OPEN(2)                   OpenBSD 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 or for data to become available
           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, do not wait for the device or file to be ready or avail-
     able.  If 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.  This flag also has the effect of making all subsequent I/O
     on the open file non-blocking.

     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(2) 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(3) 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 {NAME_MAX} characters,
                   or an entire path name exceeded {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 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.

     [EINVAL]      The flags specified for opening the file are not valid.

     [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).

     [EAGAIN]      O_NONBLOCK and either O_EXLOCK or O_SHLOCK are set and the
                   file is already locked.

SEE ALSO
     chmod(2),  close(2),  dup(2),  flock(2),  lseek(2),  read(2),  umask(2),
     write(2),  getdtablesize(3)

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

OpenBSD 2.6                    November 16, 1993                             3