Working with the Unix OS

Intro to System Concepts
! File Subsystem
User File Descriptor Table
File Table
Inode Table
- allocated per process
- global kernel structure
- index node , describes disk
layout file data, file owner,
access permissions, access times
Introduction to kernel
user fd table file table i-node table
When a process creates a new file, the kernel assigns it an unused inode. Inodes are stored in the file system, but
the kernel reads them into an in-core inode table.
The file table keeps track of the byte offset in the file where the user's next read or write will start, and the access
rights allowed to the opening process.
The user file descriptor table identifies all open files for a process. The kernel returns a file descriptor for the open
system call, which is an index into the user fd table.
! File System Layout
boot block - occupies the beginning of the file system: first sector, bootstrap code
super block - describes state of file system: size, number of files, free space
inode list - kernel references inodes by index, the root inode is used by mount
data blocks - an allocated data block can belong to one and only one file in the file system
Processes
A process is the execution of a program and consists of bytes that the CPU interprets as machine instructions.
Processes communicate with other processes and with the rest of the world via system calls.
A process on a UNIX system is created by the" fork" system call. Every process except process 0 is created by
"fork". Process 0 is the swapper, process 1, known as init is the parent of all other processes.
Executable File contents:
- set of headers that describe the attributes of the file
- the program text
- machine language representation of data initial values when much memory space for uninitialized data (bss =
block started
- other sections, such as a symbol table
The kernel loads an executable file into memory during an "exec" system call. The three regions are: text, data and
stack.
The stack region is automatically created and its size is dynamically adjusted by t_ kernel at run time.
#include /* program to copy a file */
char buffer[2048];
int version = 1;
main(int argc, char *argv[])
{
int fdold, fdnew;
if (argc != 3)
{
printf("need 2 arguments for copy program\n");
exit(1);
}
fdold = open(argv[l], O_RDONLY); /* open source file */
if (fdold == -1)
{
printf("can't open file %s\n", argv[l]);
exit(1);
}
fdnew = creat(argv[2], 0666); /* create target file */
if (fdnew == -1)
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