The functions listed in Table 5-1 perform basic operations on files:
Table 5-1 Basic File I/O Functions
Function Name |
Purpose |
---|---|
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.
#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); }
Always close(2) a file when you are done reading or writing it, but never close(2) a file descriptor that you did not open(2).
Offset into an open file are changed by read(2)s, write(2)s, or by calls to lseek(2). Some examples of using lseek(2) are:
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));