fcntlFlags

fcntlFlags refer to the flags that control the behavior of a file descriptor when using the fcntl interface in POSIX systems. The fcntl function can query and modify two kinds of flags: file descriptor flags (FD flags) and file status flags.

File descriptor flags are accessed with the commands F_GETFD and F_SETFD. The primary flag in this category is FD_CLOEXEC, which marks a descriptor to be automatically closed when an exec-family function is invoked. Different platforms may expose additional FD flags, but FD_CLOEXEC is the standard example.

File status flags are accessed with F_GETFL and F_SETFL. These flags include the access mode bits (O_RDONLY, O_WRONLY, O_RDWR) and a set of status flags such as O_APPEND (append mode), O_NONBLOCK (non-blocking I/O), and various synchronous or direct I/O flags (for example O_SYNC, O_DSYNC, O_RSYNC, O_DIRECT on some systems). The exact set of status flags is platform-dependent. Importantly, F_SETFL can modify non-blocking and other behavior flags but cannot change the open access mode; changing that typically requires reopening the file.

In addition to these, fcntl supports advisory file locking via F_SETLK, F_SETLKW, and F_GETLK, which use struct flock to describe locks. These are separate capabilities from the standard FD and status flags but are commonly used alongside fcntlFlags to coordinate access to shared files.

Typical usage examples include obtaining current flags with fcntl(fd, F_GETFL), then modifying them (for example enabling non-blocking) with fcntl(fd, F_SETFL, newFlags). The exact behavior and available flags depend on the operating system and its headers (such as fcntl.h or sys/file.h).