bg.c

#include "headers.h"

void bg(int argc, char** argv) {
    if(argc != 2) {
        fprintf(stderr, "bg: Syntax error, bg command takes 1 argument.\n");
        return;
    }

    errno = 0;

    char *str, *endptr;
    long val;

    str = argv[1];
    val = strtol(str, &endptr, 10);

    if ((errno == ERANGE && (val == LONG_MAX || val == LONG_MIN)) || (errno != 0)) {
        perror("bg: Invalid arguments, either not integer or overflow.");
        return;
    } 

    if (endptr == str || *endptr != '\0') {
        fprintf(stderr, "bg: Characters other than numbers found.\n");
        return;
    }

    if(val <= 0) {
        fprintf(stderr, "bg: Invalid job no., since input is <= 0.\n");
        return;
    }

// ----------------------------------------------------------------------------------

    int job_pid = 0;
    int pid, status, job_no = 0;
    char* job_name = (char*) calloc(const_size, sizeof(char));

    struct Node* p = head;

    while(p != NULL) {

        pid = waitpid(p->pid, &status, WNOHANG);
        
        if(pid == 0) { 
            job_no++;

            if(job_no == val) {
                job_pid = p->pid;
                strcpy(job_name, p->name);
                break;
            }
        }
        
        p = p->next;
    }

    if(p == NULL) {
        fprintf(stderr, "bg: Invalid Job Number.\n");
        return;
    }

// ----------------------------------------------------------------------------------

    // continue the process from stopped state
    if(kill(job_pid, 18) < 0) {
        perror("fg: Execution of command failed.");
        return;
    }
}