Operating Systems 2019W Lecture 6 - Revision history

Soma: /* Signals */

2019-01-24T03:51:58Z

Signals

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	* The standard library can do different things with interrupted system calls. The SA_RESTART flag says to run the system call again. It only applies to a few system calls, primarily read-like ones. (So, when a read is interrupted, it is tried again.)		* The standard library can do different things with interrupted system calls. The SA_RESTART flag says to run the system call again. It only applies to a few system calls, primarily read-like ones. (So, when a read is interrupted, it is tried again.)

	~~Observations on~~ reactive.c:		'''reactive.c'''
	* reactive.c waits forever, but says ouch when it receives SIGUSR1		* reactive.c waits forever, but says ouch when it receives SIGUSR1
	* Note the sleep calls (which use the nanosleep system call) get interrupted when signals arrive.		* Note the sleep calls (which use the nanosleep system call) get interrupted when signals arrive.
	* The behavior of sleep does not change by setting SA_RESTART because sleep system calls are never restarted.		* The behavior of sleep does not change by setting SA_RESTART because sleep system calls are never restarted.
	* When nanosleep is interrupted, it returns the time left in the sleep.		* When nanosleep is interrupted, it returns the time left in the sleep.

			'''3000shell-errno.c'''
			* This is a corrected version of the attempt in lecture to prevent backgrounded processes from causing the shell to exit when SA_RESTART is removed.
			* Note how it does not exit if <tt>errno==EINTR</tt> (see highlighted lines)

	===I/O redirection===		===I/O redirection===

Soma: /* 3000shell-errno.c */

2019-01-24T03:49:44Z

3000shell-errno.c

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	===3000shell-errno.c===		===3000shell-errno.c===

	<source lang="c" highlight="~~250~~,263-267" line>		<source lang="c" highlight="243,263-267" line>
	/* 3000shell.c */		/* 3000shell.c */
	/* v1 Sept. 24, 2017 */		/* v1 Sept. 24, 2017 */

Soma: /* 3000shell-errno.c */

2019-01-24T03:49:01Z

3000shell-errno.c

← Older revision		Revision as of 03:49, 24 January 2019
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	===3000shell-errno.c===		===3000shell-errno.c===

	<source lang="c" line>		<source lang="c" highlight="250,263-267" line>
	/* 3000shell.c */		/* 3000shell.c */
	/* v1 Sept. 24, 2017 */		/* v1 Sept. 24, 2017 */

Soma: /* Code */

2019-01-24T03:45:07Z

Code

Show changes

Soma: /* Notes */

2019-01-24T03:36:56Z

Notes

← Older revision		Revision as of 03:36, 24 January 2019
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	* The standard library can do different things with interrupted system calls. The SA_RESTART flag says to run the system call again. It only applies to a few system calls, primarily read-like ones. (So, when a read is interrupted, it is tried again.)		* The standard library can do different things with interrupted system calls. The SA_RESTART flag says to run the system call again. It only applies to a few system calls, primarily read-like ones. (So, when a read is interrupted, it is tried again.)

			Observations on reactive.c:
			* reactive.c waits forever, but says ouch when it receives SIGUSR1
			* Note the sleep calls (which use the nanosleep system call) get interrupted when signals arrive.
			* The behavior of sleep does not change by setting SA_RESTART because sleep system calls are never restarted.
			* When nanosleep is interrupted, it returns the time left in the sleep.

	===I/O redirection===		===I/O redirection===

Soma: /* reactive.c */

2019-01-24T03:35:41Z

reactive.c

← Older revision		Revision as of 03:35, 24 January 2019
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	===reactive.c===		===reactive.c===

	<source lang="c" line>		<source lang="c" line>
	#include <stdio.h>		#include <stdio.h>

Soma: /* Notes */

2019-01-24T03:33:42Z

Notes

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	==Notes==		==Notes==

	Signals		===Signals===
	* processes can register signal handler functions for specific signals		* processes can register signal handler functions for specific signals
	* When the kernel delivers a signal to a process, it runs the specified handler		* When the kernel delivers a signal to a process, it runs the specified handler
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	I/O redirection		===I/O redirection===
	*		To redirect standard in, out, or error, a shell does the following when running a command:
			* The shell does a fork
			* The parent (original process) calls wait on the child when it exits in order to get its return value. This wait can be synchronous (i.e., the wait system call blocks until the child terminates) or it can be asynchronous (the parent only calls wait when it receives a SIGCHLD signal).
			* '''The child process initializes standard in, out, and error by opening files and assigning them to file descriptors 0, 1, and 2'''
			* The child calls execve

	Command-line arguments and environment variables		To implement this, the hardest part is parsing the command line in order to figure out what files, if any, the child should open for file descriptors 0, 1, and 2.

