COMP 3000 2012 Week 8 Notes - Revision history

Shahanahmed at 17:24, 31 October 2012

2012-10-31T17:24:06Z

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	Register a normal C function as a signal handler<br>		Register a normal C function as a signal handler<br>

	~~Some are implemented by~~ default (ex. sigkill)<br>		They every signal has a default signal handler defined in the C Standard Library.<br>
			Most can be overwritten to have user-defined signal handlers, however some, (e.g. sigkill) cannot be overwritten. <br>

	Signals can be called at any point in execution.<br>		Signals can be called at any point in execution.<br>

Sdp: description of malloc and sbrk

2012-10-26T13:20:52Z

description of malloc and sbrk

← Older revision		Revision as of 13:20, 26 October 2012
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	Q: What allocates the storage for environment variables and command-line arguments?<br>		Q: What allocates the storage for environment variables and command-line arguments?<br>

	A: The kernel. The kernel is the only thing that can allocate.(execve)<br>		A: The kernel. The kernel is the only thing that can allocate this space.(execve)<br>
	This also makes sense because execve is a system call.		This also makes sense because execve is a system call.

			Note that the kernel is the only thing that can change the size of a program's data segment directly (the wrapper for this system call is sbrk), but malloc is often implemented to call sbrk in advance, then divide up the space as malloc is called.

	</li>		</li>

Spenner2 at 13:07, 26 October 2012

2012-10-26T13:07:15Z

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	Q: Without errors, what does the execve system call return? # Who calls a signal handler?<br>		Q: Without errors, what does the execve system call return? # Who calls a signal handler?<br>

	A: execve ~~DOES NOT RETURN ANYTHING. Process image~~ is ~~replaced~~.<br>		A: On success, execve() does not return, on error -1 is returned, and errno is set appropriately.<br>

	The program itself calls the signal handler		The program itself calls the signal handler

Spenner2 at 13:05, 26 October 2012

2012-10-26T13:05:34Z

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	A: The former requires a library card and a telephone.		A: The former requires a library card and a telephone.
			<br> [[User:Spenner2\|Spenner2]] ([[User talk:Spenner2\|talk]]) nice
	</li>		</li>
	</ol>		</ol>

Flavoie at 02:46, 26 October 2012

2012-10-26T02:46:36Z

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	A: False. Sparse file means holes. Zeroes aren't holes.		A: False. Sparse file means holes. Zeroes aren't holes.
	~~YOu~~ need to move the file pointer. You have to use a		You need to move the file pointer. You have to use a
	seek. The data will be READ as zeros, though...		seek. The data will be READ as zeros, though...
	Look up truncate command and strace it. It ain't just writing zeros		Look up truncate command and strace it. It ain't just writing zeros

Sdp: added formatting

2012-10-25T23:34:06Z

added formatting

Show changes

Manders at 22:42, 25 October 2012

2012-10-25T22:42:50Z

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	A: Sequentially (sysV). Shell script that runs every ''command'' sequentially		A: Sequentially (sysV). Shell script that runs every ''command'' sequentially

	Upstart does it in parallel though...		Upstart does it in parallel though. This is the reason sysV is being replaced by Upstart.

	10)		10)

Cdelahou: typos

2012-10-25T21:40:00Z

typos

← Older revision		Revision as of 21:40, 25 October 2012
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	A: Sequentially (sysV). Shell script that runs every ''command'' sequentially		A: Sequentially (sysV). Shell script that runs every ''command'' sequentially

	Upstart does in ~~in parrallel~~ though...		Upstart does it in parallel though...

	10)		10)

Cdelahou: signlas

2012-10-25T21:38:05Z

signlas

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	A: True. That's the purpose of signals		A: True. That's the purpose of signals

	Register a normal C function as a signal handler		Register a normal C function as a signal handler
	Some are implemented by default (sigkill)
			Some are implemented by default (ex. sigkill)

	Signals can be called at any point in execution.		Signals can be called at any point in execution.

			"When a signal is sent, the operating system interrupts the target process's normal flow of execution. Execution can be '''interrupted during any non-atomic instruction.''' If the '''process has previously registered a signal handler, that routine is executed. Otherwise''' the default signal handler is executed." from http://en.wikipedia.org/wiki/Unix_signal

	9)		9)

Cdelahou at 21:33, 25 October 2012

2012-10-25T21:33:39Z

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	Note:		Note:

	"A system call is an entry point into the Linux kernel. Usually, system calls are not invoked directly: instead, most system calls have corresponding C library wrapper functions which perform the steps required (e.g., trapping to kernel mode) in order to invoke the system call. Thus, making a system call looks the same as invoking a normal library function." from man 2 intro		"A system call is an entry point into the Linux kernel. Usually, system calls are not invoked directly: instead, most system calls have corresponding C library wrapper functions which perform the steps required (e.g., trapping to kernel mode) in order to invoke the system call. Thus, making a system call looks the same as invoking a normal library function." from man 2 intro

	"The call to the library function itself does not cause a switch to kernel mode (if the execution was not already in kernel mode) and is usually a normal subroutine call (using, for example, a "CALL" assembly instruction in some Instruction set architectures (ISAs)). The actual system call does transfer control to the kernel (and is more implementation-dependent and platform-dependent than the library call abstracting it). For example, in Unix-like systems, "fork" and "execve" are C library functions that in turn execute instructions that invoke the "fork" and "execve" system calls. Making the system call directly in the application code is more complicated and may require embedded assembly code to be used (in C and C++) as well as knowledge of the low-level binary interface for the system call operation, which may be subject to change over time and thus not be part of the application binary interface; the library functions are meant to abstract this away." from http://en.wikipedia.org/wiki/System_call		"The call to the library function itself does not cause a switch to kernel mode (if the execution was not already in kernel mode) and is usually a normal subroutine call (using, for example, a "CALL" assembly instruction in some Instruction set architectures (ISAs)). The actual system call does transfer control to the kernel (and is more implementation-dependent and platform-dependent than the library call abstracting it). For example, in Unix-like systems, "fork" and "execve" are C library functions that in turn execute instructions that invoke the "fork" and "execve" system calls. Making the system call directly in the application code is more complicated and may require embedded assembly code to be used (in C and C++) as well as knowledge of the low-level binary interface for the system call operation, which may be subject to change over time and thus not be part of the application binary interface; the library functions are meant to abstract this away." from http://en.wikipedia.org/wiki/System_call