I / Concept about System calls

Image result for system call interface

In computing, a system call is the programmatic way in which a computer program requests a service from the kernel of the operating system it is executed on. This may include hardware-related services (for example, accessing a hard disk drive), creation and execution of new processes, and communication with integral kernel services such as process scheduling. System calls provide an essential interface between a process and the operating system.

In most systems, system calls can only be made from userspace processes, while in some systems, OS/360 and successors for example, privileged system code also issues system calls

II / File

The other broad category of system calls relates to the file system . As noted before , a major function of the operating system is to hide the peculiarities of the disks and other I/O devices and present the programmer with nice . System calls are obviously needed to create files , remove files , read files and write files . Before a file can read , it must be opened , and after it has been read it should be closed , so calls are provided to do these things .

To provide a place to keep files , MINIX 3 has the concept of a directory as a way of grouping files together . A student , for example , might have one directory for each course he is taking ( for the programs needed for that course ) , another directory for his electronic mail , and still another directory for his World Wide Web home page . System calls are then needed to create and remove directories . Calls are also provided to put an existing into a directory , and to remove a file from a directory . Directory entries may be either files or other directories .

Image result for root directory

The process and file hierarchies both are organized as trees , but the similarity stops there . Process hierarchies usually are not very deep ( more than three levels is unusual ) , whereas file hierarchies are commonly four , file , or even more levels deep . Process hierarchies are typically short-lived , generally a few minutes at most , whereas the directory hierarchy may exist for years . Ownership and protection also differ for processes and files . Typically , only a parent  process may control or even access a child process , but mechanisms nearly always exist to allow files and directories to be read by a wider group than just the owner .

Every file within the directory hierarchy can be specified by giving its path name from the top of the directory hierarchy, the root directory. Such absolute path names consist of the list of directories that must be traversed from the root directory to get to the file, with slashes separating the components .

Files and directories in MINIX 3 are protected by assigning each one an 11-bit binary protection code. The protection code consists of three 3-bit fields: one for the owner, one for other members of the owner’s group (users are divided into groups by the system administrator), one for everyone else, and 2 bits we will discuss later. Each field has a bit for read access, a bit for write access, and a bit for execute access. These 3 bits are known as the rwx bits. For example, the protection code rwxr-x–x means that the owner can read, write, or execute the file, other group members can read or execute (but not write) the file, and everyone else can execute (but not read or write) the file. For a directory (as opposed to a file), x indicates search permission. A dash means that the corresponding permission is absent (the bit is zero).

Before a file can be read or written, it must be opened, at which time the permissions are checked. If access is permitted, the system returns a small integer called a file descriptor to use in subsequent operations. If the access is prohibited, an error code (1) is returned.

Another important concept in MINIX 3 is the mounted file system. Nearly all personal computers have one or more CD-ROM drives into which CD-ROMs can be inserted and removed. To provide a clean way to deal with removable media (CD-ROMs, DVDs, floppies, Zip drives, etc.), MINIX 3 allows the file system on a CD-ROM to be attached to the main tree. Consider the situation of figure below . Before the mount call, the root file system, on the hard disk, and a second file system, on a CD-ROM, are separate and unrelated.

Image result for before mounting, the files on drive 0 are not accessible. (b) After mounting, they are part of the file hierarchy.

However, the file system on the CD-ROM cannot be used, because there is no way to specify path names on it. MINIX 3 does not allow path names to be prefixed by a drive name or number; that is precisely the kind of device dependence that operating systems ought to eliminate. Instead, the mount system call allows the file system on the CD-ROM to be attached to the root file system wherever the program wants it to be. In Fig (b) the file system on drive 0 has been mounted on directory b, thus allowing access to files /b/x and /b/y. If directory b had originally contained any files they would not be accessible while the CD-ROM was mounted, since /b would refer to the root directory of drive 0. (Not being able to access these files is not as serious as it at first seems: file systems are nearly always mounted on empty directories.) If a system contains multiple hard disks, they can all be mounted into a single tree as well.

Another important concept in MINIX 3 is the special file. Special files are provided in order to make I/O devices look like files. That way, they can be read and written using the same system calls as are used for reading and writing files. Two kinds of special files exist: block special files and character special files. Block special files are normally used to model devices that consist of a collection of randomly addressable blocks, such as disks. By opening a block special file and reading, say, block 4, a program can directly access the fourth block on the device, without regard to the structure of the file system contained on it. Similarly, character special files are used to model printers, modems, and other devices that accept or output a character stream. By convention, the special files are kept in the /dev directory. For example, /dev/lp might be the line printer.

Image result for Two processes connected by a pipe.

The last feature we will discuss in this overview is one that relates to both processes and files: pipes. A pipe is a sort of pseudofile that can be used to connect two processes . If processes A and B wish to talk using a pipe, they must set it up in advance. When process A wants to send data to process B, it writes on the pipe as though it were an output file. Process B can read the data by reading from the pipe as though it were an input file. Thus, communication between processes in MINIX 3 looks very much like ordinary file reads and writes. Stronger yet, the only way a process can discover that the output file it is writing on is not really a file, but a pipe, is by making a special system call.






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