			===Command-line arguments and environment variables===
	* They are both stored in the memory of the process.		* They are both stored in the memory of the process.
	* They are put there by execve, as part of loading a new executable into a process (and erasing everything that was in the process's address space).		* They are put there by execve, as part of loading a new executable into a process (and erasing everything that was in the process's address space).

Soma: /* Notes */

2019-01-24T03:29:32Z

Notes

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	* When the kernel delivers a signal to a process, it runs the specified handler		* When the kernel delivers a signal to a process, it runs the specified handler
	* The C library defines default handlers for all signals (except STOP and KILL)		* The C library defines default handlers for all signals (except STOP and KILL)
	* When a process gets a signal, current execution is interrupted and the handler		* When a process gets a signal, current execution is interrupted and the handler is invoked. When the handler terminates, the process continues where it was
	is invoked. When the handler terminates, the process continues where it was
	* Interrupted system calls return with an error EINTR.		* Interrupted system calls return with an error EINTR.
	* If the process was blocked on a system call, the system call is interrupted		* If the process was blocked on a system call, the system call is interrupted and the handler is run.
	and the handler is run.
	* The standard library can do different things with interrupted system calls. The SA_RESTART flag says to run the system call again. It only applies to a few system calls, primarily read-like ones. (So, when a read is interrupted, it is tried again.)		* The standard library can do different things with interrupted system calls. The SA_RESTART flag says to run the system call again. It only applies to a few system calls, primarily read-like ones. (So, when a read is interrupted, it is tried again.)

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	* They are both stored in the memory of the process.		* They are both stored in the memory of the process.
	* They are put there by execve, as part of loading a new executable into a process (and erasing everything that was in the process's address space).		* They are put there by execve, as part of loading a new executable into a process (and erasing everything that was in the process's address space).
	* execve doesn't necessarily change other aspects of process's state.		* execve doesn't necessarily change other aspects of process's state, thus they should be initialized by something else (normally, the C library)
			* In particular, the data structures associated with file descriptors (i.e., open files) are preserved across execve.

	==Code==		==Code==

Soma: /* Notes */

2019-01-24T03:27:40Z

Notes

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	==Notes==		==Notes==

	~~Topics for today~~		Signals
	* signal handlers
	* I/O redirection
	* environment variables

	~~===In Class===~~

	~~<pre>~~
	~~Lecture 6~~
	~~---------~~

	~~Topics~~
	~~- signal handlers~~
	~~- I/O redirection~~
	~~- environment variables & command line arguments~~


	~~Key ideas for signals~~
	* processes can register signal handler functions for specific signals		* processes can register signal handler functions for specific signals
	* When the kernel delivers a signal to a process, it runs the specified handler		* When the kernel delivers a signal to a process, it runs the specified handler
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	* When a process gets a signal, current execution is interrupted and the handler		* When a process gets a signal, current execution is interrupted and the handler
	is invoked. When the handler terminates, the process continues where it was		is invoked. When the handler terminates, the process continues where it was
			* Interrupted system calls return with an error EINTR.
	* If the process was blocked on a system call, the system call is interrupted		* If the process was blocked on a system call, the system call is interrupted
	and the handler is run.		and the handler is run.
	* The standard library can do different things with interrupted system calls		* The standard library can do different things with interrupted system calls. The SA_RESTART flag says to run the system call again. It only applies to a few system calls, primarily read-like ones. (So, when a read is interrupted, it is tried again.)
	</~~pre>~~

			I/O redirection
			*

			Command-line arguments and environment variables
			* They are both stored in the memory of the process.
			* They are put there by execve, as part of loading a new executable into a process (and erasing everything that was in the process's address space).
			* execve doesn't necessarily change other aspects of process's state.

	==Code==		==Code==

Soma at 02:27, 24 January 2019

2019-01-24T02:27:10Z

← Older revision		Revision as of 02:27, 24 January 2019
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	Topics for today		Topics for today
	* environment variables
	* signal handlers		* signal handlers
	* I/O redirection		* I/O redirection
			* environment variables

	===In Class===		===In Class===