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What Are System Calls

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Today, we want to discuss about system calls

What  are System Calls


•    System Calls provide the Interface between a process and the Operating System.
•    These calls are generally available as Assembly language instruction.
•    System Calls can also be made directly through HLL  programs for certain systems.
•    UNIX System calls can be invoked directly from a C or C++ program.
figure 1: structure of system calls

An Example of how system calls are used.


  • Consider a simple program to copy data from one file to another file .
  • The first input of the program will be the name of both files.
  • These names can be specified depending on the design of Operation System
  • Once the file names are obtained, the program opens an input file and  create an output file
  • Each of this operations require other System calls and may encounter possible error    conditions.




How System Calls Occur

System Calls occur in different ways depending on the computer in use:

  • More information is required than just the identify of the desired System call.
  • The exact type and amount of information varies according to the operation system and call
  • For instance to get an input we need to specify the file of device to use as source and the address and length of memory buffer into which the input should be read.

Figure 2: System Calls occur in different ways depending on the computer

Categories of System Calls

System calls can be grouped into five major categories as follows.

  • Process control
  • File management.
  • Device management
  • Information Maintenance and
  • Communication.

Some system calls under process control are:


  • End, abort
  • Load, execute
  • Create process, terminate process
  • Get process, terminate process
  • Wait for time
  • Allocate and free memory

figure 3: system calls under process control

Some system calls under file management are:


  • Create file, delete file
  • Open , close
  • Read, write, re position.
  • Get file attributes, set file attributes

Some system calls under device management are:


  • Request Device, release device
  • Read, write, re position.
  • Get device attributes and set device attributes
  • Logically attach or detach devices


Figure 4: system calls under device management

Some system calls under information maintenance are:

•    Get time or date, Set time of date
•    Logically attach or detach devices
•    Information maintenance
•    Get system data, Set Systems  data
•    Get process, file of device attributes
•    Set process, file or device attributes

Some system calls under communication are:

•    Create, delete communication connection.
•    Send, receive messages
•    Transfer status information
•    Attach or detach remote devices.




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