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Unix system calls (1/2)
Unix system calls (1/2)
Published: 2011/03/24
Channel: Brian Will
Programming Interview: System Calls in operating system
Programming Interview: System Calls in operating system
Published: 2014/05/02
Channel: saurabhschool
Explanation-System calls and System call types in operating system
Explanation-System calls and System call types in operating system
Published: 2017/07/13
Channel: Last moment Learning
Linux Tutorial: How a Linux System Call Works
Linux Tutorial: How a Linux System Call Works
Published: 2012/05/04
Channel: The Linux Foundation
Programming Interview  System Calls in operating system
Programming Interview System Calls in operating system
Published: 2016/05/05
Channel: IncTutorial Education
System Calls  Going from User to OS Code
System Calls Going from User to OS Code
Published: 2016/06/06
Channel: Udacity
Understanding fork() system call for new process creation
Understanding fork() system call for new process creation
Published: 2017/01/10
Channel: Techtud
Assembly Language Tutorial #11 Making System Calls
Assembly Language Tutorial #11 Making System Calls
Published: 2010/02/28
Channel: Debojyoti Majumder
System Call
System Call
Published: 2014/06/02
Channel: IFRO Porto Velho Zona Norte
Morgan Phillips: Adding a system call to the Linux kernel
Morgan Phillips: Adding a system call to the Linux kernel
Published: 2014/08/28
Channel: Jason Orendorff
Services (System Calls) for Process Management | Creating, Termination, FORK/JOIN, ABORT etc.
Services (System Calls) for Process Management | Creating, Termination, FORK/JOIN, ABORT etc.
Published: 2017/02/28
Channel: Easy Engineering Classes
Implementing System Calls
Implementing System Calls
Published: 2014/04/29
Channel: David Evans
Following a system call to kernel space and back again
Following a system call to kernel space and back again
Published: 2015/07/10
Channel: Oskar Thorén
Mod-03 Lec-12 System Calls
Mod-03 Lec-12 System Calls
Published: 2011/09/14
Channel: nptelhrd
Video 14 :- FORK SYSTEM CALL Questions
Video 14 :- FORK SYSTEM CALL Questions
Published: 2016/07/16
Channel: GATE Lectures Computer Forum NCR
Read() system call
Read() system call
Published: 2016/05/30
Channel: fezan rasool
Fork() system call tutorial
Fork() system call tutorial
Published: 2014/12/13
Channel: holidaylvr
System Calls in Linux - Basics
System Calls in Linux - Basics
Published: 2012/05/08
Channel: Hari Maruthachalam
Lesson1b - Using strace to View System Calls
Lesson1b - Using strace to View System Calls
Published: 2016/01/13
Channel: Grok Linux
Unix system calls (2/2)
Unix system calls (2/2)
Published: 2011/03/24
Channel: Brian Will
How to add a System Call to Kernel 3.3.8
How to add a System Call to Kernel 3.3.8
Published: 2012/10/17
Channel: Benjamin Buzbee
W3 L3 system Calls for Process Management
W3 L3 system Calls for Process Management
Published: 2016/07/31
Channel: Introduction to Operating Systems
System Calls and his Types
System Calls and his Types
Published: 2016/07/22
Channel: ALI RAZA
System call tracing and system call injection on Linux
System call tracing and system call injection on Linux
Published: 2013/11/11
Channel: Arcopix
Linux Exec System Call
Linux Exec System Call
Published: 2015/02/06
Channel: DrBFraser
Illustration on fork() system call
