Files and I/O Interfaces

Files that are organized as a sequence of data are called regular files. Regular files can contain ASCII text, text in some other binary data encoding, executable code, or any combination of text, data, and code.

A regular file is made up of the following components:

• Control data, which is called the inode. This data includes the file type, the access permissions, the owner, the file size, and the location of the data blocks.

• File contents: a nonterminated sequence of bytes.

The Solaris operating system provides the following basic forms of file input/output interfaces:

• The traditional, raw style of file I/O is described in Basic File I/O.

• The standard I/O buffering provides an easier interface and improved efficiency to an application run on a system without virtual memory. In an application running in a virtual memory environment, such as on the SunOS operating system, standard file I/O is outdated.

• The memory mapping interface is described in Memory Management Interfaces. Mapping files is the most efficient form of file I/O for most applications run under the SunOS platform.

Basic File I/O

The following interfaces perform basic operations on files and on character I/O devices.

Table 6–1 Basic File I/O Interfaces

Interface Name	Purpose
open(2)	Open a file for reading or writing
close(2)	Close a file descriptor
read(2)	Read from a file
write(2)	Write to a file
creat(2)	Create a new file or rewrite an existing one
unlink(2)	Remove a directory entry
lseek(2)	Move read/write file pointer

The following code sample demonstrates the use of the basic file I/O interface. read(2) and write(2) both transfer no more than the specified number of bytes, starting at the current offset into the file. The number of bytes actually transferred is returned. The end of a file is indicated on a read(2) by a return value of zero.

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Example 6–1 Basic File I/O Interface

#include			<fcntl.h>
#define			MAXSIZE			256

main()
{
    int     fd;
    ssize_t n;
    char	    array[MAXSIZE];

    fd = open ("/etc/motd", O_RDONLY);
    if (fd == -1) {
        perror ("open");
        exit (1);
    }
    while ((n = read (fd, array, MAXSIZE)) > 0)
        if (write (1, array, n) != n)
            perror ("write");
    if (n == -1)
        perror ("read");
    close (fd);
}

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When you are done reading or writing a file, always call close(2). Do not call close(2) for a file descriptor that was not returned from a call to open(2).

File pointer offsets into an open file are changed by using read(2), write(2), or by calls to lseek(2). The following example demonstrates the uses of lseek.

off_t     start, n;
struct    record    rec;

/* record current offset in start */
start = lseek (fd, 0L, SEEK_CUR);

/* go back to start */
n = lseek (fd, -start, SEEK_SET);
read (fd, &rec, sizeof (rec));

/* rewrite previous record */
n = lseek (fd, -sizeof (rec), SEEK_CUR);
write (fd, (char *&rec, sizeof (rec));

Advanced File I/O

The following table lists the tasks performed by advanced file I/O interfaces.

Table 6–2 Advanced File I/O Interfaces

Interface Name	Purpose
link(2)	Link to a file
access(2)	Determine accessibility of a file
mknod(2)	Make a special or ordinary file
chmod(2)	Change mode of file
chown(2), lchown(2), fchown(2)	Change owner and group of a file
utime(2)	Set file access and modification times
stat(2), lstat(2), fstat(2)	Get file status
fcntl(2)	Perform file control functions
ioctl(2)	Control device
fpathconf(2)	Get configurable path name variables
opendir(3C), readdir(3C), closedir(3C)	Perform directory operations
mkdir(2)	Make a directory
readlink(2)	Read the value of a symbolic link
rename(2)	Change the name of a file
rmdir(2)	Remove a directory
symlink(2)	Make a symbolic link to a file

File System Control

The file system control interfaces listed in the following table enable the control of various aspects of the file system.

Table 6–3 File System Control Interfaces

Interface Name	Purpose
ustat(2)	Get file system statistics
sync(2)	Update super block
mount(2)	Mount a file system
statvfs(2), fstatvfs(2)	Get file system information
sysfs(2)	Get file system type information