Basic File I/O

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

Table 5-1 Basic File I/O Interfaces

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

The following code sample demonstrates the use of the basic file I/O interface.

Example 5-1 Using the 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);
}

In this example, the read and the write interfaces both transfer the specified number of bytes, starting at the current offset into the file. The number of bytes transferred is returned. The end of a file is indicated on a read by a return value of zero. For more information, see the read(2) and write(2) man pages.

When you are done reading or writing a file, call the close interface. Do not call close for a file descriptor that was not returned from a call to open. For more information, see the close(2), and open(2) man pages.

File pointer offsets into an open file are changed by using read, write, or by calls to lseek. For more information, see the read(2), write(2) and lseek(2) man pages.

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));