In this post, I added some interesting programs written in C to implement some commands .
Our very first program is to implement a Command Interpreter. The idea to implement this is simple. Just take the input command from the user and call exec in a forked child and print the output. The code is as follows:
#include<stdio.h>
#include<signal.h>
#include<unistd.h>
#include<stdlib.h>
#include<string.h>
#include<sys/types.h>
#include<fcntl.h>
#include<sys/stat.h>
#define MAX_ARGS 5
void runcmd()
{
int c=0, take=0, give=0;
char *i[MAX_ARGS],g;
char *input_file=(char*)malloc(12*sizeof(char));
char *output_file=(char*)malloc(12*sizeof(char));
for(int j=0; j<MAX_ARGS; j++) i[j]=(char*)malloc(9*sizeof(char));
do
{
/*read all the input into a char double pointer*/
scanf("%s",i[c++]);
if(!strcmp(i[c-1], ">") || !strcmp(i[c-1], "<"))
{
if(!strcmp(i[c-1], ">"))
{
/*if the symbol is '>' then read the output
* file-name to which the data must be sent
*/
scanf("%s",output_file);
give=1;
}
else
{
/*if the symbol is '<', then we are gonna
*have an input file to take data from
*/
scanf("%s", input_file);
take=1;
}
c--;
}
/*read the nxt char of every string if its '\n'
* then terminate the loop..
*/
g=getc(stdin);
}while(g!='\n');
i[c]=NULL;
pid_t h;
if((h=fork()) < 0) printf("fork error\n");
else if(h==0)
{
if(give==1)
{
int fd1 = open(output_file, O_RDWR | O_CREAT, S_IRUSR | S_IWUSR|S_IRGRP);
dup2(fd1,1);
}
else if(take==1)
{
int fd = open(input_file, O_RDWR , S_IRUSR | S_IWUSR|S_IRGRP);
if(fd==-1)
{
printf("unsupported file or file doesn't exists\n");
return;
}
dup2(fd,STDIN_FILENO);
}
execvp(i[0], i);
/*if the control transfers here, it means
*execvp has not executed successfully
*/
printf("unknown command\n");
}
else
{
waitpid(h, NULL, NULL);
/*wait for the child to terminate*/
}
}
int main(int argc, char* argv[])
{
while(1)
{
printf("-->");
/*keep on calling this function*/
runcmd();
}
exit(0);
}
The above program works for most of the commands and I also provide the stream re-direction facility, to transfer the data into/from files.
The second program is to implement the ps command. For those who don't know what this does, it simply displays the list of the processes running from the current terminal. If you append an argument '-A', it gives the list of all the processes running on the system. Our task is to implement the later one.
The idea is to open the proc directory. This contains numbered directories and some other directories. Each numbered directory denotes a running process and the number on the directory represents its process-ID.
Inside every numbered directory lies a file called "stat". This contains the information that is needed to display the necessary contents. For example, the stat file of init process is as follows:
nitish@nitish:~$ cat /proc/1/stat
1 (init) S 0 1 1 0 -1 4202752 14453 339956 23 20 19 35 257 129 20 0 1
0 3 25030656 613 18446744073709551615 1 1 0 0 0 0 0 4096 536962595
18446744073709551615 0 0 17 0 0 0 41 0 0 0 0 0 0 0 0 0 0
The first of these is the init's process ID, second is the name of the process, third is the process status, fourth is the parent process ID(scheduler in this case).
These strings will be displayed for each numbered directory and then we are done. The C code is as follows:
#include<stdio.h>
#include<stdlib.h<
#include<dirent.h<
#include<string.h<
#include<sys/types.h<
#include<unistd.h<
int main(int argc, char *argv[])
{
DIR *d;
char *t;
int N=0,pid;
int cnt=0;
int curr[10], curr_cnt=0;
struct dirent *fl;
d=opendir("/proc");
int M=getppid();
printf("Process ID of the terminal : %d\n", M);
printf("Format: PID / PNAME / STATE / PPID \n");
while((fl=readdir(d))!=NULL)
{
int num=atoi(fl->d_name);
N=0;
if(num!=0)
{
cnt++;
t= (char*)malloc(15*sizeof(char));
strcpy(t,"/proc/");
strncat(t,fl->d_name,10);
strncat(t,"/stat",6);
FILE *ptr=fopen(t,"r");
if(ptr==NULL)
printf("error\n");
char tt[20];
char g;
/**process id**/
while((g=getc(ptr))!=' ')
N=N*10 + (g-48);
pid=N;
printf("%d / ", N);
g=getc(ptr);
/**process name**/
while((g=getc(ptr))!=')' )
{
printf("%c", g);
}
printf(" / ");
g=getc(ptr);
g=getc(ptr);
if(g=='S')
printf("Interruptible sleep / ");
else if(g=='D')
printf("Uninterruptible sleep / ");
else if(g=='R')
printf("Runnable / ");
else if(g=='T')
printf("Stopped / ");
else if(g=='X')
printf("Dead / ");
else if(g=='Z')
printf("Zombie / ");
else if(g=='W')
printf("Paging / ");
g=getc(ptr);
N=0;
while((g=getc(ptr))!=' ')
N = N*10 + (g-48);
printf("%d" , N);
if(N==M)
{
printf("**\n");
curr[curr_cnt++]=pid;
}
fclose(ptr);
printf("\n");
}
}
printf("**\nTOTAL PROCESSES:%d\n**\n", cnt);
printf("**********\n");
printf("Processes inside the terminal:%d\n",curr_cnt );
for(int i=0; i<curr_cnt; i++)
printf("%d ", curr[i]);
printf("\n");
exit(0);
}
If you want to display only the processes specific to the current terminal, you need to just store all those process IDs which have parent process ID as terminal. Sounds simple isn't it?
