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

SYNOPSIS
#include <sys/file.h>

open(path, flags, mode)
const char *path;
int flags, mode;

DESCRIPTION
Open opens the file path for reading and/or writing, as specified
by the flags argument and returns a descriptor for that file. The
flags argument may indicate the file is to be created if it does
not already 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)).

Path is the address of a string of ASCII characters representing a
path name, terminated by a null character. 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_NDELAY 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

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, then if the file already exists, the open returns an
error. This can 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, the open will fail even if the
symbolic link points to a non-existent name. If the O_NDELAY 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), the open returns immediately. The first time the process
attempts to perform i/o on the open file it will block (not
currently implemented).

Upon successful completion a non-negative integer termed a file
descriptor is returned. The file pointer used to mark the current
position within the file is set to the beginning of the file.

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

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

ERRORS
The named file is opened unless one or more of the following are
true:

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

[EINVAL] The pathname contains a character with the high-
order bit set.

[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 named flag.

[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 system limit for open file descriptors per
process has already been reached.

[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.

[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),
write(2), umask(2)