System Programming Chapter 1
  • Simple IO model
    • #include <stdio.h>

int main()
{
	int c;

	while((c = getchar()) != EOF)
		putchar(c);

	return 0;
}
  • Process ID
    • #include <stdio.h>
#include <unistd.h>

int main()
{
	printf("Process id: %d\n", getpid());

	return 0;
}
  • privileged mode, only the operating system is allowed to execute privileged instructions
  • unprivileged mode, user processes
  • Environment Variables
  • Get environment variable
    • #include <stdio.h>
#include <stdlib.h>

int main()
{
	char* home = getenv("HOME");
	printf("Home direcotry is %s\n", home);

	return 0;
}
  • System call with wrapper, the wapper in C library has same name as the function in kernel
  • System call without wrapper, such as gettid
    • #define _GUNU_SOURCE
#include <unistd.h>
#include <sys/syscall.h>
#include <sys/types.h>
#include <stdio.h>

int main()
{
	printf("Thread id %ld\n", syscall(SYS_gettid));

	return 0;
}
    The UNIX File Hierarchy
  • bin, essential binary executables including those shell commands
  • boot, Static files of the boot loader
  • dev, essential device files
  • etc, host conguration files and system administration programs
  • home, user home directories
  • lib, essential shared libraries and kernel modules
  • mnt, mount point for mounting a le system temporarily
  • opt, add-on application software packages, used for the storage of larger program packages
  • sbin, essential system binaries for system start-up
  • tmp, temporary files
  • usr, the top of a hierarchy in which non-essential binaries, libraries, and sources
  • var, variable files
  • proc, pseudo file system containing process information
  • root, system administrator home directory
    • Files and Filenames
  • regular files
  • device files (character or block)
  • FIFOs
  • sockets
  • symbolic links
  • - (regular le), d (directory), b (buered special le), c (character special le), l (symbolic link), p (pipe), or s (socket)
    • more Program
  • more file1 file2 ... filen
  • ls -l | more
  • more < file1
    • #include <stdio.h>
#include <stdlib.h>
#define SCREEN_ROWS 23
#define LINELEN 512
#define SPACEBAR 1
#define RETURN 2
#define QUIT 3
#define INVALID 4


/** do_more_of ( )
 * Given a FILE* argument fp, display up to a page of the
 * file fp, and then display a prompt and wait for user input.
 * If user input is SPACEBAR, display next page.
 * If user input is RETURN, display one more line.
 * If user input is QUIT, terminate program.
*/
void do_more_of (FILE *filep) ;

/** get_user_input ( )
 * Displays more's status and prompt and waits for user response,
 * Require that user press return key to receive input
 * Returns one of SPACEBAR, RETURN or QUIT on valid key presses
 * and INVALID for invalide key presses.
*/
int get_user_input();

int main(int argc, char *argv[])
{
	FILE *fp;
	int i = 0;
	if(argc == 1)
		do_more_of(stdin); // no args, read from standard input
	else
		while(++i < argc)
		{
			fp = fopen(argv[i], "r");
			if (NULL != fp)
			{
				do_more_of(fp);
				fclose(fp);
			}
			else
				printf("Skipping %s \n", argv[i]);
		}

	return 0;
}

void do_more_of (FILE * fp)
{
	char line[LINELEN]; // buffer to store line of input
	int num_of_lines = SCREEN_ROWS; // # of lines left on screen
	int getmore = 1; // boolean to signal when to stop
	int reply; // input from user

	while(getmore && fgets(line, LINELEN, fp)){
		//fgets() return pointr to string read or NULL
		if(num_of_lines == 0)
		{
			reply = get_user_input();
			switch(reply)
			{
				case SPACEBAR:
					num_of_lines = SCREEN_ROWS; // all full screen
					break;
				case RETURN:
					num_of_lines++; // allow one more line
					break;
				case QUIT:
					getmore = 0;
					break;
				default:
					break;
			}
		}
		if(fputs(line, stdout) == EOF)
			exit(1);
		num_of_lines--;
	}
}

int get_user_input()
{
	int c;
	// Control Sequence Introducer
	printf("\033[7m more? \033[m"); // reverse on a VT100

	while((c = getchar()) != EOF)
		switch(c)
		{
			case 'q':
				return QUIT;
			case ' ':
				return SPACEBAR;
			case '\n':
				return RETURN;
			default:
				return INVALID;
		}

	return INVALID;
}
    Device Special Files
  • Every I/O device is associated with a device special file
  • /dev/tty is the name of the terminal that the process is using
  • /dev/mem is a special le that is a character interface to memory
  • /dev/null is a special le that acts like a black hole. All data sent to it is discarded
    • tty # /dev/pts/3
echo 'Hello World!' > /dev/pts/3
    Notation
  • ls [<option>] ... [<directory_name>] ...
    • Shells
  • Bourne shell (sh), the C shell (csh), the Korn shell (ksh), the Bourne-again shell (bash), the Z shell (zsh), and the TC shell (tcsh)
  • echo $0 or echo $SHELL, check the current shell
    • Reference
  • System Programming at CNUY