open - open a file for reading or writing, or create a new file
open(path, flags, mode)
int flags, mode;
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_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_NOCTTY do not acquire as controlling terminal
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, 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_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), the open returns immediately. The first time the process
attempts to perform i/o on the open file it will block.
The flag O_NOCTTY indicates that even if the file is a terminal device, the call should
not result in acquiring the terminal device as the controlling terminal of the caller.
This flag is not the default and is currently unimplemented (it will be Real Soon Now).
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.
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.
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
[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
[EROFS] The named file resides on a read-only file system, and the file is to be
[EMFILE] The system limit for open file descriptors per process has already been
[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 fie is being placed cannot be extended because the
user's quota of disk blocks on the file system containing the directory has
[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
[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).
chmod(2), close(2), dup(2), getdtablesize(2), lseek(2), read(2), write(2), umask(2)
4th Berkeley Distribution Nov 30, 1994 OPEN(2)