open(): Opening Files#

#include <fcntl.h>

int open(const char *pathname, int flags);
int open(const char *pathname, int flags, mode_t mode);

#include <unistd.h>

int close(int fd);

Swiss Army Knife#

Swiss army knife: multiple actions, governed by bitwise-or’ed flags:

  • Create/Open/Truncate/…

  • flags: access intention - open for reading, writing, appending, …

  • Hundreds of others

  • ⟶ man -s 2 open is a very hard read

open() Flags#

Intended Access

  • O_RDONLY: can only read(); error on write() (see Reading Files)

  • O_WRONLY: can only write(); error on read() (see Writing Files)

  • O_RDWR: both

Creating a File (see Creating Files)

  • O_CREAT: create if not exists, else simply open

    • If file is created, mode is used - file permissions

  • O_CREAT|O_EXCL: exclusive creation

    • error if file exists

    • ⟶ to prevent race conditions when two parties try to create a file at the same time

    • security measure

Miscellaneous

  • O_APPEND: write access appends at the end

  • O_TRUNC: truncate file to zero length if already exists

  • O_CLOEXEC: closes the file descriptor on exec() (⟶ later)

More About Each Of flags And mode#