fork出来的父子进程对信号的处理

进程与信号处理的理解及示例

原创已于 2022-09-05 23:16:54 修改 · 1.9k 阅读

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于 2022-09-05 23:15:21 首次发布

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本文介绍了进程的fork和exec操作对信号处理的影响，以及终端上Ctrl+C与kill命令的区别。在fork过程中，子进程继承了父进程的信号处理方式，但在exec后，新程序会恢复默认的信号处理。终端上的Ctrl+C可以同时结束进程及其子进程，而kill命令仅针对指定进程。文中还给出了示例代码，展示了如何捕获和处理SIGUSR1和SIGUSR2信号，以及利用信号进行进程间通信。

假设：
SIGUSR1 用户自定义信号 默认处理：进程终止
SIGUSR2 用户自定义信号 默认处理：进程终止

当一个进程调用fork时，因为子进程在开始时复制父进程的存储映像，信号捕捉函数的地址在子进程中是有意义的，所以子进程继承父进程的信号处理方式。
但是当子进程调用exec后，因为exec运行新的程序后会覆盖从父进程继承来的存储映像，那么信号捕捉函数在新程序中已无意义，所以exec会将原先设置为要捕捉的信号都更改为默认动作。

回想我们经常在terminal上执行的ctrl+c操作可以把当前正在执行的进程以及其子进程一起杀死。原因就是上面所说的。因为当前terminal上运行的进程及其子进程都是bash的子进程。

如果使用kill -n pid，则只会把某个具体的进程杀死，当然有可能导致孤儿进程和僵尸进程的出现。这种方式是针对具体的进程而言的。不像在terminal上执行的ctrl+c，实际上它也是针对具体的进程的。

