Download signals - Washington University in St. Louis

Inter-Process Communication:
Signals
David Ferry, Chris Gill
CSE 522S - Advanced Operating Systems
Washington University in St. Louis
St. Louis, MO 63130
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Signals
Asynchronous notifications:
• Generated by hardware or requested by
another process
• Delivered to a specific process
• Processes receive signals and respond
Allows for event based programming
Conceptually is very similar to hardware
interrupts and exceptions, but for processes
CSE 522S – Advanced Operating Systems
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Common Signals
•
•
•
•
•
SIGINT – keyboard interrupt (CTRL+C)
SIGTERM – terminate program
SIGKILL – kill program immediately
SIGSEV – segmentation fault
SIGUSR1 & SIGUSR2
Allows for the delivery of a notifications,
but not for delivery of data.
(E.g. segmentation faults simply crash)
CSE 522S – Advanced Operating Systems
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Signal Receipt
For each type of signal, a process can
handle signal delivery by:
1. Ignoring it
2. Terminating
3. Invoking a signal handler
Additionally, a process may block delivery of
certain signals. Undelivered signals are said
to be pending.
CSE 522S – Advanced Operating Systems
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Signal Handler Example
#include <signal.h>
void sigusr1_handler( int signum ){
//Handle signal
}
int main( int argc, char* argv[]){
//Normal program flow
}
CSE 522S – Advanced Operating Systems
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Signal Example
Process A:
Executes code
Event happens
kill( B, SIGUSR1);
context switch
Kernel sends signal
context switch
Process B:
context switch
context switch
sigusr1_handler();
CSE 522S – Advanced Operating Systems
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Signal Default Behaviors
All signals have a default action. E.g.:
• SIGTERM – terminate program
• SIGCHLD – ignore signal
See man 7 signal for details…
Special signals that cannot be caught or
blocked:
• SIGKILL (force quit vs. orderly shutdown)
• SIGSTOP
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Signals as Hardware Events
A hardware event triggers an interrupt or
exception handler that raises a signal, e.g.:
• Divide by zero (SIGFPE)
• Segmentation fault (SIGSEV)
These are synchronous with program flow!
Note: Signals allow userspace programs to
respond to and correct hardware-level faults.
Compare to how page faults are handled
entirely within the kernel.
CSE 522S – Advanced Operating Systems
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Signal Handler Concurrency
A signal handler may be called at any point
of execution!
Creates a concurrent programming problem
even in single threaded programs!
• Deadlock
• Races
• Many of the same risks/strategies of
interrupt handlers apply here
CSE 522S – Advanced Operating Systems
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Concurrency Race Example
char* buffer;
void sig_handler( int signum ){
buffer = “Handler called\n”;
write( buffer );
}
int main( int argc, char* argv[] ){
buffer = “Main called\n”;
write( buffer );
}
CSE 522S – Advanced Operating Systems
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More Subtle Concurrency Race Example
int temp;
void swap( int *a, int *b){
temp = &a;
&a = &b;
&b = temp;
}
What race can happen if this function is called
from a signal handler and from elsewhere?
CSE 522S – Advanced Operating Systems
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