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Organization of the file system

processes at all times while the file remains open to the process. But it was not flexible and would be inconsistent with the nature of the task. The main ways the temporary seizure of the means of a file or record (part of the file between the positions), as well as blocking some kernel structures responsible for working with files. Thus, the OS Unix system call while conducting a transaction file, usually occurs blocking index node containing the addresses of file data blocks. It would seem that the organization holds or prohibiting more than one process to work with the file at runtime system call was unnecessary, because in the vast majority of cases, implementation of system calls and so is not interrupted, that is the kernel works in nevytesnyayuschey multitasking. But in this case, this is not quite the case. Operation reading and writing took a long time and only come central processor, as determined by the independent channels, so installing locks at the time of system call is a necessary guarantee atomicity of operations reading and writing. The systemic challenge to install and test lock on the file is always an issue with modern multi-OS. In principle, it would be logical to link the file synchronization as a whole system with a challenge to open (ie, such as opening a file on the recording mode or renewal could mean its monopoly lock appropriate process, and the opening line-reading together lock). So there are many operating systems (OS starting with Multics). In the Unix operating system that is not the case, which has historical causes. The first version of Unix developed by Thompson and Ritchie, the mechanism for the capture file absent. Apply a very simple approach to the parallel (from multiple processes) to access files: the system allows any number of processes at the same time open the same file in any mode (read, write or update), and made no sinhronizatsionnyh action. The entire responsibility for the correctness of joint processing burden on the file using its processes, and the system has not even provided any special funds for the synchronization of access to the file. But then, in order to increase the attractiveness of the system for commercial users working with databases, in the version of System V have been included mechanisms capture and record the file-based system call fcntl. Allowed two options synchronization: on the expectation when the locking requirement may result in postponing the process until this requirement can be met, and without waiting, immediately notified when the process of meeting the requirements of blocking or her inability to meet at this point in time. Installed locking refer only to the process, which they found and not inherited protsessami-potomkami the process. Moreover, even if the process takes some opportunities sinhronizatsionnymi Call fcntl system, other processes can still work with the file without any synchronization. In other words, the case group processes, file sharing, to agree on a way to synchronize concurrent access.
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