In this previous question I posted most of my own shell code. My next step is to implement foreground and background process execution and properly wait for them to terminate so they don't stay as "zombies".
Before adding the possibility to run them in the background, all processes were running in the foreground. And for that, I simply called wait(NULL) after executing any process with execvp(). Now, I check for the '&' character as the last argument and if it's there, run the process in the background by not calling wait(NULL) and the process can run happily in the background will I'm returned to my shell.
This is all working properly (I think), the problem now, is that I also need to call wait() (or waitpid() ?) somehow so that the background process doesn't remain "zombie". That's my problem, I'm not sure how to do that...
I believe I have to handle SIGCHLD and do something there, but I have yet to fully understand when the SIGCHLD signal is sent because I tried to also add wait(NULL) to childSignalHandler() but it didn't work because as soon as I executed a process in the background, the childSignalHandler() function was called and consequently, the wait(NULL), meaning I couldn't do anything with my shell until the "background" process finished. Which wasn't running on the background anymore because of the wait in the signal handler.
What am I missing in all this?
One last thing, part of this exercise I also need to print the changes of the processes status, like process termination. So, any insight on that is also really appreciated.
This is my full code at the moment:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <wait.h>
#include <signal.h>
#include <sys/types.h>
#include "data.h" // Boolean typedef and true/false macros
void childSignalHandler(int signum) {
//
}
int main(int argc, char **argv) {
char bBuffer[BUFSIZ], *pArgs[10], *aPtr = NULL, *sPtr;
bool background;
ssize_t rBytes;
int aCount;
pid_t pid;
//signal(SIGINT, SIG_IGN);
signal(SIGCHLD, childSignalHandler);
while(1) {
write(1, "\e[1;31mmyBash \e[1;32m# \e[0m", 27);
rBytes = read(0, bBuffer, BUFSIZ-1);
if(rBytes == -1) {
perror("read");
exit(1);
}
bBuffer[rBytes-1] = '\0';
if(!strcasecmp(bBuffer, "exit")) {
exit(0);
}
sPtr = bBuffer;
aCount = 0;
do {
aPtr = strsep(&sPtr, " ");
pArgs[aCount++] = aPtr;
} while(aPtr);
background = FALSE;
if(!strcmp(pArgs[aCount-2], "&")) {
pArgs[aCount-2] = NULL;
background = TRUE;
}
if(strlen(pArgs[0]) > 1) {
pid = fork();
if(pid == -1) {
perror("fork");
exit(1);
}
if(pid == 0) {
execvp(pArgs[0], pArgs);
exit(0);
}
if(!background) {
wait(NULL);
}
}
}
return 0;
}
This should get you started. The major difference is that I got rid of the child handler and added
waitpid
in the main loop with some feedback. Tested and working, but obviously needs more TLC.EDIT: Adding back in signal handling isn't difficult with
waitpid()
using WNOHANG. It's as simple as moving thewaitpid()
stuff from the top of the loop into the signal handler. You should be aware of two things, though:First, even "foreground" processes will send SIGCHLD. Since there can be only one foreground process you can simply store the foreground pid (parent's return value from
fork()
) in a variable visible to the signal handler if you want to do special handling of foreground vs. background.Second, you're currently doing blocking I/O on standard input (the
read()
at main loop top). You are extremely likely to be blocked onread()
when SIGCHLD occurs, resulting in an interrupted system call. Depending on OS it may restart the system call automatically, or it may send a signal that you must handle.There are various options to
waitpid()
to help you (quotes from the POSIX standard):In particular, WNOHANG will allow you to see whether there are any corpses to collect without causing your process to block waiting for a corpse.
You probably don't want to be ignoring SIGCHLD, etc, and your signal handler should probably be setting a flag to tell your main loop "Oops; there's dead child - go collect that corpse!".
The SIGCONT and SIGSTOP signals will also be of relevance to you - they are used to restart and stop a child process, respectively (in this context, at any rate).
I'd recommend looking at Rochkind's book or Stevens' book - they cover these issues in detail.
Instead of using a global variable, I thought of a different solution:
If I'm running a foreground process "delete" the handler for SIGCHLD so it doesn't get called. Then, after waitpid(), set the handler again. This way, only the background processes will be handled.
Do you think there's anything wrong with this solution?
You may use:
This way, the process blocks until it receives the
SIGCHLD
signal, and the signal handler will do the wait stuff.