Illustration on fork() system call
Published: 2017/01/10
Channel: Techtud
Video 13 :-  FORK SYSTEM CALL Theory
Video 13 :- FORK SYSTEM CALL Theory
Published: 2016/07/16
Channel: GATE Lectures Computer Forum NCR
fd, dup()/dup2() system call tutorial
fd, dup()/dup2() system call tutorial
Published: 2014/12/13
Channel: holidaylvr
Understanding usage of fork() System call in C program : Advanced Linux Programming # Tutorial - 6
Understanding usage of fork() System call in C program : Advanced Linux Programming # Tutorial - 6
Published: 2016/11/19
Channel: HowTo
Add Hello World System Call in Ubuntu 17.04
Add Hello World System Call in Ubuntu 17.04
Published: 2017/05/11
Channel: IT & Stuff
Execv() system call
Execv() system call
Published: 2016/05/22
Channel: Akshata Deore
Adding system Call in xv6
Adding system Call in xv6
Published: 2016/05/10
Channel: Ghanshyam Yadav
Telephone System Call Routing
Telephone System Call Routing
Published: 2010/12/17
Channel: Eli the Computer Guy
fork system call in Unix part 1
fork system call in Unix part 1
Published: 2017/06/06
Channel: Kashif Shariff
Operating systems Example 1.002 GATE CS 2012 (fork system call)
Operating systems Example 1.002 GATE CS 2012 (fork system call)
Published: 2013/06/05
Channel: Harshit Pande
Lecture 1: Introduction to UNIX System Calls Part 1
Lecture 1: Introduction to UNIX System Calls Part 1
Published: 2017/04/06
Channel: OsLectures ForAll
Fork() System call
Fork() System call
Published: 2015/08/04
Channel: Abhay Agrawal
OS Unit 2  Computer Organization, Structure of Operating Systems and System Calls
OS Unit 2 Computer Organization, Structure of Operating Systems and System Calls
Published: 2016/08/03
Channel: Vidya-mitra
strace - know and troubleshoot the system calls
strace - know and troubleshoot the system calls
Published: 2017/01/25
Channel: dolastack devops
xv6 -- Adding a system call
xv6 -- Adding a system call
Published: 2017/04/13
Channel: Tong Yu
Pipe() tutorial for linux
Pipe() tutorial for linux
Published: 2014/12/12
Channel: holidaylvr
Linux System Calls
Linux System Calls
Published: 2012/10/09
Channel: linuxkernellearning
Linux/ Unix Tutorial ||  System  calls in Linux OS by Shiva
Linux/ Unix Tutorial || System calls in Linux OS by Shiva
Published: 2016/11/10
Channel: Durga Software Solutions
Syscalls 4 Dummies "Lesson #3 Dup();"
Syscalls 4 Dummies "Lesson #3 Dup();"
Published: 2014/12/18
Channel: PixelPattern
Hijack linux system calls
Hijack linux system calls
Published: 2012/07/14
Channel: Hussein El-Sayed
W4 L3 Software Interrupts and System calls
W4 L3 Software Interrupts and System calls
Published: 2016/08/07
Channel: Introduction to Operating Systems
fork system call in C programs
fork system call in C programs
Published: 2017/01/17
Channel: Antonio Pierro
Operating systems Example 1.005 GATE CS 2005 (fork system call)
Operating systems Example 1.005 GATE CS 2005 (fork system call)
Published: 2013/06/05
Channel: Harshit Pande
QEEE Lecture 2- UNIX system calls
QEEE Lecture 2- UNIX system calls
Published: 2014/08/25
Channel: Engineering & Technology
Write your own Operating System 20: System calls, POSIX compliance
Write your own Operating System 20: System calls, POSIX compliance
Published: 2016/08/06
Channel: Write your own Operating System
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WIKIPEDIA ARTICLE

