Operating Systems 2017F Lecture 3 - Revision history

Ssarazin: /* Adding the execve() system call to the mix */

2017-10-05T00:47:32Z

Adding the execve() system call to the mix

← Older revision		Revision as of 00:47, 5 October 2017
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	=== Adding the execve() system call to the mix ===		=== Adding the execve() system call to the mix ===
			<syntaxhighlight lang="c">
	#include <stdio.h>		#include <stdio.h>
	#include <unistd.h>		#include <unistd.h>
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	return 42;		return 42;
	}		}
			</syntaxhighlight>
	- int execve(const char filename, char const argv[], char *const envp[])		- int execve(const char filename, char const argv[], char *const envp[])
	- this system call executes the program pointed to by filename		- this system call executes the program pointed to by filename

Ssarazin: /* What is fork? */

2017-10-05T00:46:48Z

What is fork?

← Older revision		Revision as of 00:46, 5 October 2017
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	- More on creating a process using fork().		- More on creating a process using fork().
			<syntaxhighlight lang="c">
	#include <stdio.h>		#include <stdio.h>
	#include <unistd.h>		#include <unistd.h>
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	return 42;		return 42;
	}		}
			</syntaxhighlight>
	- sleep(int)		- sleep(int)
	- causes the current process to do nothing for the specified number of seconds.		- causes the current process to do nothing for the specified number of seconds.

Ssarazin: /* Notes */

2017-10-05T00:11:59Z

Notes

← Older revision		Revision as of 00:11, 5 October 2017
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	==== Manipulating environment variables====		==== Manipulating environment variables====
	*argc tells the number of argument in command line, while argv[] contains the actual command obtained in the command line		*argc tells the number of argument in command line, while argv[] contains the actual command obtained in the command line

			=== Adding the execve() system call to the mix ===

			#include <stdio.h>
			#include <unistd.h>
			#include <sys/types.h>
			#include <sys/wait.h>


			int main(int argc, char* argv[], char* envp[]) {
			int child;
			int status;
			pid_t retval;
			int i;

			printf("You called me with %d arguments.\n", argc);

			for (i=0; i<argc; i++) {
			printf("Arg %d: %s\n", i, argv[i]);
			}

			child = fork();

			printf("Hello! I am %d, my parent is %d.\n",
			getpid(), getppid());

			if (child == 0) {
			/* We're the child */

			execve("/bin/ls", argv, envp);
			sleep(3);
			printf("Child all done!\n");
			} else {
			/* We're the parent */
			retval = wait(&status);
			printf("Parent all done! Child %d returned %d.\n",
			(int) retval, WEXITSTATUS(status));
			}

			return 42;
			}

			- int execve(const char filename, char const argv[], char *const envp[])
			- this system call executes the program pointed to by filename
			- The child process becomes independent from its parent -- a new program
			- argv[] is an array of strings passed to the new program. The values are loaded into the array by execve not the kernel.
			- envp[] is an array of strings of the form key=value which are passed as environment to the new program

			- What happens when the if statement executes
			- execve creates a new program -- an independent process and passes arguments to the "ls" program.
			- execve stops the new process after it has completed.

			- Some interesting points about main(int argc, char* argv[], char* envp[])
			- Notice that the size of the arrays are not defined. We have to use argc to get this information. Not good programming practice but is
			accepted because it is grandfathered from the early days of the c language.
			- arg[0] is always the name of the program.

Ssarazin: /* What Happen to the child process after the parent process died? */

2017-10-05T00:04:38Z

What Happen to the child process after the parent process died?

← Older revision		Revision as of 00:04, 5 October 2017
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	* In the demo showed in class, “system d user” became its new parent		* In the demo showed in class, “system d user” became its new parent
	**Use command "Pstree" to show process hierarchy		**Use command "Pstree" to show process hierarchy
			* Every process must have a valid process parent. Use "pstree" to see the tree structure
			* When a parent dies before a child "init" will adopt every orphan on the system

	==== What is zombie process? ====		==== What is zombie process? ====

Ssarazin: /* What is fork? */

2017-10-05T00:03:10Z

What is fork?

