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 V7

pidof

PIDOF(8)						Linux System Administrator's Manual						  PIDOF(8)

NAME

       pidof -- find the process ID of a running program.

SYNOPSIS

       pidof [-s] [-c] [-n] [-x] [-o omitpid[,omitpid..]]  [-o omitpid[,omitpid..]..]  program [program..]

DESCRIPTION

       Pidof  finds  the  process  id's (pids) of the named programs. It prints those id's on the standard output. This program is on some systems
       used in run-level change scripts, especially when the system has a System-V like rc structure. In that case these scripts  are  located	in
       /etc/rc?.d, where ? is the runlevel. If the system has a start-stop-daemon (8) program that should be used instead.

OPTIONS

       -s     Single shot - this instructs the program to only return one pid.

       -c     Only  return  process ids that are running with the same root directory.	This option is ignored for non-root users, as they will be
	      unable to check the current root directory of processes they do not own.

       -n     Avoid stat(2) system function call on all binaries which are located on network based file systems like NFS.  Instead of using  this
	      option the the variable PIDOF_NETFS may be set and exported.

       -x     Scripts too - this causes the program to also return process id's of shells running the named scripts.

       -o omitpid
	      Tells  pidof  to omit processes with that process id. The special pid %PPID can be used to name the parent process of the pidof pro-
	      gram, in other words the calling shell or shell script.

EXIT STATUS

       0      At least one program was found with the requested name.

       1      No program was found with the requested name.

NOTES

       pidof is actually the same program as killall5; the program behaves according to the name under which it is called.

       When pidof is invoked with a full pathname to the program it should find the pid of, it is reasonably safe. Otherwise it is  possible  that
       it  returns  pids  of  running programs that happen to have the same name as the program you're after but are actually other programs. Note
       that that the executable name of running processes is calculated with readlink(2), so symbolic links to executables will also match.

SEE ALSO

       shutdown(8), init(8), halt(8), reboot(8), killall5(8)

AUTHOR

       Miquel van Smoorenburg, miquels@cistron.nl

								    01 Sep 1998 							  PIDOF(8)

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