open - open a file for reading or writing, or create a new file
int open(const char *path, int flags [, mode_t 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
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, the open returns immediately.
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
The system imposes a limit on the number of descriptors open
simultaneously by one process.
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.
[ENAMETOOLONG] The path name exceeds PATH_MAX 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
[EACCES] Search permission is denied for a component of the path
[EACCES] The required permissions (for reading and/or writing) are
denied for the named file.
[EACCES] O_CREAT is specified, the file does not exist, and the
directory in which it is to be created does not permit
[EACCES] A device to be opened for writing is physically write
[ELOOP] Too many symbolic links were encountered in translating
the pathname. (Minix-vmd)
[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
[EIO] An I/O error occurred while making the directory entry or
allocating the inode for O_CREAT.
[EFAULT] Path points outside the process's allocated address space.
[EEXIST] O_CREAT and O_EXCL were specified and the file exists.
chmod(2), close(2), dup(2), fcntl(2), lseek(2), read(2), write(2),