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A high-level overview of the Linux kernel's system call interface, which handles communication between its various components and the userspace

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.[1]

Privileges[edit]

The architecture of most modern processors, with the exception of some embedded systems, involves a security model. For example, the rings model specifies multiple privilege levels under which software may be executed: a program is usually limited to its own address space so that it cannot access or modify other running programs or the operating system itself, and is usually prevented from directly manipulating hardware devices (e.g. the frame buffer or network devices).

However, many normal applications obviously need access to these components, so system calls are made available by the operating system to provide well defined, safe implementations for such operations. The operating system executes at the highest level of privilege, and allows applications to request services via system calls, which are often initiated via interrupts. An interrupt automatically puts the CPU into some elevated privilege level, and then passes control to the kernel, which determines whether the calling program should be granted the requested service. If the service is granted, the kernel executes a specific set of instructions over which the calling program has no direct control, returns the privilege level to that of the calling program, and then returns control to the calling program.

The library as an intermediary[edit]

Generally, systems provide a library or API that sits between normal programs and the operating system. On Unix-like systems, that API is usually part of an implementation of the C library (libc), such as glibc, that provides wrapper functions for the system calls, often named the same as the system calls they invoke. On Windows NT, that API is part of the Native API, in the ntdll.dll library; this is an undocumented API used by implementations of the regular Windows API and directly used by some system programs on Windows. The library's wrapper functions expose an ordinary function calling convention (a subroutine call on the assembly level) for using the system call, as well as making the system call more modular. Here, the primary function of the wrapper is to place all the arguments to be passed to the system call in the appropriate processor registers (and maybe on the call stack as well), and also setting a unique system call number for the kernel to call. In this way the library, which exists between the OS and the application, increases portability.

The call to the library function itself does not cause a switch to kernel mode (if the execution was not already in kernel mode) and is usually a normal subroutine call (using, for example, a "CALL" assembly instruction in some Instruction set architectures (ISAs)). The actual system call does transfer control to the kernel (and is more implementation-dependent and platform-dependent than the library call abstracting it). For example, in Unix-like systems, fork and execve are C library functions that in turn execute instructions that invoke the fork and exec system calls. Making the system call directly in the application code is more complicated and may require embedded assembly code to be used (in C and C++) as well as knowledge of the low-level binary interface for the system call operation, which may be subject to change over time and thus not be part of the application binary interface; the library functions are meant to abstract this away.

On exokernel based systems, the library is especially important as an intermediary. On exokernels, libraries shield user applications from the very low level kernel API, and provide abstractions and resource management.

IBM operating systems descended from OS/360 and DOS/360, including z/OS and z/VSE, implement system calls through a library of assembly language macros. This reflects their origin at a time when programming in assembly language was more common than high-level language usage. IBM system calls are therefore not directly executable by high-level language programs, but require a callable assembly language wrapper subroutine.

Examples and tools[edit]

On Unix, Unix-like and other POSIX-compliant operating systems, popular system calls are open, read, write, close, wait, exec, fork, exit, and kill. Many modern operating systems have hundreds of system calls. For example, Linux and OpenBSD each have over 300 different calls,[2][3] NetBSD has close to 500,[4] FreeBSD has over 500,[5] Windows 7 has close to 700,[citation needed] while Plan 9 has 51.[6]

Tools such as strace and truss allow a process to execute from start and report all system calls the process invokes, or can attach to an already running process and intercept any system call made by said process if the operation does not violate the permissions of the user. This special ability of the program is usually also implemented with a system call, e.g. strace is implemented with ptrace or system calls on files in procfs.

Typical implementations[edit]

Implementing system calls requires a control transfer from user space to kernel space, which involves some sort of architecture-specific feature. A typical way to implement this is to use a software interrupt or trap. Interrupts transfer control to the operating system kernel so software simply needs to set up some register with the system call number needed, and execute the software interrupt.

This is the only technique provided for many RISC processors, but CISC architectures such as x86 support additional techniques. For example, the x86 instruction set contains the instructions SYSCALL/SYSRET and SYSENTER/SYSEXIT (these two mechanisms were independently created by AMD and Intel, respectively, but in essence they do the same thing). These are "fast" control transfer instructions that are designed to quickly transfer control to the kernel for a system call without the overhead of an interrupt.[7] Linux 2.5 began using this on the x86, where available; formerly it used the INT instruction, where the system call number was placed in the EAX register before interrupt 0x80 was executed.[8][9]

An older x86 mechanism is the call gate. It allows a program to call a kernel function directly using a safe control transfer mechanism, which the operating system sets up in advance. This approach has been unpopular, presumably due to the requirement of a far call (a call to a procedure located in a different segment than the current code segment[10]) which uses x86 memory segmentation and the resulting lack of portability it causes, and existence of the faster instructions mentioned above.

