C program is interrupted during the execution of “system” function

Question

I have a problem with my C code, basically I need to send email via mutt program. It must be send when an interrupt comes up from GPIO pin. My sendMail function is listed below. I realized this by using system function. Main contains loop with logAlarm func which contains sendMail. The thing is when system(cmd) function finishes, the whole C program stops. For instance when i put sendMail function at the begining of the main it works and is sending email to my mailbox without stopping whole program, in the loop it manages to send it but it terminates program. I tried to using & sign to run it in background but it didnt help.

P.S i dont know if it matters but im also using system call from setitimer with 2 sec intervals to check few things but i guess it has no impact for this case.

Any ideas?

Thanks in advance :)

sendMail function:

void sendMail(char * msg, char * recipientMail){
 char cmd[100];
 char subject[30];
 char body[60];

 sprintf(body, "Intruder detected!!!\n%s", msg);
 strcpy(subject, "\"ALARM - SECURITY BREACH\"");
 sprintf(cmd,"echo \"%s\" | mutt -s %s %s &", body, subject, recipientMail);
 printf("%s\n\n", cmd);
 system(cmd);

}

Here is a piece of my main function:

while(1){

        sleep(1);

        if(prev_state == triggered && !emailDetach){
            if(!logAlarm()){
                printf("Error writing to log file!!!\n");
            }
            emailDetach = true;
        }
        //printf("Czas od poprzedniego alarmu: %d", millis() - alarmTriggeredTime);
        if((prev_state == triggered) && (millis() - alarmTriggeredTime >= ALARM_TIME)){

            digitalWrite(ALARM_ON_DIODE, LOW);
            digitalWrite(ALARM_OFF_DIODE, HIGH);
            //warunek czasowy osobno na syrene TODO
            if(!silentMode && (millis() - alarmTriggeredTime >= siren_alarm_time)){
                digitalWrite(SIREN, LOW);
            }
            prev_state = nottriggered;
        }

}

What dues the debugger tell? Where does it exactly stop? See [ask] and provide a [mcve]. — too honest for this site, Dec 31 '16 at 09:19
system() is synchronous so sendmail() will not return until cmd is finished. — Marichyasana, Dec 31 '16 at 09:39
thanks @Olaf for suggestion of using debugger :D. It helped a lot, it was fault of SIGSEGV. The cmd variable was to small in one case and it was crashing program. Thanks! — Prometeusz, Dec 31 '16 at 14:40

score 0 · Accepted Answer · answered Dec 31 '16 at 12:53

Good question. As per description, I consider that sendMail function works properly. I've not worked with mutt. But i've worked with system().

What system does is, it forks a child process of your current process and executes the command using execve(). So the problem should be in the returning of the system.

So, first you should check the return status of system function. If you are able to make printf() below the system(), then you're not having problem with system(). If you are not able get printf below the system(), then system() is killing your process. (by sending sigkill or similer signals, but not sigint or sigquit, since it is ignored by system()). You are creating a child process in the cmd itself (echo output piping to mutt). May be this should be root cause.

If you find problem here, then the problem is critical, and you will find directions from the "NOTES" section of "man system", since you have implemented the same logic mentioned there. First just try to wait for conditions mentioned there. If you're still unable to do this, try to fork two new child process, run execl or any other exec family function ("man 3 exec") from that child process to run echo and mutt.

If system() is ok, then check logAlarm(). is it giving the right arguments to the sendMail? if you are getting "Error writing to log file!!!", then entire sequence is ok.

thank you very much for your anwser, im doing more research, and as @Olaf proposed in comment i used strace command to debug and i noticed that system command invokes SIGSEGV, which terminates my program, the thing is why my program violates memory i guess? I need to test code more. I will put logs soon — Prometeusz, Dec 31 '16 at 13:17
well... the error was in the sendMail function, the cmd variable was too small to contain large string.. and it was invoking SIGSEGV. I wasnt testing it for the fixed length of the message so sometimes it was ok because cmd var was less than 100 chars but in main program the message was larger than 100 chars. What a shame :D. — Prometeusz, Dec 31 '16 at 14:38

C program is interrupted during the execution of “system” function

1 Answers1