UNIX Filesystems: Evolution, Design, and Implementation

File Descriptors

In order to give a more practical edge to the descriptions that follow, it is necessary to provide some examples in C. Therefore, before describing the various file properties, it is necessary to show how to access them; thus, the need to introduce file descriptors. Consider the following example:

$ cat open.c
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>

main()
{
     int fd;

     fd = open(“/etc/passwd”, O_RDONLY);
     printf(“fd = %d\n”, fd);
     close(fd);
}
$ make open
cc     open.c     -o open
$ ./open
fd = 3

To access a file's data, the file must first be opened. In this case, the open() system call is used. Looking at the manual page for open(), it shows that three header files must be included as the following excerpt shows:

NAME
     open open a file

SYNOPSIS
     #include <sys/types.h>
     #include <sys/stat.h>
     #include <fcntl.h>

     int open(const char *path, int oflag, …);

DESCRIPTION
     The open() function establishes the connection between a
     file and a file descriptor. It creates an …

The result of a successful open is a file descriptor that is a handle through which the file can then be subsequently accessed. The file descriptor is required in calls such as read(), write(), and lseek(). The value of the file descriptor is not important although how the value is assigned will be explained in the section File Descriptors and the File Table in Chapter 6. What is important is that it is used consistently. Following the open() call shown above, every time the passwd file is ...