System calls in UNIX Post: 302594626

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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

01-31-2012

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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 DEBIAN

getpgid

GETPGRP(2)						      BSD System Calls Manual							GETPGRP(2)

NAME

     getpgrp -- get process group

LIBRARY

     Standard C Library (libc, -lc)

SYNOPSIS

     #include <unistd.h>

     pid_t
     getpgrp(void);

     pid_t
     getpgid(pid_t pid);

DESCRIPTION

     The process group of the current process is returned by getpgrp().  The process group of the process identified by pid is returned by
     getpgid().  If pid is zero, getpgid() returns the process group of the current process.

     Process groups are used for distribution of signals, and by terminals to arbitrate requests for their input: processes that have the same
     process group as the terminal are foreground and may read, while others will block with a signal if they attempt to read.

     This system call is thus used by programs such as csh(1) to create process groups in implementing job control.  The tcgetpgrp() and
     tcsetpgrp() calls are used to get/set the process group of the control terminal.

RETURN VALUES

     The getpgrp() system call always succeeds.  Upon successful completion, the getpgid() system call returns the process group of the specified
     process; otherwise, it returns a value of -1 and sets errno to indicate the error.

COMPATIBILITY

     This version of getpgrp() differs from past Berkeley versions by not taking a pid_t pid argument.	This incompatibility is required by
     ISO/IEC 9945-1:1990 (``POSIX.1'').

     From the ISO/IEC 9945-1:1990 (``POSIX.1'') Rationale:

     4.3BSD provides a getpgrp() system call that returns the process group ID for a specified process.  Although this function is used to support
     job control, all known job-control shells always specify the calling process with this function.  Thus, the simpler AT&T System V UNIX
     getpgrp() suffices, and the added complexity of the 4.3BSD getpgrp() has been omitted from POSIX.1.  The old functionality is available from
     the getpgid() system call.

ERRORS

     The getpgid() system call will succeed unless:

     [ESRCH]		there is no process whose process ID equals pid

SEE ALSO

     getsid(2), setpgid(2), termios(4)

STANDARDS

     The getpgrp() system call is expected to conform to ISO/IEC 9945-1:1990 (``POSIX.1'').

HISTORY

     The getpgrp() system call appeared in 4.0BSD.  The getpgid() system call is derived from its usage in AT&T System V Release 4 UNIX.

BSD
								   June 4, 1993 							       BSD

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