ok@u20:~/test/signalTest$ ps -ef
UID          PID    PPID  C STIME TTY          TIME CMD
root           1       0  0 15:56 ?        00:00:04 /sbin/init splash
root           2       0  0 15:56 ?        00:00:00 [kthreadd]
root           3       2  0 15:56 ?        00:00:00 [rcu_gp]
root           4       2  0 15:56 ?        00:00:00 [rcu_par_gp]
root           5       2  0 15:56 ?        00:00:00 [netns]
root           7       2  0 15:56 ?        00:00:00 [kworker/0:0H-events_highpri]
root          10       2  0 15:56 ?        00:00:00 [mm_percpu_wq]
root          11       2  0 15:56 ?        00:00:00 [rcu_tasks_rude_]
root          12       2  0 15:56 ?        00:00:00 [rcu_tasks_trace]
root          13       2  0 15:56 ?        00:00:00 [ksoftirqd/0]
root          14       2  0 15:56 ?        00:00:58 [rcu_sched]
root          15       2  0 15:56 ?        00:00:00 [migration/0]
root          16       2  0 15:56 ?        00:00:00 [idle_inject/0]
root          17       2  0 15:56 ?        00:00:00 [cpuhp/0]
root          18       2  0 15:56 ?        00:00:00 [cpuhp/1]
root          19       2  0 15:56 ?        00:00:00 [idle_inject/1]
root          20       2  0 15:56 ?        00:00:00 [migration/1]
root          21       2  0 15:56 ?        00:00:00 [ksoftirqd/1]
root          23       2  0 15:56 ?        00:00:00 [kworker/1:0H-events_highpri]
root          24       2  0 15:56 ?        00:00:00 [cpuhp/2]
root          25       2  0 15:56 ?        00:00:00 [idle_inject/2]
root          26       2  0 15:56 ?        00:00:00 [migration/2]
root          27       2  0 15:56 ?        00:00:00 [ksoftirqd/2]
root          29       2  0 15:56 ?        00:00:00 [kworker/2:0H-events_highpri]
root          30       2  0 15:56 ?        00:00:00 [cpuhp/3]
root          31       2  0 15:56 ?        00:00:00 [idle_inject/3]
root          32       2  0 15:56 ?        00:00:00 [migration/3]
root          33       2  0 15:56 ?        00:00:00 [ksoftirqd/3]
root          35       2  0 15:56 ?        00:00:00 [kworker/3:0H-events_highpri]
root          36       2  0 15:56 ?        00:00:00 [kdevtmpfs]
root          37       2  0 15:56 ?        00:00:00 [inet_frag_wq]
root          38       2  0 15:56 ?        00:00:00 [kauditd]
root          42       2  0 15:56 ?        00:00:00 [khungtaskd]
root          43       2  0 15:56 ?        00:00:00 [oom_reaper]
root          44       2  0 15:56 ?        00:00:00 [writeback]
root          45       2  0 15:56 ?        00:00:08 [kcompactd0]
root          46       2  0 15:56 ?        00:00:00 [ksmd]
root          47       2  0 15:56 ?        00:00:00 [khugepaged]
root          94       2  0 15:56 ?        00:00:00 [kintegrityd]
root          95       2  0 15:56 ?        00:00:00 [kblockd]
root          96       2  0 15:56 ?        00:00:00 [blkcg_punt_bio]
root          98       2  0 15:56 ?        00:00:00 [tpm_dev_wq]
root          99       2  0 15:56 ?        00:00:00 [ata_sff]
root         100       2  0 15:56 ?        00:00:00 [md]
root         101       2  0 15:56 ?        00:00:00 [edac-poller]
root         102       2  0 15:56 ?        00:00:00 [devfreq_wq]
root         103       2  0 15:56 ?        00:00:00 [watchdogd]
root         105       2  0 15:56 ?        00:00:01 [kworker/2:1H-kblockd]
root         107       2  0 15:56 ?        00:00:03 [kswapd0]
root         108       2  0 15:56 ?        00:00:00 [ecryptfs-kthrea]
root         110       2  0 15:56 ?        00:00:00 [kthrotld]
root         111       2  0 15:56 ?        00:00:00 [irq/24-pciehp]
root         112       2  0 15:56 ?        00:00:00 [irq/25-pciehp]
root         113       2  0 15:56 ?        00:00:00 [irq/26-pciehp]
root         114       2  0 15:56 ?        00:00:00 [irq/27-pciehp]
root         115       2  0 15:56 ?        00:00:00 [irq/28-pciehp]
root         116       2  0 15:56 ?        00:00:00 [irq/29-pciehp]
root         117       2  0 15:56 ?        00:00:00 [irq/30-pciehp]
root         118       2  0 15:56 ?        00:00:00 [irq/31-pciehp]
root         119       2  0 15:56 ?        00:00:00 [irq/32-pciehp]
root         120       2  0 15:56 ?        00:00:00 [irq/33-pciehp]
