How to check if a process is ignoring or handling a signal?

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Using signal() or, preferably, sigaction(), we can choose to ignore or explicitly handle most of the POSIX signals. For example, in order to ignore SIGCHLD we could do the following:

struct sigaction sa_chld = { .sa_handler = SIG_IGN };
sigaction(SIGCHLD, &sa_chld, NULL);

Is there also a way to figure out if a given signal is being ignored and/or caught by the program?

Ideally, I'd like to distinguish if a signal...

  • is being handled as per the default actions
  • is being ignored explicitly
  • is being caught explicitly
signals linux c
2 Answers

For a running process, you can check out /proc/PID/status and check the fields SigBlk, SigIgn, and SigCgt for blocked, ignored, and caught signals respectively.

Someone wrote a utility script handy to "decode" it which I personally find very useful and been using it.

See proc documentation for details and more relevant fields.

user_12751690

Yes.

Example:

#include <signal.h>
#include <stdio.h>
void h(int Sig){(void)Sig; }
int main()
{
    signal(1,SIG_IGN); //note: setting ign/dfl is more compat (and still safe) with signal()
    sigaction(2, &(struct sigaction){.sa_handler=h},0);

    for(int i=1; i<8; i++){
        struct sigaction oldsa;
        sigaction(i, 0, &oldsa);
        if(!(oldsa.sa_flags&SA_SIGINFO)){
            if(oldsa.sa_handler==SIG_IGN) printf("%d ignored\n", i);
            else if(oldsa.sa_handler==SIG_DFL) printf("%d defaulted\n", i);
            else goto caught;
        }else caught:
            printf("%d caught\n", i);
    }

}

Example output:

1 ignored
2 caught
3 defaulted
4 defaulted
5 defaulted
6 defaulted
7 defaulted

Nonportably on Linux, you can also parse /proc/$PID/stat (or status, which has the human-readable version) to see if the signal is in the caught/ignored mask for the process (if(mask&(1ul<<(signalnumber-1)) printf("%d in the mask", signalnumber); ).

Linux maintains the list of caught/ignored signals as bitmasks, but POSIX doesn't expose those and instead requires you to obtain each individual disposition seperately vi a non-setting call to sigaction.

user_10847740
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