c - Program run in child process doesn't loop

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I have a specific question regarding
the parent process reading the stdout from child. My problem is that when I run the
program, the child program should execute a new program multiple times in a loop but it
runs it only once and exits to the parent process. The child process is running a simple
program that prints a message to stdout.
Thanks in
advance.

#include
            
#include 
#include
            
#include 
#include
            
#include 
#include
            
#include 
#include
            
#include
            


#define ssec 1
#define fsec
            0
#define secBuf 5
#define decbase 10
#define fgbuf
            60


volatile sig_atomic_t aborting =
            0;

void chld_handler (int sig)

            {
 if(sig == SIGINT){


            aborting++;

 }


            }


int rand_start(int
            low, int high){
 int s;
 int r = 0;

            srand(time(NULL));

s =
            rand();

r = s % high +
            low;

return
            r;

}

void
            Usage(){

printf("Usage :schedule [-s] [-f]
            \n");
 exit(1);
            

}

int
            main(int argc, char *argv[]){

/getopt
            variablen/
 int opt;
 int
            index;

int sflag = 0;
 int fflag =
            0;

char *svalue;
 char
            *fvalue;

int sseconds = ssec;
 int
            fseconds = fsec;

char *reactor;
 char
            *emergency;
 char
            *logfile;

/**/
 char
            *endptr;
 /Pipe and
            child/

int pipepc1[2];
 int
            pipepc2[2];

int child1_fd;
 int
            child2_fd;

FILE *log;
 FILE
            *pipe_reader;

char *p;
char
            *buf;
 int waitFc;
 int status;

            /prgm/
 int prg_r;
 int
            prg_e;

/termination/

            int cnt = 0;
 int start_period;
 p = malloc(fgbuf *
            sizeof(char));
 buf = malloc(fgbuf *
            sizeof(char));

(void)signal(SIGINT,chld_handler);

if(argc
            <
            4){

Usage();


}else
            if(argc == 4){

reactor =
            argv[1];
emergency = argv[2];
logfile =
            argv[3];


}else if(argc
            >
            4){

/*argumentenbehandlung*/


while((opt
            = getopt(argc,argv,":s:f:")) != -1){
 printf("sflag %d fflag %d
            \n",sflag,fflag);
 printf("opt %c \n", opt);
 printf("optind %d \n
            ",optind);
 switch(opt){

 case 's':
 if(sflag
            == 0){
 sflag++;
 svalue = optarg;
 }else{

            fprintf(stderr,"Widerholung der option -s\n");
 Usage();

            }

 break;

 case 'f':


            if(fflag == 0){

 fflag++;
 fvalue = optarg;

            }else {
 fprintf(stderr,"Widerholung der option -f\n");

            Usage();
 }
 break;

 case ':'
            :

 fprintf(stderr,"Option -%c brauch ein
            argument\n",optopt);
 Usage();
 break;

 case
            '?' : 
fprintf(stderr,"Nicht bekannte option -%c
            \n",optopt);
Usage();
break;

 default
            :
assert(0);


            }/*switch*/




}/*while
            getopt*/


for(index = optind; index < argc;
            index++){

 if((argc - index) == 3){

reactor =
            argv[index];

 }else if((argc - index) ==
            2){

emergency = argv[index];

 }else if((argc -
            index) == 1){

logfile = argv[index];


            }


}/*for schleife*/


/*
            */
if(sflag){

 sseconds =
            (int)strtol(svalue,&endptr,decbase);
 printf("%d ssec\n",
            sseconds);
}

if(fflag){

 fseconds =
            (int)strtol(fvalue,&endptr,decbase);
 printf("%d fsec\n",
            fseconds);

}


}

/pipeing/

if(pipe(pipepc1)
            == -1){

fprintf(stderr,"Fehler bei
            der Erzeugung der
            pipe\n");
exit(1);


}else
            {printf("Pipe created\n");}

/erzeuge
            child1/
 child1_fd =
            fork();

if(child1_fd <
            0){

fprintf(stderr,"Fehler beim
            asfuehren von
            fork\n");
exit(0);


}

if(child1_fd
            == 0){
 printf("CHILD\n");

            /close pipe read/
 if(close(pipepc1[0]) ==
            -1){

 fprintf(stderr,"Konnte nicht
            das Read-Ende vom pipepc1 schliessen\n");

            exit(1);
}

if(close(1) == -1){


            fprintf(stderr,"Konnte nicht das Read-Ende vom pipepc1 schliessen\n");

            exit(1);

}



if(dup(pipepc1[1])
            !=STDOUT_FILENO){

 fprintf(stderr,"Das setzen des Read-Endes als
            stdout is Fehlgeschlagen\n");

            exit(1);

}

if(fseconds ==
            0){

 start_period = sseconds; 

}else
            start_period = rand_start(sseconds,(sseconds + fseconds));

for(cnt
            = 0; cnt < 5; cnt++){
 sleep(start_period);

            fflush(stdout);
 prg_r = execl(reactor,"",NULL);


            //printf("prg_r ist %d \n",
            prg_r);
}

if(close(pipepc1[1]) ==
            -1){

 fprintf(stderr,"Das neue stdout konnte nich geschlossen
            werden\n");

            exit(1);

}


}else{

printf("**PARENT**\n");

log
            = fopen(logfile,"w");

/*schliesse pipe
            read*/
close(pipepc1[1]);

pipe_reader =
            fdopen(pipepc1[0],"r");


while((buf =
            fgets(p,fgbuf,pipe_reader)) != NULL){

 printf("from Child : %s
            \n",buf);
 fflush(pipe_reader);
} 

fclose(log);
            

waitFc= waitpid(child1_fd,&status,0);


            if(waitFc == -1){

fprintf(stderr,"Das Warten ist
            fehlgeschlagen\n");
exit(1);

 }


            printf("child is done\n und waitFc = %d\n und satus %d",waitFc,status);

            fclose(pipe_reader);

            close(pipepc1[1]);


}

printf("argc
            = %d \n", argc);
 exit(0);
            

}


and
            the reactor program
            :



#include
            
#include 
#include
            


int
            main(){

srand(time(NULL));
 int
            i;
 int s;

s = rand() %
            7;
/*printf("%d \n",s);*/

if(s !=
            6){

 printf("OK\n");

            fflush(stdout);

}else {

 printf("PRESSURE TOO
            HIGH - IMMEDIATE SHUTDOWN REQUIRED");

            exit(EXIT_FAILURE);

}

}

Answer

Well... you use fork() call to create a child
process then execl() to execute another program. The problem is execl() replaces current
proces image (your child process) with the image of the command you are executing. So at
the end of first pass of for() loop your program gets replaced by the executing command.
When it finishes it's job, it just simply quits, terminating the child
process.

I think what you need to do is to call
popen() instead of execl(). This will cause command to get executed in a separate
process, rather than replacing your current process. If you want, you can even start all
the commands at once (calling popen() in a loop) and then use select()/epool() to read
data from child processes as soon as it becomes available.