← Older revision		Revision as of 00:03, 5 October 2017
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	- '''What happens in the conditional?'''		- '''What happens in the conditional?'''
	- When the parent process calls fork(), a child process is created and begins executing at the fork() statement. The OS returns a value from fork() which indicates which process is		- When the parent process calls fork(), a child process is created and begins executing at the fork() statement. The OS returns a value from fork() which indicates which process
	currently running.		is currently running.
	- If fork() returns zero then the child sleeps for 3 seconds before printing. The sleep system call forces the child process to do nothing for 3 seconds.		- If fork() returns zero then the child sleeps for 3 seconds before printing. The sleep system call forces the child process to do nothing for 3 seconds.
	- if fork() returns anything else, it is assumed to be the parent process. In this case the wait() system call is run by the parent which causes the parent process to wait for its ~~child~~		- if fork() returns anything else, it is assumed to be the parent process. In this case the wait() system call is run by the parent which causes the parent process to wait for its
	to finish; then the printf executes and prints the child process id and its return value from main. The macro WEXITSTATUS retrieves 42.		child to finish; then the printf executes and prints the child process id and its return value from main. The macro WEXITSTATUS retrieves 42.

	==== What Happen to the child process after the parent process died? ====		==== What Happen to the child process after the parent process died? ====

Ssarazin: /* What is fork? */

2017-10-05T00:01:09Z

What is fork?

@@ Line 15: / Line 15: @@
 ==== What is fork? ====
 * Fork creates a new program but contains the same code (call the clone system call) and only argv[] and envp[] passed on to the new born program
 ==== What Happen to the child process after the parent process died? ====

Jamesklau at 04:27, 4 October 2017

2017-10-04T04:27:47Z

← Older revision		Revision as of 04:27, 4 October 2017
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	Code and files from the lecture (captured as they were at the end) are available [http://homeostasis.scs.carleton.ca/~soma/os-2017f/code/lec03/ here].		Code and files from the lecture (captured as they were at the end) are available [http://homeostasis.scs.carleton.ca/~soma/os-2017f/code/lec03/ here].


			==Notes==
			====Commands ====
			*-wall: show all warning
			*less: allow screen navigation, hit q to quit

			==== What is fork? ====
			* Fork creates a new program but contains the same code (call the clone system call) and only argv[] and envp[] passed on to the new born program

			==== What Happen to the child process after the parent process died? ====
			*Every child process must have a currently running parent. When the parent process die, kernel collect the dead body(return value) and re-parent the process
			* In the demo showed in class, “system d user” became its new parent
			**Use command "Pstree" to show process hierarchy

			==== What is zombie process? ====
			*When a process die it become 'zombie' (s = sleep, z = zombie)
			*We can use wait to be a 'good' parent

			==== How to load a new binary manually====
			*Use the execve command.
			**We can fork child process to handle dangerous task (“Suicide Squad”), or handle all incoming request

			==== Manipulating environment variables====
			*argc tells the number of argument in command line, while argv[] contains the actual command obtained in the command line

Soma at 21:02, 15 September 2017

2017-09-15T21:02:11Z

← Older revision		Revision as of 21:02, 15 September 2017
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	==Video==		==Video==

	Video from the lecture given on September 14, 2017 [http://homeostasis.scs.carleton.ca/~soma/os-2017f/lectures/comp3000-2017f-lec03-14Sep2017.~~m4a~~ is now available].		Video from the lecture given on September 14, 2017 [http://homeostasis.scs.carleton.ca/~soma/os-2017f/lectures/comp3000-2017f-lec03-14Sep2017.mp4 is now available].

	==Code==		==Code==

	Code and files from the lecture (captured as they were at the end) are available [http://homeostasis.scs.carleton.ca/~soma/os-2017f/code/lec03/ here].		Code and files from the lecture (captured as they were at the end) are available [http://homeostasis.scs.carleton.ca/~soma/os-2017f/code/lec03/ here].

Soma: Created page with "==Video== Video from the lecture given on September 14, 2017 [http://homeostasis.scs.carleton.ca/~soma/os-2017f/lectures/comp3000-2017f-lec03-14Sep2017.m4a is now available]...."

2017-09-15T00:25:28Z

Created page with "==Video== Video from the lecture given on September 14, 2017 [http://homeostasis.scs.carleton.ca/~soma/os-2017f/lectures/comp3000-2017f-lec03-14Sep2017.m4a is now available]...."

New page

==Video==

Video from the lecture given on September 14, 2017 [http://homeostasis.scs.carleton.ca/~soma/os-2017f/lectures/comp3000-2017f-lec03-14Sep2017.m4a is now available].

==Code==

Code and files from the lecture (captured as they were at the end) are available [http://homeostasis.scs.carleton.ca/~soma/os-2017f/code/lec03/ here].