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Unix Interface

UNIX provides functions which can be used by the program. These functions are defined in "syscalls.h".

File System

In a file system, even the peripheral devices are treated as files. This allows handling of different resources, either peripherals or files through a homogeneous interface.

File Descriptors

If a program wants access to a file then it has to let the operating system know about the intent. This could be in the form of either

  • Writing to a file
  • Reading from a file

etc

When the file is opened, it returns a non zero integer called the file descriptor. Whenever an input or output operation on this file needs to be done, it will be done using the file descriptor.
When the shell runs the program, it opens three file descriptors.

  • 0 - stdin
  • 1 - stdout
  • 2 - stderr

By default, stdin is associated with the keyboard and stdout is associated with screen usually.
The direction of stdin can be redirected using < and stdout can be redirected using >.

program1 < input_file // (1)!
program2 > output_file // (2)!

Assume program1 takes input from the keyboard, using <, the input is now taken from the input_file instead.
Similarly, assume program2 outputs characters to the screen. Using >, we are making sure that it outputs the characters to a output_file instead.

read() And write()

read(int file_descriptor, char* buffer, int size);
write(int file_descriptor, char* buffer, int size);

open()

int open(char* file_name, int flags, int perms); // returns a file descriptor

The flags determine how the file should be opened.

  • O_RDONLY for reading files
  • O_WROLY for writing files
  • O_RDWR for both writing and reading files

These constants are defined in

  • <fcntl.h> on System V UNIX systems.
  • <sys/file.h> on Berkeley (BSD) versions.

The perms is always 0 for open().

creat()

int creat(char* name, int perms); // (1)!
  1. Returns a file descriptor if it was able to create a file. Otherwise, it returns -1.

If the file already exists, the previous contents are discarded.
In UNIX, there are 9 bits of permission information associated with the files. These are usually passed in octal notation.

close()

close(int file_descriptor); // disconnects the file from the program

unlink(char* file_name); // (1)!
  1. It removes the filename from the file system. Equivalent to the library function remove().

Random Access

long lseek(int file_descriptor, long offset, int origin); // (1)!
  1. Sets the position for read() and write() relative to origin.

Directory Listing

The directory is a file which contains a list. Each entry contains the filename and an inode1 index.

References

  1. Read more about inode. ↩