root         121       2  0 15:56 ?        00:00:00 [irq/34-pciehp]
root         122       2  0 15:56 ?        00:00:00 [irq/35-pciehp]
root         123       2  0 15:56 ?        00:00:00 [irq/36-pciehp]
root         124       2  0 15:56 ?        00:00:00 [irq/37-pciehp]
root         125       2  0 15:56 ?        00:00:00 [irq/38-pciehp]
root         126       2  0 15:56 ?        00:00:00 [irq/39-pciehp]
root         127       2  0 15:56 ?        00:00:00 [irq/40-pciehp]
root         128       2  0 15:56 ?        00:00:00 [irq/41-pciehp]
root         129       2  0 15:56 ?        00:00:00 [irq/42-pciehp]
root         130       2  0 15:56 ?        00:00:00 [irq/43-pciehp]
root         131       2  0 15:56 ?        00:00:00 [irq/44-pciehp]
root         132       2  0 15:56 ?        00:00:00 [irq/45-pciehp]
root         133       2  0 15:56 ?        00:00:00 [irq/46-pciehp]
root         134       2  0 15:56 ?        00:00:00 [irq/47-pciehp]
root         135       2  0 15:56 ?        00:00:00 [irq/48-pciehp]
root         136       2  0 15:56 ?        00:00:00 [irq/49-pciehp]
root         137       2  0 15:56 ?        00:00:00 [irq/50-pciehp]
root         138       2  0 15:56 ?        00:00:00 [irq/51-pciehp]
root         139       2  0 15:56 ?        00:00:00 [irq/52-pciehp]
root         140       2  0 15:56 ?        00:00:00 [irq/53-pciehp]
root         141       2  0 15:56 ?        00:00:00 [irq/54-pciehp]
root         142       2  0 15:56 ?        00:00:00 [irq/55-pciehp]
root         143       2  0 15:56 ?        00:00:00 [acpi_thermal_pm]
root         145       2  0 15:56 ?        00:00:00 [scsi_eh_0]
root         146       2  0 15:56 ?        00:00:00 [scsi_tmf_0]
root         147       2  0 15:56 ?        00:00:00 [scsi_eh_1]
root         148       2  0 15:56 ?        00:00:00 [scsi_tmf_1]
root         150       2  0 15:56 ?        00:00:00 [vfio-irqfd-clea]
root         151       2  0 15:56 ?        00:00:00 [mld]
root         152       2  0 15:56 ?        00:00:00 [ipv6_addrconf]
root         162       2  0 15:56 ?        00:00:00 [kstrp]
root         165       2  0 15:56 ?        00:00:00 [zswap-shrink]
root         168       2  0 15:56 ?        00:00:00 [kworker/u257:0-hci0]
root         173       2  0 15:56 ?        00:00:00 [charger_manager]
root         195       2  0 15:56 ?        00:00:01 [kworker/0:1H-kblockd]
root         222       2  0 15:56 ?        00:00:01 [kworker/3:1H-kblockd]
root         223       2  0 15:56 ?        00:00:00 [mpt_poll_0]
root         224       2  0 15:56 ?        00:00:00 [mpt/0]
root         226       2  0 15:56 ?        00:00:00 [scsi_eh_2]
root         227       2  0 15:56 ?        00:00:00 [scsi_tmf_2]
root         228       2  0 15:56 ?        00:00:00 [scsi_eh_3]
root         229       2  0 15:56 ?        00:00:00 [scsi_tmf_3]
root         230       2  0 15:56 ?        00:00:00 [scsi_eh_4]
root         231       2  0 15:56 ?        00:00:00 [scsi_tmf_4]
root         232       2  0 15:56 ?        00:00:00 [scsi_eh_5]
root         233       2  0 15:56 ?        00:00:00 [scsi_tmf_5]
root         234       2  0 15:56 ?        00:00:00 [scsi_eh_6]
root         235       2  0 15:56 ?        00:00:00 [scsi_tmf_6]
root         236       2  0 15:56 ?        00:00:00 [scsi_eh_7]
root         237       2  0 15:56 ?        00:00:00 [scsi_tmf_7]
root         238       2  0 15:56 ?        00:00:00 [scsi_eh_8]
root         239       2  0 15:56 ?        00:00:00 [scsi_tmf_8]
root         240       2  0 15:56 ?        00:00:00 [scsi_eh_9]
root         241       2  0 15:56 ?        00:00:00 [scsi_tmf_9]
root         242       2  0 15:56 ?        00:00:00 [scsi_eh_10]
root         243       2  0 15:56 ?        00:00:00 [scsi_tmf_10]
root         244       2  0 15:56 ?        00:00:00 [scsi_eh_11]
root         245       2  0 15:56 ?        00:00:00 [scsi_tmf_11]
root         246       2  0 15:56 ?        00:00:00 [scsi_eh_12]
root         247       2  0 15:56 ?        00:00:00 [scsi_tmf_12]
root         248       2  0 15:56 ?        00:00:00 [scsi_eh_13]
root         249       2  0 15:56 ?        00:00:00 [scsi_tmf_13]
root         250       2  0 15:56 ?        00:00:00 [scsi_eh_14]
