Open System Call

Arguments: Filename (String)

Return Value:

File Descriptor (Integer)	Success, the return value is the file descriptor for the opened file.
-1	File not found or file is not a data or root file
-2	System has reached its limit of open files
-3	Process has reached its limit of resources

Description : For a process to read/write a file, it must first open the file. Only data and root files can be opened. The Open operation returns a file descriptor which identifies the open instance of the file. An application can open the same file several times and each time, a different descriptor will be returned by the Open operation.

The OS associates a seek position with every open instance of a file. The seek position indicates the current location of file access (read/write). The Open system call initilizes the seek position to 0 (beginning of the file). The seek position can be modified using the Seek system call.

The root file can be opened for Reading by specifying the filename as "root". Note that the Root file is different from the other files - It has a reserved memory page copy. So this will be treated as a special case in all related system calls.

	Set the MODE_FLAG in the process table entry to 2, 
	indicating that the process is in the open system call.

	//Switch to Kernel Stack - See Kernel Stack Management during System Calls. 
	Save the value of SP to the USER SP field in the Process Table entry of the process.
	Set the value of SP to the beginning of User Area Page.

 	Find a free Per-Process Resource Table entry. 

                Find the PID of the current process from the System Status Table.
                Find the User Area page number from the Process Table entry.
                The  Per-Process Resource Table is located at the  RESOURCE_TABLE_OFFSET from the base of the  User Area Page .              
		Find a free Resource Table entry.  
		If there is no free entry, return -3.  
 
	Call the open() function from the File Manager module to get the Open File table entry.
		
	If Open fails, return the error code.

 	Set the Per-Process Resource Table entry             

		Set the Resource Identifier field to FILE . 
		Set the Open File Table index field to the free Open File Table entry found.	     

	Set the MODE_FLAG in the process table entry to 0.

	Restore SP to User SP.

	Return the index of the Per-Process Resource Table entry.   /* success */
	/* The index of this entry is the File Descriptor of the file. */

	Note:  At each point of return from the system call, remember to reset the MODE FLAG and switch back to the user stack.
     

Questions

1. Why must a free Per Process Resource Table entry be found before calling the open() module function?

2. Why should the "root" file be treated seperately? Where is this change implimented for the open system call?

3. Why do we maintain OPEN INSTANCE COUNT in Open File table and FILE OPEN COUNT in File Status table? Why do we need two tables?