This has reference to LDD3 chap3 - Char drivers (Pg 60). The following is my understanding which seems to be incorrect. Can somebody please point out what am I missing? LDD3 states that the fops->open() and fops->release() methods are called only when "struct file" is created / destroyed. It further says that "struct file" is created ONLY when a process calls open() system all on a file. To give an example, it also says that if process 1 opens file A and then forks another process B, then A and B will share the same struct file?? GD> This is correct. whenever a process opens the file, corresponding "struct file" structure is created in kernel. As you might be knowing that this is just a virtual file in kernel which keeps track of attributes of actual file, like where to read/write, pointer to file operation structures etc. Theseattributes makes sense when a file is opened, till the file is not closed, at which time fops->release() function is called. This seems quite strange to me as I had an understanding that two processes working on the same file will always have different file structure. Also, this would mean that a read by one of the processes on the file would affect the file position pointer in the other process? GD> Wherever a process forks another process all the "struct file" strucutres are shared between both processes unless and untill any one of those processes does not write to it at which time a seperate copy is created, this is also know as COW (Copy On Write). When a process is forked, no other "struct file" structure is created, rather the refference count for the "struct file" structures are incremented and there pointers are also saved in new process's file table (struct file_struct).
Hi Gaurav, Yes I understand the CoW procedure. So what you are syaing that when the process A forks process B, both of them point to same "struct file" (with a ref count of 2). What I am wondering is that what happens once both the processes actually start writing to those files. 1) In case a seperate copy is created (CoW) at this point for the forked thread, is open() method of the driver called? 2) In case both the processes continue to use the same "struct file" wouldn't reads of one file affect the reads of another (since the location ptr in file will be affected)? Thanks, Rajat - To unsubscribe from this list: send the line "unsubscribe linux-newbie" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.linux-learn.org/faqs