root         251       2  0 15:56 ?        00:00:00 [scsi_tmf_14]
root         252       2  0 15:56 ?        00:00:00 [scsi_eh_15]
root         253       2  0 15:56 ?        00:00:00 [scsi_tmf_15]
root         254       2  0 15:56 ?        00:00:00 [scsi_eh_16]
root         255       2  0 15:56 ?        00:00:00 [scsi_tmf_16]
root         256       2  0 15:56 ?        00:00:00 [scsi_eh_17]
root         257       2  0 15:56 ?        00:00:00 [scsi_tmf_17]
root         258       2  0 15:56 ?        00:00:00 [scsi_eh_18]
root         259       2  0 15:56 ?        00:00:00 [scsi_tmf_18]
root         260       2  0 15:56 ?        00:00:00 [scsi_eh_19]
root         261       2  0 15:56 ?        00:00:00 [scsi_tmf_19]
root         262       2  0 15:56 ?        00:00:00 [scsi_eh_20]
root         263       2  0 15:56 ?        00:00:00 [scsi_tmf_20]
root         264       2  0 15:56 ?        00:00:00 [scsi_eh_21]
root         265       2  0 15:56 ?        00:00:00 [scsi_tmf_21]
root         266       2  0 15:56 ?        00:00:00 [scsi_eh_22]
root         267       2  0 15:56 ?        00:00:00 [scsi_tmf_22]
root         268       2  0 15:56 ?        00:00:00 [scsi_eh_23]
root         269       2  0 15:56 ?        00:00:00 [scsi_tmf_23]
root         270       2  0 15:56 ?        00:00:00 [scsi_eh_24]
root         271       2  0 15:56 ?        00:00:00 [scsi_tmf_24]
root         272       2  0 15:56 ?        00:00:00 [scsi_eh_25]
root         273       2  0 15:56 ?        00:00:00 [scsi_tmf_25]
root         274       2  0 15:56 ?        00:00:00 [scsi_eh_26]
root         275       2  0 15:56 ?        00:00:00 [scsi_tmf_26]
root         276       2  0 15:56 ?        00:00:00 [scsi_eh_27]
root         277       2  0 15:56 ?        00:00:00 [scsi_tmf_27]
root         278       2  0 15:56 ?        00:00:00 [scsi_eh_28]
root         279       2  0 15:56 ?        00:00:00 [scsi_tmf_28]
root         280       2  0 15:56 ?        00:00:00 [scsi_eh_29]
root         281       2  0 15:56 ?        00:00:00 [scsi_tmf_29]
root         282       2  0 15:56 ?        00:00:00 [scsi_eh_30]
root         283       2  0 15:56 ?        00:00:00 [scsi_tmf_30]
root         284       2  0 15:56 ?        00:00:00 [scsi_eh_31]
root         285       2  0 15:56 ?        00:00:00 [scsi_tmf_31]
root         313       2  0 15:56 ?        00:00:00 [scsi_eh_32]
root         314       2  0 15:56 ?        00:00:00 [scsi_tmf_32]
root         320       2  0 15:56 ?        00:00:00 [kworker/1:1H-kblockd]
root         349       2  0 15:56 ?        00:00:01 [jbd2/sda6-8]
root         350       2  0 15:56 ?        00:00:00 [ext4-rsv-conver]
root         391       1  0 15:56 ?        00:00:04 /lib/systemd/systemd-journald
root         411       1  0 15:56 ?        00:00:01 /lib/systemd/systemd-udevd
root         414       2  0 15:56 ?        00:00:00 [ttm_swap]
root         416       2  0 15:56 ?        00:00:05 [irq/16-vmwgfx]
root         418       2  0 15:56 ?        00:00:00 [card0-crtc0]
root         419       2  0 15:56 ?        00:00:00 [card0-crtc1]
root         420       2  0 15:56 ?        00:00:00 [card0-crtc2]
root         421       2  0 15:56 ?        00:00:00 [card0-crtc3]
root         422       2  0 15:56 ?        00:00:00 [card0-crtc4]
root         423       2  0 15:56 ?        00:00:00 [card0-crtc5]
root         424       2  0 15:56 ?        00:00:00 [card0-crtc6]
root         426       2  0 15:56 ?        00:00:00 [card0-crtc7]
root         493       1  0 15:56 ?        00:00:00 vmware-vmblock-fuse /run/vmblock-
root         619       2  0 15:56 ?        00:00:00 [kworker/u257:1-hci0]
root         692       2  0 15:56 ?        00:00:00 [cryptd]
root         827       2  0 15:56 ?        00:00:00 [jbd2/sda1-8]
root         828       2  0 15:56 ?        00:00:00 [ext4-rsv-conver]
systemd+     853       1  0 15:56 ?        00:00:01 /lib/systemd/systemd-resolved
systemd+     854       1  0 15:56 ?        00:00:00 /lib/systemd/systemd-timesyncd
root         865       1  0 15:56 ?        00:00:00 /usr/bin/VGAuthService
root         869       1  0 15:56 ?        00:00:46 /usr/bin/vmtoolsd
root         886       1  0 15:56 ?        00:00:00 /usr/lib/accountsservice/accounts
