open(2) - NetBSD Manual Pages

OPEN(2)                   NetBSD 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 are specified by or'ing the following values.
     Applications must specify exactly one of the first three values (file ac-
     cess modes):

           O_RDONLY    Open for reading only.

           O_WRONLY    Open for writing only.

           O_RDWR      Open for reading and writing.

     Any combination of the following may be used:

           O_NONBLOCK  Do not block on open or for data to become available.

           O_APPEND    Append to the file on each write.

           O_CREAT     Create the file if it does not exist, 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)).

           O_TRUNC     Truncate size to 0.

           O_EXCL      Error if O_CREAT and the file already exists.

           O_SHLOCK    Atomically obtain a shared lock.

           O_EXLOCK    Atomically obtain an exclusive lock.

           O_DSYNC     If set, write operations will be performed according to
                       synchronized I/O data integrity completion: each write
                       will wait for the file data to be committed to stable
                       storage.

           O_SYNC      If set, write operations will be performed according to
                       synchronized I/O file integrity completion: each write
                       will wait for both the file data and file status to be
                       committed to stable storage.

           O_RSYNC     If set, read operations will complete at the same level
                       of integrity which is in effect for write operations:
                       if specified together with O_SYNC, each read will wait
                       for the file status to be committed to stable storage.

                       Combining O_RSYNC with O_DSYNC only, or specifying it
                       without any other synchronized I/O integrity completion
                       flag set, has no further effect.

     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 lock-
     ing 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 available.  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 mak-
     ing 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.  Calling 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.

     [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).


SEE ALSO

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


STANDARDS

     The open() function conforms to IEEE Std1003.1-1990 (``POSIX'').  The
     flags values O_DSYNC, O_SYNC and O_RSYNC are extensions defined in IEEE
     Std1003.1b-1993 (``POSIX'').


HISTORY

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

NetBSD 1.4                     November 16, 1993                             3