background.cpp

#include "headers.h"

// PIDs of jobs started with a trailing '&'. Reaped in reapBackgroundProcesses().
static vector<pid_t> backgroundPids;

static char **buildArgv(const vector<string> &args)
{
    char **argv = new char *[args.size() + 1];
    for (size_t i = 0; i < args.size(); ++i)
    {
        argv[i] = new char[args[i].size() + 1];
        strcpy(argv[i], args[i].c_str());
    }
    argv[args.size()] = nullptr;
    return argv;
}

void executeBackgroundProcess(vector<string> args, bool runInBackground, int &foregroundJobPid, bool &foregroundJobRunning)
{
    if (args.empty())
        return;

    pid_t pid = fork();

    if (pid == 0)
    {
        // Child: restore default signal handling before exec.
        signal(SIGINT, SIG_DFL);
        signal(SIGTSTP, SIG_DFL);

        if (runInBackground)
        {
            // Detach from the terminal so background jobs don't steal input
            // or get killed by Ctrl-C/Ctrl-Z aimed at the shell.
            setpgid(0, 0);
        }

        char **argv = buildArgv(args);
        execvp(argv[0], argv);
        cerr << args[0] << ": command not found" << endl;
        _exit(127);
    }
    else if (pid > 0)
    {
        if (!runInBackground)
        {
            foregroundJobPid = pid;
            foregroundJobRunning = true;

            int status = 0;
            while (waitpid(pid, &status, WUNTRACED) == -1 && errno == EINTR)
                ;

            foregroundJobRunning = false;
            foregroundJobPid = 0;

            if (WIFSTOPPED(status))
            {
                backgroundPids.push_back(pid);
                cout << "[stopped] " << pid << " " << args[0] << endl;
            }
        }
        else
        {
            backgroundPids.push_back(pid);
            cout << "[bg] " << pid << endl;
        }
    }
    else
    {
        perror("fork");
    }
}

void reapBackgroundProcesses(int signum)
{
    (void)signum;
    for (size_t i = 0; i < backgroundPids.size();)
    {
        int status = 0;
        pid_t result = waitpid(backgroundPids[i], &status, WNOHANG | WUNTRACED | WCONTINUED);

        if (result == 0)
        {
            ++i; // still running
        }
        else if (result == -1)
        {
            backgroundPids.erase(backgroundPids.begin() + i); // already gone
        }
        else if (WIFEXITED(status) || WIFSIGNALED(status))
        {
            cout << backgroundPids[i] << " done" << endl;
            backgroundPids.erase(backgroundPids.begin() + i);
        }
        else
        {
            ++i; // stopped/continued: keep tracking
        }
    }
}