Programming Interfaces Guide

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 



Open a file for reading or writing 


Close a file descriptor 


Read from a file 


Write to a file 


Create a new file or rewrite an existing one 


Remove a directory entry 


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.

Example 6–1 Basic File I/O Interface

#include			<fcntl.h>
#define			MAXSIZE			256

    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);

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));