OPEN(2)

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

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

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 which case 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

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() call would result in the process being blocked for some reason (e.g., waiting for carrier on a dialup line), open() returns immediately. The first time the process attempts to perform I/O on the open file it will block (not currently implemented).

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 filesystem supports locking).

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. Getdtablesize(2) returns the current system limit.

IMPLEMENTATION NOTES
In the non-threaded library open() is implemented as the open syscall.

In the threaded library, the open syscall is assembled to _thread_sys_open() and open() is implemented as a function which disables thread rescheduling and calls _thread_sys_open(). Before returning, open() enables thread rescheduling.

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.

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

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

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

[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() call requests write access.

[EFAULT]
    Path 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).

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

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