The normal ps command displays some other information like tty and command name and these can also be found in each numbered directory which I leave it as an exercise.
The extension of this would be to display all the PIDs, hierarchically starting with the init process and also to find all the file names opened by a given process(of course you must explore the proc directory a bit to this one).
Lastly, I would like to post the code for the thread library, which I have described in my previous posts. Here is the code. (Don't panic by seeing the length, most of them were for indentations and comments).
#include<stdio.h>
#include<stdlib.h>
#include<signal.h>
#include<sys/types.h>
#include<sys/time.h>
#include<string.h>
#include<errno.h>
#include<ucontext.h>
#include "thqueue.c"
#define MAX_THREADS 10
#define STACK_SIZE 64000
#define TIME_SLOT 500000 //time slot given to each thread in round robin scheduling (0.25sec here..)
typedef struct Task* Tptr;
int CURRENT_THREAD_COUNT=0;
int TERMINATED=0;
int Global_id;
ucontext_t Main; //context for saving main...
ucontext_t Delete[MAX_THREADS]; //context for thread recycling...
int alloc[MAX_THREADS];
struct itimerval alarm;
struct sigaction sched_signal;
struct Tque ready_queue;
struct Task *Current_Thread=NULL;
void schedule()
{
//cs printf("activated\n");
if(Current_Thread==NULL) // Main Thread was executing previously..
{
TERMINATED=0;
if(!isempty(&ready_queue)) // If there are more threads...
{
Current_Thread=top(&ready_queue);
//c printf("yes!! %p\n", Current_Thread);
Global_id=Current_Thread->thread_id;
swapcontext(&Main, &(Current_Thread->threads_context));
}
else // If all threads are dead...
{
//c printf("main is getting set!\n");
setcontext(&Main);
}
}
else //if someother thread was executing...
{
struct Task* tmp=next(&ready_queue, Current_Thread);
//c printf("other way around and %p and current:%p\n", tmp, Current_Thread);
if(tmp==NULL) //if this was the last thread in the queue....
{ //execute main fn.
//c printf("it was null\n");
if(TERMINATED==1)
{
//c printf("its gone!!\n");
TERMINATED=0;
remov(&ready_queue, Global_id);
Current_Thread=NULL;
setcontext(&Main);
}
else
{
struct Task* tmp1=Current_Thread;
Current_Thread=NULL;
swapcontext(&(tmp1->threads_context), &Main);
}
}
else
{
struct Task* tmp2=Current_Thread;
Current_Thread=tmp;
if(TERMINATED==1)
{
TERMINATED=0;
remov(&ready_queue, Global_id);
Global_id=tmp->thread_id;
//c printf("context set for %p\n", tmp);
setcontext(&(tmp->threads_context));
}
else
{
Global_id=tmp->thread_id;
//c printf("running:%p\n", tmp);
swapcontext(&(tmp2->threads_context), &(tmp->threads_context));
}
}
}
}
void Kill_Task(void *arg)
{
//c printf("its coming!!\n");
int t_id=*((int*)arg);
//c printf("terminator for %d started\n", Global_id);
TERMINATED=1;
schedule();
}
void Init_Task()
{
sched_signal.sa_handler=schedule;
sigemptyset(&sched_signal.sa_mask);
sched_signal.sa_flags=0;
sigaction(SIGPROF, &sched_signal, NULL);
ready_queue.count=0;
int i=-1;
while(++i<MAX_THREADS)
alloc[i]=0;
//for the first time alarm after 0 sec....
//for every TIME_SLOT secs...
alarm.it_value.tv_sec=0;
alarm.it_value.tv_usec=TIME_SLOT;
//next time alarm after every TIME_SLOT secs...
