UNIX File I/O

File I/O system calls

open

Example (taken from man open in Linux):

#include <fcntl.h>
...
int fd;
mode_t mode = S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH;
char *pathname = "/tmp/file";
...
fd = open(pathname, O_WRONLY | O_CREAT | O_TRUNC, mode);
...

Another example – creating a lock file:

fd = open("/var/run/myprog.pid", O_WRONLY | O_CREAT | O_EXCL, 0644);

creat

Redundant: creat(path, mode) is equivalent to open(path, O_WRONLY|O_CREAT|O_TRUNC, mode)

And it should have been called create, says Ken Thompson

close

int close(int fildes);

lseek

off_t lseek(int fildes, off_t offset, int whence);

• If whence is SEEK_SET, the file offset shall be set to offset bytes.
• If whence is SEEK_CUR, the file offset shall be set to its current location plus offset.
• If whence is SEEK_END, the file offset shall be set to the size of the file plus offset.

read

ssize_t read(int fildes, void *buf, size_t nbyte);

• returns number of bytes read, 0 if end of file, -1 on error

Note:

• Number of bytes read may be less than the requested nbyte

• read() may block forever on a “slow” read from pipes, FIFOs (aka named pipes), sockets, or keyboard

• For sockets, read(socket, buf, nbyte) is equivalent to recv(socket, buf, nbyte, 0)

  recv(int socket, void *buffer, size_t length, int flags)
  

  • normally, recv() blocks until it has received at least 1 byte
  • returns num bytes received, 0 if connection closed, -1 if error

write

ssize_t write(int fildes, const void *buf, size_t nbyte);

• returns number of bytes written, -1 on error

Note:

• Number of bytes written may be less than the requested nbyte – ex) filling up a disk

• write() may block forever on a “slow” write into pipes, FIFOs, or sockets

• For sockets, write(socket, buf, nbyte) is equivalent to send(socket, buf, nbyte, 0)

  send(int socket, const void *buffer, size_t length, int flags)
  

  • normally, send() blocks until it sends all bytes requested
  • returns num bytes sent or -1 for error

• If the file was opened with O_APPEND flag, the file offset gets set to the end of the file prior to each write

  • setting of the offset and writing happen in an atomic operation

File sharing

1. Kernel data structures for open files

   

2. Two independent processes with the same file open

   

3. Kernel data structures after dup(1)

   

4. Sharing of open files between parent and child after fork

   

Last updated: 2019–01–28