# Sending signal from child to parent process! **URL:** \ **Category:** UNIX for Dummies Questions & Answers\ **Created:** [November 5, 2009, 3:13pm UTC](https://community.unix.com/t/sending-signal-from-child-to-parent-process/252973 "2009-11-05T15:13:08Z")\ **Posts on this page:** 5\ **Page:** 1
**Author:** ![vkn\_1985](https://community.unix.com/letter_avatar/vkn_1985/32/5_5575768a8748004e209b776fc1b2916d.png) [@vkn\_1985](https://community.unix.com/u/vkn_1985)\ **Post date:** [November 5, 2009, 3:13pm UTC](https://community.unix.com/t/sending-signal-from-child-to-parent-process/252973/1 "2009-11-05T15:13:08Z")
Hi All, I facing a problem in handling signals between parent process communication. I am trying to send a signal(SIGINT) from child to parent. I am using kill function to do so and I am trying to read the signal using sigaction(). But the program is ending abruptly and I am not able to figure out why! Could anyone of you please help me? The code is as follows : ```nohighlight #include #include #include #include void sig_usr(int signo){ if(signo == SIGINT) printf("Signal caught!"); return; } int main(void){ pid_t pid, ppid; ppid = getpid(); printf("%d ", ppid); if((pid = fork()) == 0){ //Child kill(ppid, SIGINT); } else{ printf("%d %d ",ppid, pid); struct sigaction sig; sigemptyset(&sig.sa_mask); sig.sa_flags = 0; sig.sa_handler = sig_usr; if(sigaction(SIGINT,&sig,NULL) == 0) printf("Signal processed OKay "); sleep(10); } } ``` ---
**Author:** ![jim\_mcnamara](https://community.unix.com/letter_avatar/jim_mcnamara/32/5_5575768a8748004e209b776fc1b2916d.png) [@jim\_mcnamara](https://community.unix.com/u/jim_mcnamara)\ **Post date:** [November 5, 2009, 4:12pm UTC](https://community.unix.com/t/sending-signal-from-child-to-parent-process/252973/2 "2009-11-05T16:12:42Z")
start by setting up the signal handler before you create the child ```plaintext int main(void) { pid_t pid, ppid; ppid = getpid(); struct sigaction sig; sigemptyset(&sig.sa_mask); sig.sa_flags = 0; sig.sa_handler = sig_usr; printf("%d ", ppid); if((pid = fork()) == 0) { //Child kill(ppid, SIGINT); } else { printf("%d %d ",ppid, pid); if(sigaction(SIGINT,&sig,NULL) == 0) printf("Signal processed OKay "); sleep(10); } } ``` And... consider not using functions like printf in a signal handler. See async-signal safe functions: [System Interfaces Chapter 2](http://www.opengroup.org/onlinepubs/009695399/functions/xsh_chap02_04.html#tag_02_04_04) ---
**Author:** ![vkn\_1985](https://community.unix.com/letter_avatar/vkn_1985/32/5_5575768a8748004e209b776fc1b2916d.png) [@vkn\_1985](https://community.unix.com/u/vkn_1985)\ **Post date:** [November 5, 2009, 4:17pm UTC](https://community.unix.com/t/sending-signal-from-child-to-parent-process/252973/3 "2009-11-05T16:17:10Z")
Thank you. But I didnt get why printf() should not be used...Looking at the link you have posted, I am guessing functions like printf cannot be interruped in the middle. Am I right? And which other function should I consider using for printing within handler? ---
**Author:** ![jim\_mcnamara](https://community.unix.com/letter_avatar/jim_mcnamara/32/5_5575768a8748004e209b776fc1b2916d.png) [@jim\_mcnamara](https://community.unix.com/u/jim_mcnamara)\ **Post date:** [November 5, 2009, 5:15pm UTC](https://community.unix.com/t/sending-signal-from-child-to-parent-process/252973/4 "2009-11-05T17:15:57Z")
The reverse - printf CAN be interrupted so you get a hashcan result. write() - which is listed in the functions in the link. Let me rephrase this correctly - async-signal safe functions are defined to be usable in signal handlers and have them behave as advertised. When executing a call that is not async-signal safe you get undefined results when a second signal hits a signal handler that is currently executing that function. ie., if the function call works it is purely by accident not design. printf is a large chunk of code made from a lot of functions and system calls. write() is a single base system call that is not made of sub-functions and code in user space like printf. It is a single kernel mode call. ---
**Author:** ![vkn\_1985](https://community.unix.com/letter_avatar/vkn_1985/32/5_5575768a8748004e209b776fc1b2916d.png) [@vkn\_1985](https://community.unix.com/u/vkn_1985)\ **Post date:** [November 5, 2009, 9:05pm UTC](https://community.unix.com/t/sending-signal-from-child-to-parent-process/252973/5 "2009-11-05T21:05:35Z")
Thank you. Now I get it. Hey, I have two more questions. I was trying sigaction before fork(), but I see some unexpected results! 1. I trigger SIGINT signal only once using kill function. I expect this to trigger respective signal handler and do the operation. But even if I am sending SIGINT only once, I am getting "Signal processed OKay" printed twice! Shouldn't this be printed only once as I am sending SIGINT only once? Output : ```nohighlight Signal processed OKay 7006 0 Success SIGINT signal caught!! Signal processed OKay 7006 ``` Source code : ```nohighlight #include #include #include #include #include static void sig_usr(int signo){ if(signo == SIGINT){ char buf[] = "SIGINT signal caught!!\n"; int wr = strlen(buf); write(STDOUT_FILENO, buf, wr); } return; } int main(void) { pid_t pid, ppid; ppid = getpid(); struct sigaction sig; sigemptyset(&sig.sa_mask); sig.sa_flags = 0; sig.sa_handler = sig_usr; if(sigaction(SIGINT,&sig,NULL) == 0) printf("Signal processed OKay "); sleep(10); printf("%d ", ppid); if((pid = fork()) == 0) { //Child kill(ppid, SIGINT); } } ``` 1. Secondly, I am facing some problems when I try to send 2 SIGINT signals one after the other continuously from child process! i.e, when another kill() function is used after the first one in above code. The result I expected was two "SIGINT signal caught!!" messages. But I am getting only one instead. In general, if I am sending signals of same kind continuously of same kind from child to parent, anything in particular I need to do? Sorry for the elongated mail. Please help. Regards!