alarm.it_interval.tv_sec=0;
alarm.it_interval.tv_usec=TIME_SLOT;
//c printf("Initialized\n");
}
void Init_Semaphore(struct Task_Semaphore* mut, int value)
{
mut->count=value;
}
void change_Semaphore(struct Task_Semaphore* mut, int value)
{
sigset_t old, new;
sigemptyset(&new);
sigaddset(&new, SIGPROF);
sigprocmask(SIG_BLOCK, &new, &old);
if(mut->count > value)
{
mut->count-=value;
sigprocmask(SIG_SETMASK, &old, NULL);
return;
}
else
{
insert(&(mut->wait_queue), rem_and_poi(&ready_queue, Current_Thread));
sigprocmask(SIG_SETMASK, &old,NULL);
schedule();
}
}
void create_task(struct Task* target, void (*fn)(void*), void * param)
{
if(getcontext(&(target->threads_context))==-1)
{
printf("getcontext error!\n");
exit(1);
}
(target->threads_context).uc_stack.ss_sp=malloc(STACK_SIZE);
(target->threads_context).uc_stack.ss_size=STACK_SIZE;
(target->threads_context).uc_stack.ss_flags=0;
int id=-1;
while(++id<10 && alloc[id]!=0);
if(id==10)
{
printf("allocation error\n");
exit(1);
}
target->thread_id=id;
alloc[id]=1;
//Delete[id]=malloc(sizeof(ucontext_t));
//c printf("yyyy\n");
if(getcontext(&Delete[id])==-1)
{
printf("delete context error!\n");
exit(1);
}
Delete[id].uc_link=0;
Delete[id].uc_stack.ss_sp=malloc(STACK_SIZE);
Delete[id].uc_stack.ss_size=STACK_SIZE;
Delete[id].uc_stack.ss_flags=0;
(target->threads_context).uc_link=&Delete[id];
makecontext(&Delete[id], (void*)&Kill_Task, 0);
makecontext(&(target->threads_context), fn, 0);
target->next=NULL;
//if this is the first thread, then set the timer...
if(CURRENT_THREAD_COUNT++==0)
{
if(setitimer(ITIMER_PROF, &alarm, NULL)==-1)
{
//some shit is happening with setitimer
printf("Itimer error:%s", strerror(errno));
exit(1);
}
//c printf("timer set!\n");
}
//thread should be added to queue...
//so block all the SIGPROF signal until this is added....
sigset_t new,old;
sigemptyset(&new);
sigaddset(&new, SIGPROF);
sigprocmask(SIG_BLOCK, &new, &old);
if(isempty(&ready_queue)!=0)
{
//c printf("first added to queue %p\n", target);
insert(&ready_queue, target);
}
else
{
//c printf("next added to queue %p\n", target);
insert(&ready_queue, target);
}
//sleep(5);
//restore back the old mask...
sigprocmask(SIG_SETMASK, &old, NULL);
}
int Sigh=0;
void sample()
{
printf("success!!\n");
while(Sigh==0);
printf("completed!!\n");
//setcontext(&Main);
}
void samps()
{
printf("success2\n");
Sigh=1;
}
void sam3()
{
printf("started3\n");
//while(1);
printf("success3\n");
}
int main(int argc, char *argv[])
{
Init_Task();
struct Task t,p,q;
int a=5;
create_task(&t, (void*)&sample, (void*)&a);
create_task(&p, (void*)&samps, (void*)&a);
create_task(&q, (void*)&sam3, (void*)&a);
//getcontext(&Main);
printf("yeasss\n");
while(1);
//exit(0);
}
The associated "thque.c" is as follows:
struct Task
{
int user_priority;
ucontext_t threads_context;
struct Task *next;
int thread_id;
};
struct Tque
{
struct Task *head;
struct Task *tail;
int count;
};
struct Task_Semaphore
{
struct Tque wait_queue;
int count;
};
void display(struct Tque *u)
{
struct Task *tmp=u->head;
while(tmp!=NULL)
{
printf("%p ", tmp);
tmp=tmp->next;
}
printf("\n");
}
void insert(struct Tque *q, struct Task *t)
{
if(q->count==0)
{
t->next=NULL;
q->head=q->tail=t;
}
else
{
t->next=NULL;
(q->tail)->next=t;
q->tail=(q->tail)->next;
}
q->count++;
//c printf("insert:");
// display(q);
}
void remov(struct Tque *q, int id)
{
struct Task *tmp=q->head, *prev;
while(tmp!=NULL && tmp->thread_id!=id)
{
prev=tmp;
tmp=tmp->next;
}
if(tmp!=NULL)
{
//c printf("getting deleted\n");
if(tmp==q->head)
{
//c printf("first one!\n");
q->head=(q->head)->next;
//free(tmp);
}
else
{
prev->next=(prev->next)->next;
//free(tmp);
}
q->count--;
}
else
{
printf("node with id=%d not found!\n",id);
}
//c printf("delete:");
// display(q);
}
int isempty(struct Tque *q)
{
//c printf("count:%d\n",q->count );
return q->count==0;
}
struct Task* top(struct Tque* q)
{
if(q!=NULL)
{
//c printf("correct!\n");
return q->head;
}
else
{
return NULL;
}
}
struct Task* next(struct Tque* q, struct Task* cur)
{
return cur->next;
}
struct Task* rem_and_poi(struct Tque* q, struct Task* to)
{
struct Task *tmp=q->head, *tmp1=NULL;
while(tmp!=to)
{
tmp1=tmp;
tmp=tmp->next;
}
if(tmp1==NULL) //node is at head...
{
q->head=tmp->next;
return tmp;
}
else //node is in middle or may be end...
{
tmp1->next=tmp->next;
return tmp;
}
}
Note that in that thread library, the mutexes and sempahores weren't implemented fully.
