Open File Table Tutorial - 3 смотреть онлайн

To understand the need of open file table in the File System Interface module of Operating System.
see the complete series File System Implementation https://www.youtube.com/playlist?list=PLskQvPDUk0sKgGGReUz7nMwYkDTbevUC1
File System Interface https://www.youtube.com/playlist?list=PLskQvPDUk0sLsU1Vv5jSr-P-1yTt0AqdJsee the complete series File System Implementation https://www.youtube.com/playlist?list=PLskQvPDUk0sKgGGReUz7nMwYkDTbevUC1
File System Interface https://www.youtube.com/playlist?list=PLskQvPDUk0sLsU1Vv5jSr-P-1yTt0AqdJ
see the complete series Operating System-process synchronization at https://www.youtube.com
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Operating System-Memory Management
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Most of the file operations require a search in the directory with the specified file name. To avoid this constant searching in the directory which resides on the disk the operating system maintains a table called open file table which contains information about all the open files.

When is file is referred to by a process an open system call is used. Some system implicitly open the file in some other system the programmer should explicitly open the file with the open system call.This open operation search the directory with the specified filename and the information about the file are copied to the open file table.

But the same file can be used by many processes at the same time and there are certain information which are process specific.For example the current file position pointer which point to the location where the next read or write should take place .Thus a per process table is also maintained which keeps track of all the files opened by the particular process.

When the file is referred to by any process for the first time an open file table entry is created for it and a per process table entry is also created. The process table entry will be containing a pointer to the corresponding file entry in the system wide table. And if the same file been opened by another process at the same time an entry is created in its process table with a pointer pointing to the same entry in the system table. The other information in the process table is the file pointer which points to the location where the next read or write should take place by the process on that file.

So before a process is active on a file it has to be opened and if the file is not at all open in the system ,a system table entry and per process table entry is created.And if already opened by any process a process specific entry is created and a pointer to that entry will be returned by the open call .Later on this pointer will be used avoiding any further search.

Open call will return a pointer which point to the corresponding entry in the per process table which contains information regarding the activity of that process on that file, access right information and a pointer to the file entry in the system table. That country contains other Global information regarding the file such a disk location,size of the file and so on. That one other information is the file open count. When a file is opened by any process, file open count is incremented. When a file is closed the per process table entry if freed and counter in the system wide table is decremented. When the file open count reaches zero system wide table entry is also freed.