For IA-64 architecture, EPC (Enter Privileged Code) instruction is used. The first eight system call arguments are passed in registers, and the rest are passed on the stack.

In the IBM System/360 mainframe family, a Supervisor Call instruction implements a system call for legacy facilities; the Program Call (PC) instruction is used for newer facilities. In particular, PC is used when the caller might be in SRB mode.

Categories of system calls[edit]

System calls can be roughly grouped into five major categories:

  1. Process Control
  2. File management
    • create file, delete file
    • open, close
    • read, write, reposition
    • get/set file attributes
  3. Device Management
    • request device, release device
    • read, write, reposition
    • get/set device attributes
    • logically attach or detach devices
  4. Information Maintenance
    • get/set time or date
    • get/set system data
    • get/set process, file, or device attributes
  5. Communication
    • create, delete communication connection
    • send, receive messages
    • transfer status information
    • attach or detach remote devices

Processor mode and context switching[edit]

System calls in most Unix-like systems are processed in kernel mode, which is accomplished by changing the processor execution mode to a more privileged one, but no process context switch is necessary – although a privilege context switch does occur. The hardware sees the world in terms of the execution mode according to the processor status register, and processes are an abstraction provided by the operating system. A system call does not generally require a context switch to another process; instead, it is processed in the context of whichever process invoked it.[11][12]

In a multithreaded process, system calls can be made from multiple threads. The handling of such calls is dependent on the design of the specific operating system kernel and the application runtime environment. The following list shows typical models followed by operating systems:[13][14]

  • Many-to-one model: All system calls from any user thread in a process are handled by a single kernel-level thread. This model has a serious drawback – any blocking system call (like awaiting input from user) can freeze all the other threads. Also, since only one thread can access the kernel at a time, this model cannot utilize multiple cores of processor.
  • One-to-one model: Every user thread gets attached to a distinct kernel-level thread during a system call. This model solves the above problem of blocking system calls. It is found in all major distribution of Linux, recent Windows and Solaris versions.
  • Many-to-many model: In this model a pool of user threads is mapped to a pool of kernel threads. All system calls from a user thread pool are handled by the threads in their corresponding kernel thread pool
  • Hybrid model: This model implements both many to many and one to one model depending upon choice made by the kernel. This is found in old versions of IRIX, HP-UX and Solaris.

See also[edit]

References[edit]

  1. ^ IBM (March 1967). "Writing SVC Routines". IBM System/360 Operating System System Programmer's Guide (PDF). Third Edition. pp. 32–36. C28-6550-2. 
  2. ^ "syscalls(2) - Linux manual page". 
  3. ^ OpenBSD (2013-09-14). "System call names (kern/syscalls.c)". BSD Cross Reference. 
  4. ^ NetBSD (2013-10-17). "System call names (kern/syscalls.c)". BSD Cross Reference. 
  5. ^ "FreeBSD syscalls.c, the list of syscall names and IDs". 
  6. ^ "Plan 9 sys.h, the list of syscall names and IDs". 
  7. ^ "SYSENTER (OSDev wiki)". 
  8. ^ Anonymous (2002-12-19). "Linux 2.5 gets vsyscalls, sysenter support". KernelTrap. Retrieved 2008-01-01. 
  9. ^ Manu Garg (2006). "Sysenter Based System Call Mechanism in Linux 2.6". 
  10. ^ "Liberation: x86 Instruction Set Reference". renejeschke.de. Retrieved 4 July 2015. 
  11. ^ Bach, Maurice J. (1986), The Design of the UNIX Operating System, Prentice Hall, pp. 15-16.
  12. ^ Elliot, John (2011). "Discussion of system call implementation at ProgClub including quote from Bach 1986". 
  13. ^ "Threads". 
  14. ^ "Threading Models" (PDF). 

External links[edit]

This article is based on material taken from the Free On-line Dictionary of Computing prior to 1 November 2008 and incorporated under the "relicensing" terms of the GFDL, version 1.3 or later.

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