POSIX:fcntl

lastmod 2024-07-09created 2016-12-0510 edits

Syntax

#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
int open(const char *pathname, int flags);
int open(const char *pathname, int flags, mode_t mode);

#include <unistd.h>
#include <fcntl.h>
int fcntl(int fd, int cmd, ... /* arg */ );

open

Flags

Open option	Description
O_RDONLY	Open read-only
O_WRONLY	Open write-only
O_RDWR	Both reading and writing are allowed
O_CREAT	Creates the file if it does not exist.
O_EXCL	When `O_CREAT` is used, the file can be opened even if it already exists, and writing to it erases the previous contents.
O_TRUNC	Deletes all existing contents of the file.
O_APPEND	After open, the write pointer is placed at the end of the file so that writes append to the file.
O_NOCITTY	If the target being opened is a terminal, the terminal is not assigned as the program's controlling terminal. (It prevents it from becoming the controlling TTY so that the program is not terminated when a `CTRL`-`C` character arrives during data transfer.)
O_NONBLOCK	When there is nothing to read, returns immediately instead of waiting until there is something to read.
O_SYNC	When writing, waits until the actual write is complete. In other words, it returns only after the write has physically completed.

O_NONBLOCK

In BLOCK mode: When writing - waits until another thread reads the file with read.; When reading - waits until another thread writes to the file with write.

In NON BLOCK mode: When writing - if no other thread is waiting in a read state, it ends without delay by returning an error.; When reading - if there is something to read, it returns the number of bytes read; otherwise it returns -1. Either way it ends immediately without delay.

Let's take serial communication as an example. Without the O_NONBLOCK option:

fd = open( "/dev/ttyS3", O_RDWR | O_NOCTTY );

res = read(fd, buf, 255);

If you read() data this way, the program stays blocked until data arrives. It is easiest to think of the program as stopped. In the case of write(), it waits until the other side enters read().

With the O_NONBLOCK option:

fd = open( "/dev/ttyS3", O_RDWR | O_NOCTTY | O_NONBLOCK );

res = read(fd, buf, 255);

It returns without waiting, whether or not there is data. If there is data to read, it writes the data read into buf and returns the number of bytes read in res. If not, it returns -1 in res. write() also behaves the same way as read().