root         887       1  0 15:56 ?        00:00:00 /usr/sbin/acpid
avahi        890       1  0 15:56 ?        00:00:00 avahi-daemon: running [u20.local]
root         891       1  0 15:56 ?        00:00:00 /usr/lib/bluetooth/bluetoothd
root         894       1  0 15:56 ?        00:00:00 /usr/sbin/cron -f
root         895       1  0 15:56 ?        00:00:00 /usr/sbin/cupsd -l
message+     896       1  0 15:56 ?        00:00:03 /usr/bin/dbus-daemon --system --a
root         897       1  0 15:56 ?        00:00:32 /usr/sbin/NetworkManager --no-dae
root         903       1  0 15:56 ?        00:00:01 /usr/sbin/irqbalance --foreground
root         907       1  0 15:56 ?        00:00:00 /usr/bin/python3 /usr/bin/network
root         910       1  0 15:56 ?        00:00:00 /usr/lib/policykit-1/polkitd --no
syslog       917       1  0 15:56 ?        00:00:00 /usr/sbin/rsyslogd -n -iNONE
root         919       1  0 15:56 ?        00:00:05 /usr/lib/snapd/snapd
root         921       1  0 15:56 ?        00:00:00 /usr/libexec/switcheroo-control
root         926       1  0 15:56 ?        00:00:00 /lib/systemd/systemd-logind
root         935       1  0 15:56 ?        00:00:12 /usr/sbin/thermald --no-daemon --
root         938       1  0 15:56 ?        00:00:00 /usr/lib/udisks2/udisksd
root         940       1  0 15:56 ?        00:00:00 /sbin/wpa_supplicant -u -s -O /ru
avahi        945     890  0 15:56 ?        00:00:00 avahi-daemon: chroot helper
root         972       1  0 15:56 ?        00:00:00 /usr/sbin/cups-browsed
root         983       1  0 15:56 ?        00:00:00 /usr/sbin/ModemManager --filter-p
root         990       1  0 15:56 ?        00:00:09 /usr/bin/python3 /usr/bin/supervi
root        1014       1  0 15:56 ?        00:00:00 /usr/bin/python3 /usr/share/unatt
root        1025       1  0 15:56 ?        00:00:00 /usr/sbin/gdm3
root        1039    1025  0 15:56 ?        00:00:00 gdm-session-worker [pam/gdm-autol
kernoops    1088       1  0 15:56 ?        00:00:00 /usr/sbin/kerneloops --test
kernoops    1098       1  0 15:56 ?        00:00:00 /usr/sbin/kerneloops
ok          1127       1  0 15:56 ?        00:00:03 /lib/systemd/systemd --user
ok          1130    1127  0 15:56 ?        00:00:00 (sd-pam)
ok          1153    1127  1 15:56 ?        00:06:05 /usr/bin/pulseaudio --daemonize=n
ok          1156    1127  0 15:56 ?        00:00:00 /usr/libexec/tracker-miner-fs
ok          1158       1  0 15:56 ?        00:00:00 /usr/bin/gnome-keyring-daemon --d
ok          1162    1039  0 15:56 tty2     00:00:00 /usr/lib/gdm3/gdm-x-session --run
ok          1164    1162  1 15:56 tty2     00:04:31 /usr/lib/xorg/Xorg vt2 -displayfd
ok          1169    1127  0 15:56 ?        00:00:04 /usr/bin/dbus-daemon --session --
rtkit       1170       1  0 15:56 ?        00:00:00 /usr/libexec/rtkit-daemon
ok          1178    1127  0 15:56 ?        00:00:00 /usr/libexec/gvfsd
ok          1194    1127  0 15:56 ?        00:00:00 /usr/libexec/gvfsd-fuse /run/user
ok          1201    1127  0 15:56 ?        00:00:01 /usr/libexec/gvfs-udisks2-volume-
ok          1219    1127  0 15:56 ?        00:00:00 /usr/libexec/gvfs-mtp-volume-moni
root        1220       2  0 15:56 ?        00:00:00 [krfcommd]
ok          1224    1127  0 15:56 ?        00:00:00 /usr/libexec/gvfs-goa-volume-moni
ok          1228    1127  0 15:56 ?        00:00:00 /usr/libexec/goa-daemon
ok          1235    1127  0 15:56 ?        00:00:00 /usr/libexec/goa-identity-service
ok          1240    1127  0 15:56 ?        00:00:01 /usr/libexec/gvfs-afc-volume-moni
ok          1246    1127  0 15:56 ?        00:00:00 /usr/libexec/gvfs-gphoto2-volume-
root        1252       1  0 15:56 ?        00:00:00 /usr/lib/upower/upowerd
ok          1331    1162  0 15:56 tty2     00:00:00 /usr/libexec/gnome-session-binary
ok          1405    1331  0 15:56 ?        00:00:00 /usr/bin/ssh-agent /usr/bin/im-la
ok          1422       1  0 15:56 ?        00:00:19 /usr/bin/ibus-daemon --daemonize 
ok          1426    1422  0 15:56 ?        00:00:00 /usr/libexec/ibus-memconf
ok          1427    1422  0 15:56 ?        00:00:01 /usr/libexec/ibus-ui-gtk3
ok          1428    1422  0 15:56 ?        00:00:07 /usr/libexec/ibus-extension-gtk3
