LIBRARY

Lb libc

SYNOPSIS

In fcntl.h Ft int Fn open const char *path int flags ... Ft int Fn openat int fd const char *path int flags ...

DESCRIPTION

The file name specified by Fa path is opened for either execution or reading and/or writing as specified by the argument Fa flags and the file descriptor returned to the calling process. The Fa flags argument may indicate the file is to be created if it does not exist (by specifying the O_CREAT flag). In this case Fn open and Fn openat require an additional argument Fa mode_t mode , and the file is created with mode Fa mode as described in chmod(2) and modified by the process' umask value (see umask(2)).

The Fn openat function is equivalent to the Fn open function except in the case where the Fa path specifies a relative path. In this case the file to be opened is determined relative to the directory associated with the file descriptor Fa fd instead of the current working directory. The Fa flag parameter and the optional fourth parameter correspond exactly to the parameters of Fn open . If Fn openat is passed the special value AT_FDCWD in the Fa fd parameter, the current working directory is used and the behavior is identical to a call to Fn open .

The flags specified are formed by or 'ing the following values

O_RDONLY        open for reading only
O_WRONLY        open for writing only
O_RDWR          open for reading and writing
O_EXEC          open for execute only
O_NONBLOCK      do not block on open
O_APPEND        append on each write
O_CREAT         create file if it does not exist
O_TRUNC         truncate size to 0
O_EXCL          error if create and file exists
O_SHLOCK        atomically obtain a shared lock
O_EXLOCK        atomically obtain an exclusive lock
O_DIRECT        eliminate or reduce cache effects
O_FSYNC         synchronous writes
O_SYNC          synchronous writes
O_NOFOLLOW      do not follow symlinks
O_NOCTTY        don't assign controlling terminal
O_TTY_INIT      restore default terminal attributes
O_DIRECTORY     error if file is not a directory
O_CLOEXEC       set FD_CLOEXEC upon open

Opening a file with O_APPEND set causes each write on the file to be appended to the end. If O_TRUNC is specified and the file exists, the file is truncated to zero length. If O_EXCL is set with O_CREAT and the file already exists, Fn open returns an error. This may be used to implement a simple exclusive access locking mechanism. If O_EXCL is set and the last component of the pathname is a symbolic link, Fn open will fail even if the symbolic link points to a non-existent name. If the O_NONBLOCK flag is specified and the Fn open system call would result in the process being blocked for some reason (e.g., waiting for carrier on a dialup line), Fn open returns immediately. The descriptor remains in non-blocking mode for subsequent operations.

If O_FSYNC is used in the mask, all writes will immediately be written to disk, the kernel will not cache written data and all writes on the descriptor will not return until the data to be written completes.

O_SYNC is a synonym for O_FSYNC required by POSIX

If O_NOFOLLOW is used in the mask and the target file passed to Fn open is a symbolic link then the Fn open will fail.

When opening a file, a lock with flock(2) semantics can be obtained by setting O_SHLOCK for a shared lock, or O_EXLOCK for an exclusive lock. If creating a file with O_CREAT the request for the lock will never fail (provided that the underlying file system supports locking).

O_DIRECT may be used to minimize or eliminate the cache effects of reading and writing. The system will attempt to avoid caching the data you read or write. If it cannot avoid caching the data, it will minimize the impact the data has on the cache. Use of this flag can drastically reduce performance if not used with care.

O_NOCTTY may be used to ensure the OS does not assign this file as the controlling terminal when it opens a tty device. This is the default on Fx , but is present for POSIX compatibility. The Fn open system call will not assign controlling terminals on Fx .

O_TTY_INIT may be used to ensure the OS restores the terminal attributes when initially opening a TTY. This is the default on Fx , but is present for POSIX compatibility. The initial call to Fn open on a TTY will always restore default terminal attributes on Fx .

O_DIRECTORY may be used to ensure the resulting file descriptor refers to a directory. This flag can be used to prevent applications with elevated privileges from opening files which are even unsafe to open with O_RDONLY such as device nodes.

O_CLOEXEC may be used to set FD_CLOEXEC flag for the newly returned file descriptor.

If successful, Fn open returns a non-negative integer, termed a file descriptor. It returns -1 on failure. The file pointer used to mark the current position within the file is set to the beginning of the file.

If a sleeping open of a device node from devfs(5) is interrupted by a signal, the call always fails with Er EINTR , even if the SA_RESTART flag is set for the signal. A sleeping open of a fifo (see mkfifo(2)) is restarted as normal.

When a new file is created it is given the group of the directory which contains it.

Unless O_CLOEXEC flag was specified, the new descriptor is set to remain open across execve(2) system calls; see close(2), fcntl(2) and O_CLOEXEC description.

The system imposes a limit on the number of file descriptors open simultaneously by one process. The getdtablesize(2) system call returns the current system limit.

RETURN VALUES

If successful, Fn open and Fn openat return a non-negative integer, termed a file descriptor. They return -1 on failure, and set errno to indicate the error.

HISTORY

The Fn open function appeared in AT&T System v6 . The Fn openat function was introduced in Fx 8.0 .