Introduction

System Calls: Set or function provided to other programs provided by the program.

• Used to request access to the resources of the machine, to communicate with other currently-running programs, and to start new programs.
• Sometimes referred to as an application programming interface (API).
• Different operating systems provide different sets of system calls.
• Over the years standard have been developed to regularize at least a subset of system calls common to all systems (e.g., POSIX)

Taxonomy of System Calls

Modern UNIX kernels provide several hundred system calls, typically broken into families of functions.

Some of the common taxonomy of system calls:

1. Memory Management System Calls:
   • e.g., mmap(),shmget(),mprotect(),mlock(), and shmctl()
2. Time Management System Calls:
   • e.g., time() ,gettimer() ,settimer() ,settimeofday() ,and alarm()
3. File System Calls:
   • e,g., open(),read(),write(),close(),creat(), lseek(), link()
4. Process System Calls:
   • e.g., fork() ,exec(), execl(),execv(), and wait()
5. Socket System Calls:
   • e.g., socket(),bind(),connect(),listen(),accept(),send(), and recv()

Note: Other system calls include those used for message passing, shared memory, semaphores, and thread management.

Libraries and System Calls

Many standard library functions are built on top of system calls.

• e.g., The malloc() family of functions is built on mmap() and brk(); sleep() is built on a few time management system calls like alarm(), fopen(), fread(), fwrite(), and fclose() are built on open(), read(), write(), and close().
• This is a common hierarchical relationship between library functions and system calls.