ok          1433       1  0 15:56 ?        00:00:00 /usr/libexec/ibus-x11 --kill-daem
ok          1436    1127  0 15:56 ?        00:00:00 /usr/libexec/ibus-portal
ok          1458    1127  0 15:56 ?        00:00:00 /usr/libexec/at-spi-bus-launcher
ok          1463    1458  0 15:56 ?        00:00:00 /usr/bin/dbus-daemon --config-fil
ok          1468    1127  0 15:56 ?        00:00:00 /usr/libexec/at-spi2-registryd --
ok          1477    1127  0 15:56 ?        00:00:01 /usr/libexec/xdg-desktop-portal
ok          1481    1127  0 15:56 ?        00:00:00 /usr/libexec/xdg-document-portal
ok          1484    1127  0 15:56 ?        00:00:00 /usr/libexec/xdg-permission-store
ok          1494    1127  0 15:56 ?        00:00:01 /usr/libexec/xdg-desktop-portal-g
ok          1502    1127  0 15:56 ?        00:00:00 /usr/libexec/dconf-service
ok          1508    1127  0 15:56 ?        00:00:00 /usr/bin/gnome-keyring-daemon --s
ok          1516    1422  0 15:56 ?        00:00:00 /usr/libexec/ibus-engine-simple
ok          1525    1127  0 15:56 ?        00:00:00 /usr/libexec/gnome-session-ctl --
ok          1531    1127  0 15:56 ?        00:00:00 /usr/libexec/gnome-session-binary
ok          1546    1127  1 15:56 ?        00:06:54 /usr/bin/gnome-shell
ok          1570    1127  0 15:56 ?        00:00:00 /usr/libexec/gnome-shell-calendar
ok          1576    1127  0 15:56 ?        00:00:00 /usr/libexec/evolution-source-reg
ok          1585    1127  0 15:56 ?        00:00:00 /usr/libexec/evolution-calendar-f
ok          1599    1127  0 15:56 ?        00:00:00 /usr/libexec/evolution-addressboo
ok          1616    1127  0 15:56 ?        00:00:00 /usr/bin/gjs /usr/share/gnome-she
ok          1619    1178  0 15:56 ?        00:00:00 /usr/libexec/gvfsd-trash --spawne
ok          1640    1127  0 15:56 ?        00:00:00 /usr/libexec/gsd-a11y-settings
ok          1641    1127  0 15:56 ?        00:00:01 /usr/libexec/gsd-color
ok          1644    1127  0 15:56 ?        00:00:00 /usr/libexec/gsd-datetime
ok          1646    1127  0 15:56 ?        00:00:01 /usr/libexec/gsd-housekeeping
ok          1647    1127  0 15:56 ?        00:00:01 /usr/libexec/gsd-keyboard
ok          1650    1127  0 15:56 ?        00:00:01 /usr/libexec/gsd-media-keys
ok          1651    1127  0 15:56 ?        00:00:01 /usr/libexec/gsd-power
ok          1652    1127  0 15:56 ?        00:00:00 /usr/libexec/gsd-print-notificati
ok          1654    1127  0 15:56 ?        00:00:00 /usr/libexec/gsd-rfkill
ok          1656    1127  0 15:56 ?        00:00:00 /usr/libexec/gsd-screensaver-prox
ok          1657    1127  0 15:56 ?        00:00:00 /usr/libexec/gsd-sharing
ok          1658    1127  0 15:56 ?        00:00:00 /usr/libexec/gsd-smartcard
ok          1665    1127  0 15:56 ?        00:00:00 /usr/libexec/gsd-sound
ok          1671    1127  0 15:56 ?        00:00:00 /usr/libexec/gsd-usb-protection
ok          1674    1127  0 15:56 ?        00:00:00 /usr/libexec/gsd-wacom
ok          1677    1127  0 15:56 ?        00:00:00 /usr/libexec/gsd-wwan
ok          1678    1127  0 15:56 ?        00:00:01 /usr/libexec/gsd-xsettings
ok          1711    1127  0 15:56 ?        00:00:46 /usr/bin/vmtoolsd -n vmusr --bloc
ok          1719    1531  0 15:56 ?        00:00:00 /usr/libexec/gsd-disk-utility-not
ok          1722    1531  0 15:56 ?        00:00:01 /usr/libexec/evolution-data-serve
colord      1764       1  0 15:56 ?        00:00:00 /usr/libexec/colord
ok          1785    1127  0 15:56 ?        00:00:00 /usr/libexec/gsd-printer
ok          1826    1422  0 15:56 ?        00:00:05 /usr/lib/ibus/ibus-engine-libpiny
ok          1871    1127  0 15:56 ?        00:00:01 /opt/sogoupinyin/files/bin/sogoup
ok          1970    1127  0 15:56 ?        00:00:35 /usr/libexec/gnome-terminal-serve
ok          2054    1127  8 15:57 ?        00:37:09 /usr/lib/firefox/firefox -new-win
ok          2144    2054  0 15:57 ?        00:00:00 /usr/lib/firefox/firefox -content
ok          2190    2054  0 15:57 ?        00:00:02 /usr/lib/firefox/firefox -content
ok          2232    2054  0 15:57 ?        00:00:01 /usr/lib/firefox/firefox -content
