Read carefully USP: 4.1 - 4.3, 4.6 - 4.7
UNIX I/O is done with file descriptors.
Normally, when your program starts, there are 3 file descriptors open:
STDIN_FILENO
STDOUT_FILENO
STDERR_FILENO
You must use these symbolic values in your code.
read
#include <unistd.h>
ssize_t read(int fildes, void *buf, size_t nbyte);
- It is not an error if read returns fewer bytes than requested - see Assignment 0.
- It is the programmer's responsibility to make sure all bytes neede have been read.
- r_read does not fix this problem.
- Look at the implementation of readline.
write
#include <unistd.h>
ssize_t write(int fildes, const void *buf, size_t nbyte);
- It is not an error if this returns a value greater than 0 and less than
nbyte
- You must restart the write if it returns fewer bytes than
requested.
- r_write can be used to make sure you write all requested bytes.
open and close
#include <fcntl.h>
#include <sys/stat.h>
int open(const char *path, int oflag);
int open(const char *path, int oflag, mode_t mode);
Important values of the
flag include:
O_RDONLY: read only
O_WRONLY: write only
O_RDWR: read and write
O_APPEND: writes always write to end
O_CREAT: create the file if it does not exist
O_EXCL: used with O_CREAT, return an error if file exists
O_NONBLOCK: do not block if not ready to open, also affects reads and writes
O_TRUNC: discard previous contents
You must use the 3-parameter form of
open if the
O_CREAT
flag is used. This specifies permissions.
Permissions are given by an integer (
mode_t), traditionally given in octal.
For this course you must use symbolic values for the permissions:
| S_IRUSR | read permission bit for owner |
| S_IWUSR | write permission bit for owner |
| S_IXUSR | execute permission bit for owner |
| S_IRWXU | read, write, execute for owner |
| |
| S_IRGRP | read permission bit for group |
| S_IWGRP | write permission bit for group |
| S_IXGRP | execute permission bit for group |
| S_IRWXG | read, write, execute for group |
| |
| S_IROTH | read permission bit for others |
| S_IWOTH | write permission bit for others |
| S_IXOTH | execute permission bit for others |
| S_IRWXO | read, write, execute for others |
| |
| S_ISUID | set user ID on execution |
| S_ISGID | set group ID on execution |
Table 4.1, Page 105: POSIX symbolic names for file permission.
#include <unistd.h>
int close(int fildes);
Open files are closed when your program exits normally.
Continue with main Chapter 4 notes.