C shell printing output infinitely without stopping at gets() - shell

Question

I am trying to use the SIGCHLD handler but for some reason it prints of the command I gave infinitely. If I remove the struct act it works fine.
Can anyone take a look at it, I am not able to understand what the problem is.
Thanks in advance!!
/* Simplest dead child cleanup in a SIGCHLD handler. Prevent zombie processes
but dont actually do anything with the information that a child died. */
#include <sys/types.h>
#include <sys/wait.h>
#include <signal.h>
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
typedef char *string;
/* SIGCHLD handler. */
static void sigchld_hdl (int sig)
{
/* Wait for all dead processes.
* We use a non-blocking call to be sure this signal handler will not
* block if a child was cleaned up in another part of the program. */
while (waitpid(-1, NULL, WNOHANG) > 0) {
}
}
int main (int argc, char *argv[])
{
struct sigaction act;
int i;
int nbytes = 100;
char my_string[nbytes];
string arg_list[5];
char *str;
memset (&act, 0, sizeof(act));
act.sa_handler = sigchld_hdl;
if (sigaction(SIGCHLD, &act, 0)) {
perror ("sigaction");
return 1;
}
while(1){
printf("myshell>> ");
gets(my_string);
str=strtok(my_string," \n");
arg_list[0]=str;
i =1;
while ( (str=strtok (NULL," \n")) != NULL){
arg_list[i]= str;
i++;
}
if (i==1)
arg_list[i]=NULL;
else
arg_list[i+1]=NULL;
pid_t child_pid;
child_pid=fork();
if (child_pid == (pid_t)-1){
printf("ERROR OCCURED");
exit(0);
}
if(child_pid!=0){
printf("this is the parent process id is %d\n", (int) getpid());
printf("the child's process ID is %d\n",(int)child_pid);
}
else{
printf("this is the child process, with id %d\n", (int) getpid());
execvp(arg_list[0],arg_list);
printf("this should not print - ERROR occured");
abort();
}
}
return 0;
}

Answer 1

I haven't run your code, and am merely hypothesizing:
SIGCHLD is arriving and interrupting fgets (I'll just pretend you didn't use gets). fgets returns before actually reading any data, my_string contains the tokenized list that it had on the previous loop, you fork again, enter fgets, which is interrupted before reading any data, and repeat indefinitely.
In other words, check the return value of fgets. If it is NULL and has set errno to EINTR, then call fgets again. (Or set act.sa_flags = SA_RESTART.)