OPEN(2) BSD OPEN(2)
NAME
open - open a file for reading or writing, or create a new file
SYNOPSIS
#include <sys/file.h>
open(path, flags, mode)
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
pathname, terminated by a null character. The flags specified are formed
by ORing 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
succeed even if the symbolic link points to an existing name. If the
O_NDELAY flag is specified and the open call would result in the process
being blocked for some reason (for example waiting for a carrier on a
dial-up 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.
[ENAMETOOLONG] A component of a pathname exceeded 255 characters, or an
entire pathname exceeded 1023 characters.
[ENOENT] O_CREAT is not set and the named file does not exist.
[ENOENT] A component of the pathname 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 writting.
[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.
[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' 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)
NOTES
Not including any of O_RDONLY, O_WRONLY, or O_RDWR among the values ORed
to obtain flags implies opening the file pointed to by path for reading
only.
On other implementations, 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.
The following errors, not returned under Domain/OS BSD, may be returned
under other implementations:
[EINVAL] The pathname contains a character with the high-order
bit set.
[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.