System call implementation in Pintos

Question

I want to implement the already defined system calls in PintOS ( halt(), create()...etc defined in pintos/src/lib/user/syscall.c ). The current system call handler in pintos/src/userprog/syscall.c does not do anything. How do I make a process that makes system calls. Further I need to myself add a few system calls. How do I proceed in that too. But first I need to implement the existing system calls.

Answer 1

The default implementation in pintos terminates the calling process.
goto this link.There is explanation on where to modify the code to implement the system calls.

The "src/examples" directory contains a few sample user programs.
The "Makefile" in this directory compiles the provided examples, and you can edit it compile your own programs as well.
This program/process when run will inturn make a system call.
Use gdb to follow the execution of one such program a simple printf statement will eventually call write system call to STDOUT file.
The link given also has pointers on how to run pintos on gdb, my guess is you are using either bochs or qemu.In any case just run the gdb once with a simple hello world program running on pintos.
This will give u an idea of how the system call is made.

static void
syscall_handler (struct intr_frame *f)// UNUSED) 
{
  int *p=f->esp;
 switch(*p)
case *p=SYS_CREATE  // NUMBER # DEFINED

      const char *name=*(p+1); //extract the filename 
      if(name==NULL||*name==NULL)
            exit(-1);

      off_t size=(int32_t)*(p+2);//extract file size

      f->eax=filesys_create(name,size,_FILE); //call filesys_create
      //eax will have the return value
}

This is pseudo code for sys_create .. all file system related system call are very trivial, Filesys realted system calls like open read write close needs you to translate file to their corresponding fd (file descriptor). You need to add a file table for each process to keep track this, this can either be preprocess data or a global data.(UR choice),

case (*p==SYS_WRITE)
    {
    //  printf("wite syscall\n");
      char *buffer=*(p+2); 
     unsigned size=*(p+3);
     int fd=*(p+1);
      // getiing the fd of specified file       
     struct file *fil= thread_current()->fdtable[fd];/ my per thread fdtable
     if(fd==1)  goto here;

     if(is_directory(fil->inode)){
      exit(-1);
      goto done;
     }
 here:
  if(buffer>=PHYS_BASE)exit(-1);
  if(fd<0||fd>=128){exit(-1);}
  if(fd==0){exit(-1);} // writing to STDIN
  if(fd==1)     //writing to STDOUT
  {
    int a=(int)size;
    while(a>=100)
    {
      putbuf(buffer,100);
      buffer=buffer+100;
      a-=100;
    }
    putbuf(buffer,a);
    f->eax=(int)size;
  }
  else 
      if(thread_current()->fdtable[fd]==NULL)
        {f->eax=-1;}
     else
     {
        f->eax=file_write(thread_current()->fdtable[fd],buffer,(off_t)size);
     }
done: ;

  }//printf("write");}                  /* Write to a file. */

Open - adds anew entry to fdtable and return the fd number u give to the file,
close - remove that entry from fd table
read - similar to write.

The process_create ,wait are not simple to implement...

Cheers :)