C Programming

File handling using open(), read(), write() and close()
The previous examples of file handling deal with File Control Blocks (FCB). Under MSDOS v3.x (or greater) and UNIX systems, file handling is often done using handles, rather than file control blocks.

Writing programs using handles ensures portability of source code between different operating systems. Using handles allows the programmer to treat the file as a stream of characters.

open()


	#include <fcntl.h>
	int  open(  char  *filename,  int  access,  int  permission  );

The available access modes are


	O_RDONLY		O_WRONLY		O_RDWR
	O_APPEND		O_BINARY		O_TEXT

The permissions are


	S_IWRITE	S_IREAD	S_IWRITE | S_IREAD

The open() function returns an integer value, which is used to refer to the file. If un- successful, it returns -1, and sets the global variable errno to indicate the error type.

read()


	#include  <fcntl.h>
	int  read(  int  handle,  void  *buffer,  int  nbyte );

The read() function attempts to read nbytes from the file associated with handle, and places the characters read into buffer. If the file is opened using O_TEXT, it removes carriage returns and detects the end of the file.

The function returns the number of bytes read. On end-of-file, 0 is returned, on error it returns -1, setting errno to indicate the type of error that occurred.

write()


	#include  <fcntl.h>
	int  write(  int  handle,  void  *buffer,  int  nbyte  );

The write() function attempts to write nbytes from buffer to the file associated with handle. On text files, it expands each LF to a CR/LF.

The function returns the number of bytes written to the file. A return value of -1 indicates an error, with errno set appropriately.

close()


	#include  <fcntl.h>
	int  close(  int  handle  );

The close() function closes the file associated with handle. The function returns 0 if successful, -1 to indicate an error, with errno set appropriately.