl+\). | Yes | | ||
| | 4 | SIGILL | Dump | Illegal instruction. | Yes | | ||
| | 5 | SIGTRAP | Dump | Breakpoint for debugging. | No | | ||
| | 6 | SIGABRT | Dump | Abnormal termination. | Yes | | ||
| | 6 | SIGIOT | Dump | Equivalent to `SIGABRT`. | No | | ||
| | 7 | SIGBUS | Dump | Bus error (hardware fault). | No | | ||
| | 8 | SIGFPE | Dump | Floating-point exception. | Yes | | ||
| | 9 | SIGKILL | Terminate | Forced process termination (cannot be caught/ignored). | Yes | | ||
| | 10 | SIGUSR1 | Terminate | User-defined signal 1. | Yes | | ||
| | 11 | SIGSEGV | Dump | Invalid memory reference (segmentation fault). | Yes | | ||
| | 12 | SIGUSR2 | Terminate | User-defined signal 2. | Yes | | ||
| | 13 | SIGPIPE | Terminate | Writing to a pipe with no readers. | Yes | | ||
| | 14 | SIGALRM | Terminate | Real-time timer expired. | Yes | | ||
| | 15 | SIGTERM | Terminate | Termination request (default signal for `kill`). | Yes | | ||
| | 16 | SIGSTKFLT | Terminate | Stack fault on coprocessor. | No | | ||
| | 17 | SIGCHLD | Ignore | Child process stopped, terminated, or received signal. | Yes | | ||
| | 18 | SIGCONT | Continue | Resume execution, if stopped. | Yes | | ||
| | 19 | SIGSTOP | Stop | Stop process execution (cannot be caught/ignored). | Yes | | ||
| | 20 | SIGTSTP | Stop | Stop signal from terminal (Ctrl+Z). | Yes | | ||
| | 21 | SIGTTIN | Stop | Background process trying to read from terminal. | Yes | | ||
| | 22 | SIGTTOU | Stop | Background process trying to write to terminal. | Yes | | ||
| | 23 | SIGURG | Ignore | Urgent condition on socket. | No | | ||
| | 24 | SIGXCPU | Dump | CPU time limit exceeded. | No | | ||
| | 25 | SIGXFSZ | Dump | File size limit exceeded. | No | | ||
| | 26 | SIGVTALRM | Terminate | Virtual timer expired. | No | | ||
| | 27 | SIGPROF | Terminate | Profiling timer expired. | No | | ||
| | 28 | SIGWINCH | Ignore | Window size change. | No | | ||
| | 29 | SIGIO | Terminate | I/O now possible. | No | | ||
| | 29 | SIGPOLL | Terminate | Equivalent to `SIGIO`. | No | | ||
| | 30 | SIGPWR | Terminate | Power failure. | No | | ||
| | 31 | SIGSYS | Dump | Bad system call. | No | | ||
| | 31 | SIGUNUSED | Dump | Equivalent to `SIGSYS`. | No | | ||
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| ** sending signals | ||
| - checkout ~man kill~ | ||
| - can send all signals irrespective of what the name indicates | ||
| - default is to terminate if no number provided | ||
| ** raising signals | ||
| - can raise signals within a process using the `raise()` or `kill()` functions, both declared in the `signal.h` header file in C. | ||
| - ~man 2 signal~ and check out the "SEE ALSO" section | ||
| - ~man 3 raise~ | ||
| - ~man 2 kill~ | ||
| - and more ... | ||
| ** masking signals | ||
| - fetch and/or change the signal mask of the calling thread | ||
| - set of signals whose delivery is currently blocked for the caller | ||
| - ~man 2 sigprocmask~ | ||
| ** catching signals | ||
| ~SIGUSR1~ and ~SIGUSR2~ are user-defined signals. Here's a basic example using ~SIGUSR1~ to toggle a flag in a C program: | ||
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| #+begin_src C | ||
| #include <stdio.h> | ||
| #include <unistd.h> | ||
| #include <signal.h> | ||
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| volatile int flag = 0; | ||
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| void signal_handler(int signum) { | ||
| if (signum == SIGUSR1) { | ||
| flag = !flag; // Toggle the flag | ||
| printf("Signal received, flag toggled to: %d\n", flag); | ||
| } | ||
| } | ||
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| int main() { | ||
| signal(SIGUSR1, signal_handler); // Register signal handler | ||
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| while (1) { | ||
| printf("Waiting for signal (flag: %d)...\n", flag); | ||
| sleep(1); // Check every second | ||
| } | ||
| return 0; | ||
| } | ||
| #+end_src | ||
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| #+begin_src sh | ||
| gcc signal_example.c -o signal_example | ||
| ./signal_example | ||
| #+end_src | ||
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| #+begin_src sh | ||
| ps aux | grep signal_example | ||
| kill -USR1 <process_id> | ||
| #+end_src | ||
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| * Resources | ||
| - Signals https://faculty.cs.niu.edu/~hutchins/csci480/signals.htm | ||
| - ~man 7 signal~ | ||
| - ~man kill~ |