OPEN(2) MachTen Programmer’s Manual 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 cre-
ated 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 ap-
pended 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 per-
form 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 de-
scriptor. 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 system calls; see
close(2) and fcntl(2).

The system imposes a limit on the number of file descriptors open simul-
taneously by one process. Getdtablesize(2) returns the current system
limit.

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 ex-
ist.

[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 di-
rectory in which it is to be created does not permit writ-
ing.

[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 de-
scriptors.

[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 di-
rectory 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 di-
rectory 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 ex-
hausted.

[EDQUOT] O_CREAT is specified, the file does not exist, and the us-
er’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 be-
ing 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 imple-
mented).

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

HISTORY
An open function call appeared in Version 6 AT&T UNIX.

4th Berkeley Distribution November 16, 1993 3