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Full Discussion: System calls in UNIX
Top Forums UNIX for Dummies Questions & Answers System calls in UNIX Post 302594626 by Corona688 on Tuesday 31st of January 2012 08:05:45 PM
Old 01-31-2012
When you make a function call, your program jumps to a different location in memory. It's just a program jump with a little extra stack work, nothing magic, just running around in its own private little universe (process).

To make a system call, you set registers to some particular values, call INT 0x20, and bang, it's done. Your own program doesn't jump anywhere or do anything -- the system call just happens, like magic, then leaves your program right where it started.

Code does execute, of course, but not in your program. INT 0x20 just passes a message into the operating system. When that happens, the OS literally stops your program, rearranges that program's private universe in the manner requested. Once it's finished, the kernel starts your program running again. This isn't necessarily instant. If you do a read() on a pipe or socket with no data in it, your program might be sleeping entire seconds for whatever's on the other end to write into it.

So. Function calls: Runs instructions inside your program. These instructions can't do anything except read or write memory or alter certain unprivileged registers.

System calls: Instantly stops your program and sends a message to the operating system, asking it to do something. The OS decides what to do with your request and does it, in kernel mode, with much higher privileges. Once finished, starts your program again.
 

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LEARN ABOUT NETBSD

siginterrupt

SIGINTERRUPT(3) 					   BSD Library Functions Manual 					   SIGINTERRUPT(3)

NAME

     siginterrupt -- allow signals to interrupt system calls

LIBRARY

     Standard C Library (libc, -lc)

SYNOPSIS

     #include <signal.h>

     int
     siginterrupt(int sig, int flag);

DESCRIPTION

     The siginterrupt() function is used to change the system call restart behavior when a system call is interrupted by the specified signal.	If
     the flag is false (0), then system calls will be restarted if they are interrupted by the specified signal and no data has been transferred
     yet.  System call restart is the default behavior on 4.2BSD.

     If the flag is true (1), then restarting of system calls is disabled.  If a system call is interrupted by the specified signal and no data
     has been transferred, the system call will return -1 with the global variable errno set to EINTR.	Interrupted system calls that have started
     transferring data will return the amount of data actually transferred.  System call interrupt is the signal behavior found on 4.1BSD and AT&T
     System V UNIX systems.

     Note that the new 4.2BSD signal handling semantics are not altered in any other way.  Most notably, signal handlers always remain installed
     until explicitly changed by a subsequent sigaction(2) call, and the signal mask operates as documented in sigaction(2).  Programs may switch
     between restartable and interruptible system call operation as often as desired in the execution of a program.

     Issuing a siginterrupt(3) call during the execution of a signal handler will cause the new action to take place on the next signal to be
     caught.

NOTES

     This library routine uses an extension of the sigaction(2) system call that is not available in 4.2BSD, hence it should not be used if back-
     ward compatibility is needed.

RETURN VALUES

     A 0 value indicates that the call succeeded.  A -1 value indicates that an invalid signal number has been supplied.

SEE ALSO

     sigaction(2), sigprocmask(2), sigsuspend(2)

HISTORY

     The siginterrupt() function appeared in 4.3BSD.

BSD
								   June 4, 1993 							       BSD
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