OPEN(2)

NAME
open - open or create a file for reading or writing

LIBRARY
libc

SYNOPSIS
#include <fcntl.h>
int
open(const char *path, int flags, ...);

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

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_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_NOFOLLOW	do not follow symlinks

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, 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, open() will fail even if the symbolic link points to a non-existent name. If the O_NONBLOCK flag is specified and the open() system call would result in the process being blocked for some reason (e.g., waiting for carrier on a dialup line), 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.

If O_NOFOLLOW is used in the mask and the target file passed to open() is a symbolic link then the 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.

If successful, 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.

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

The new descriptor is set to remain open across execve(2) system calls; see close(2) and fcntl(2).

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, open() returns a non-negative integer, termed a file descriptor. It returns -1 on failure, and sets errno to indicate the error.

ERRORS
The named file is opened unless:

[ENOTDIR]
    A component of the path prefix is not a directory.

[ENAMETOOLONG]
    A component of a pathname exceeded 255 characters, or an entire path name exceeded 1023 characters.

[ENOENT]
    O_CREAT is not set and the named file does not exist.

[ENOENT]
    A component of the path name that must exist does not exist.

[EACCES]
    Search permission is denied for a component of the path prefix.

[EACCES]
    The required permissions (for reading and/or writing) are denied for the given flags.

[EACCES]
    O_CREAT is specified, the file does not exist, and the directory in which it is to be created does not permit writing.

[ELOOP]
    Too many symbolic links were encountered in translating the pathname.

[EISDIR]
    The named file is a directory, and the arguments specify it is to be opened for writing.

[EROFS]
    The named file resides on a read-only file system, and the file is to be modified.

[EMFILE]
    The process has already reached its limit for open file descriptors.

[ENFILE]
    The system file table is full.

[EMLINK]
    O_NOFOLLOW was specified and the target is a symbolic link.

[ENXIO]
    The named file is a character special or block special file, and the device associated with this special file does not exist.

[ENXIO]
    The named file is a fifo, no process has it open for reading, and the arguments specify it is to be opened for writing.

[EINTR]
    The open() operation was interrupted by a signal.

[EOPNOTSUPP]
    O_SHLOCK or O_EXLOCK is specified but the underlying file system does not support locking.

[EOPNOTSUPP]
    The named file is a special file mounted through a filesystem that does not support access to it (e.g. NFS).

[EWOULDBLOCK]
    O_NONBLOCK and one of O_SHLOCK or O_EXLOCK is specified and the file is locked.

[ENOSPC]
    O_CREAT is specified, the file does not exist, and the directory in which the entry for the new file is being placed cannot be extended because there is no space left on the file system containing the directory.

[ENOSPC]
    O_CREAT is specified, the file does not exist, and there are no free inodes on the file system on which the file is being created.

[EDQUOT]
    O_CREAT is specified, the file does not exist, and the directory in which the entry for the new file is being placed cannot be extended because the user's quota of disk blocks on the file system containing the directory has been exhausted.

[EDQUOT]
    O_CREAT is specified, the file does not exist, and the user's quota of inodes on the file system on which the file is being created has been exhausted.

[EIO]
    An I/O error occurred while making the directory entry or allocating the inode for O_CREAT.

[ETXTBSY]
    The file is a pure procedure (shared text) file that is being executed and the open() system call requests write access.

[EFAULT]
    The path argument points outside the process's allocated address space.

[EEXIST]
    O_CREAT and O_EXCL were specified and the file exists.

[EOPNOTSUPP]
    An attempt was made to open a socket (not currently implemented).

[EINVAL]
    An attempt was made to open a descriptor with an illegal combination of O_RDONLY, O_WRONLY, and O_RDWR.

SEE ALSO
chmod(2), close(2), dup(2), getdtablesize(2), lseek(2), read(2), umask(2), write(2), fopen(3)

HISTORY
The open() function appeared in AT&T UNIX.