The following interfaces perform basic operations on files and on character I/O devices.
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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 5-1 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);
}
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));