ok          2289    2054  0 15:57 ?        00:00:33 /usr/lib/firefox/firefox -content
ok          2292    2054  0 15:57 ?        00:00:06 /usr/lib/firefox/firefox -content
ok          2326    2054  0 15:57 ?        00:00:05 /usr/lib/firefox/firefox -content
ok          2454    2054  0 15:57 ?        00:00:01 /usr/lib/firefox/firefox -content
ok          2498    2054  0 15:57 ?        00:00:29 /usr/lib/firefox/firefox -content
ok          2657    1531  0 15:57 ?        00:00:01 update-notifier
ok          2660    1127  0 15:57 ?        00:00:00 /usr/libexec/gvfsd-metadata
ok          3416    1127  0 15:58 ?        00:00:01 /opt/sogoupinyin/files/bin/sogoup
ok          3469    1127  0 15:59 ?        00:00:04 /usr/lib/speech-dispatcher-module
ok          3473    1127  0 15:59 ?        00:00:04 /usr/lib/speech-dispatcher-module
ok          3479    1127  0 15:59 ?        00:00:00 /usr/bin/speech-dispatcher --spaw
ok          3483    2054  0 15:59 ?        00:00:36 /usr/lib/firefox/firefox -content
ok          3593     990  0 16:02 ?        00:00:00 bash test.sh
ok          3596    3593  0 16:02 ?        00:00:01 ./a.out
root        3641    1127  0 16:03 ?        00:00:00 dbus-launch --autolaunch=d5d6bd10
root        3642    1127  0 16:03 ?        00:00:00 /usr/bin/dbus-daemon --syslog-onl
root        3646    1127  0 16:03 ?        00:00:00 /usr/libexec/xdg-desktop-portal
root        3651    1127  0 16:03 ?        00:00:00 /usr/libexec/xdg-document-portal
root        3655    1127  0 16:03 ?        00:00:00 /usr/libexec/xdg-permission-store
root        3665    1127  0 16:03 ?        00:00:00 /usr/libexec/xdg-desktop-portal-g
root        3669    1127  0 16:03 ?        00:00:00 /usr/libexec/gvfsd
root        3674    1127  0 16:03 ?        00:00:00 /usr/libexec/gvfsd-fuse /root/.ca
root        3685    1127  0 16:03 ?        00:00:00 /usr/libexec/dconf-service
root        3691    1127  0 16:03 ?        00:00:00 /usr/bin/gnome-keyring-daemon --s
ok          4118    1127  0 16:57 ?        00:00:11 /usr/bin/gedit --gapplication-ser
ok          4964    1970  0 21:07 pts/2    00:00:00 bash
ok          5088    1127  0 21:10 ?        00:00:01 /usr/bin/gnome-calendar --gapplic
ok          5199    2054  0 21:11 ?        00:00:01 /usr/lib/firefox/firefox -content
ok          5219    2054  0 21:11 ?        00:00:00 /usr/lib/firefox/firefox -content
ok          5255    2054  0 21:11 ?        00:00:00 /usr/lib/firefox/firefox -content
ok          5296    2054  0 21:12 ?        00:00:00 /usr/lib/firefox/firefox -content
ok          7011    1178  0 21:17 ?        00:00:00 /usr/libexec/gvfsd-network --spaw
ok          7101    1178  0 21:18 ?        00:00:00 /usr/libexec/gvfsd-dnssd --spawne
root        7237       2  0 21:40 ?        00:00:04 [kworker/2:2-events]
ok          7273    1970  0 21:41 pts/1    00:00:00 bash
root        7279       2  0 21:41 ?        00:00:01 [kworker/1:1-rcu_par_gp]
ok          7407    1127  0 21:44 ?        00:00:00 /usr/bin/seahorse --gapplication-
root        7896       2  0 22:05 ?        00:00:00 [kworker/0:0-rcu_par_gp]
ok          7922    1970  0 22:05 pts/0    00:00:00 bash
root        8065       2  0 22:09 ?        00:00:00 [kworker/1:0-rcu_par_gp]
root        8114       2  0 22:10 ?        00:00:00 [kworker/0:2-cgroup_destroy]
root        8155       2  0 22:11 ?        00:00:00 [kworker/3:1-rcu_gp]
root        8338       2  0 22:33 ?        00:00:00 [kworker/2:1-cgroup_destroy]
root        8340       2  0 22:34 ?        00:00:00 [kworker/3:2-rcu_par_gp]
root        8345       2  0 22:35 ?        00:00:00 [kworker/u256:0-events_unbound]
root        8356       2  0 22:41 ?        00:00:00 [kworker/u256:2-ext4-rsv-conversi
root        8364       2  0 22:46 ?        00:00:00 [kworker/u256:1-events_unbound]
root        8375       2  0 22:52 ?        00:00:00 [kworker/1:2-events]
root        8376       2  0 22:52 ?        00:00:00 [kworker/0:1-events]
ok          8399    7273  0 22:52 pts/1    00:00:00 ./sig0
root        8411       2  0 22:52 ?        00:00:00 [kworker/3:0-mm_percpu_wq]
root        8414       2  0 22:52 ?        00:00:00 [kworker/2:0-events]
ok          8418    7922  0 22:53 pts/0    00:00:00 ps -ef
ok@u20:~/test/signalTest$

