Openflags

Openflags are a set of bitwise options passed to a file opening system call to control how a file is opened. They are most commonly associated with the POSIX open function, where the flags modify access mode, creation behavior, and I/O characteristics of the resulting file descriptor. Openflags are typically combined using bitwise OR to produce a single integer argument.

The flags fall into several categories. The access mode flags determine how the file will be accessed: O_RDONLY for read-only, O_WRONLY for write-only, and O_RDWR for read-write. Other flags modify behavior without changing the basic access mode, such as O_CREAT to create the file if it does not exist, O_EXCL to fail if the file already exists (useful for safely creating), and O_TRUNC to truncate an existing file. Additional modifiers include O_APPEND (write at end), O_NONBLOCK (non-blocking I/O), and various synchronization flags like O_SYNC, O_DSYNC, and O_RSYNC. Platform-specific flags include O_CLOEXEC (do not keep the descriptor across exec), O_NOFOLLOW, and O_DIRECTORY, among others.

A typical example is open("file.txt", O_WRONLY | O_CREAT | O_TRUNC, 0644), which opens for writing, creates the file if needed, and truncates it to zero length. The exact set of available flags and their values can vary by operating system and architecture, so portable code often relies on the standard constants provided by the system headers.

Cross-platform notes: while POSIX defines a core set of flags, some environments (such as Windows) provide different mechanisms or additional flags. When writing portable code, check the target platform’s documentation for the exact open flags and any caveats.

Security considerations include using O_EXCL with O_CREAT to avoid TOCTOU races, and using O_CLOEXEC to prevent file descriptors from leaking into child processes.