我们看一下sig0的pid为8399，ppid为7273；

7273对应的是bash，其ppid为1970；

1970对应的是/usr/libexec/gnome-terminal-serve，其ppid为1127；

1127对应的是/lib/systemd/systemd --user，其ppid为1；

通过示例，我们可以发现，在terminal上按下ctrl+c时（相当于sigNum=2），父子进程都能捕捉到该信号；但使用kill -2 pid时则只有一个进程能捕捉到。


#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <signal.h>

void handel(int num)
{
	if(num == 2)
	{
		printf("%d catch the 2 signal\n",getpid());
	}
	
	else if(num == 3)
	{
		printf("%d catch the 3 signal\n",getpid());
	}
	else if(num == 4)
	{
		printf("%d catch the 4 signal\n",getpid());
	}
}

int main()
{
	//捕捉2,3,4号信号，并且执行相对应的函数
	signal(2,handel);
	signal(3,handel);
	signal(4,handel);
	//创建一个子进程
	pid_t id = fork();

	if(id<0)
	{
		perror("fork failed");
		return -1;
	}
	else if(id==0)
	{
		while(1)
		{
			printf("i am child and pid = %d\n",getpid());
			sleep(2);
		}
		
		
	}
	else
	{
		while(1)
		{
			printf("i am parent and pid = %d\n",getpid());
			sleep(1);
		}
	}
	//退出程序并且刷新缓冲区
	exit(0);
}

#include <stdio.h>
#include <stdlib.h>
#include <signal.h>
#include <unistd.h>
#include <sys/wait.h>
 
void handler(int signo)
{
    switch(signo) {
    case SIGUSR1: //处理信号 SIGUSR1
        printf("Parent : catch SIGUSR1\n");
        break;
    case SIGUSR2: //处理信号 SIGUSR2
        printf("Child : catch SIGUSR2\n");
        break;
    default:      //本例不支持
        printf("Should not be here\n");
        break;
    }
}
 
int main(void)
{
    pid_t ppid, cpid;
    //为两个信号设置信号处理函数
    if(signal(SIGUSR1, handler) == SIG_ERR)
    { //设置出错
        perror("Can't set handler for SIGUSR1\n");
        exit(1);
    }
 
    if(signal(SIGUSR2, handler) == SIG_ERR)
    { //设置出错
        perror("Can't set handler for SIGUSR2\n");
        exit(1);
    }
 
    ppid = getpid();//得到父进程ID
 
    if((cpid = fork()) < 0)
    {
        perror("fail to fork\n");
        exit(1);
    }
    else if(cpid == 0)
    {
        // 子进程内向父进程发送信号SIGUSER1
        if(kill(ppid, SIGUSR1) == -1)
        {
            perror("fail to send signal\n");
            exit(1);
        }
 
        while(1);//死循环，等待父进程的信号
    }
    else
    {
        sleep(1);//休眠，保证子进程先运行，并且发送SIGUSR1信号
        // 父进程向自己发送SIGUSER2信号
        if(kill(cpid, SIGUSR2) == -1)
        {
            perror("fail to send signal\n");
            exit(1);
        }
         
        // 必须sleep一下，否则子进程捕获不到SIGUSER2信号
        sleep(1);
 
        printf("will kill child\n");//输出提示
        if(kill(cpid, SIGKILL) == -1)
        { //发送SIGKILL信号，杀死子进程
            perror("fail to send signal\n");
            exit(1);
        }
 
        if(wait(NULL) ==-1)
        { //回收子进程状态，避免僵尸进程
            perror("fail to wait\n");
            exit(1);
        }
        printf("child has been killed.\n");
    }
    return;
}

除此之外，我们还可以在父子进程中利用信号进行通信。

参考：Linux的SIGUSR1和SIGUSR2信号 - 小楼一夜听春雨 - 博客园