MirBSD manpage: open(2)

OPEN(2)                    BSD Programmer's Manual                     OPEN(2)


     open - open or create a file for reading or writing


     #include <fcntl.h>

     open(const char *path, int flags, mode_t mode);


     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 creat-
     ed 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 bitwise-inclusive OR'ing the following
     values. Exactly one of the first three values (file access modes) must be

           O_RDONLY     Open for reading only.
           O_WRONLY     Open for writing only.
           O_RDWR       Open for reading and writing.

     Any combination of the following flags may additionally be used:

           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_SYNC       Perform synchronous I/O operations.
           O_SHLOCK     Atomically obtain a shared lock.
           O_EXLOCK     Atomically obtain an exclusive lock.
           O_NOFOLLOW   If last path element is a symlink, don't follow it.
           O_CLOEXEC    Set new file descriptor to close on exec.
           O_DIRECTORY  Error if path does not name a directory.

     Opening a file with O_APPEND set causes each write on the file to be ap-
     pended to the end. If O_TRUNC and a writing mode are 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 either
     of O_EXCL or O_NOFOLLOW are 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 making all subsequent I/O on the open file non-blocking. If
     the O_SYNC flag is set, all I/O operations on the file will be done syn-

     A FIFO should either be opened with O_RDONLY or with O_WRONLY. The
     behavior for opening a FIFO with O_RDWR is undefined.

     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 open() is successful, the file pointer used to mark the current posi-
     tion 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


     If successful, open() returns a non-negative integer, termed a file
     descriptor. Otherwise, a value of -1 is returned and errno is set to in-
     dicate the error.


     The named file is opened unless:

     [ENOTDIR]     A component of the path prefix is not a directory.

     [ENOTDIR]     O_DIRECTORY is specified and path does not name a directo-

                   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-

     [EACCES]      Search permission is denied for a component of the path

     [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
                   directory in which it is to be created does not permit

     [ELOOP]       Too many symbolic links were encountered in translating the
                   pathname, or the O_NOFOLLOW flag was specified and the tar-
                   get is a symbolic link.

     [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 filesystem, and the
                   file is to be modified.

     [EMFILE]      The process has already reached its limit for open file

     [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.

     [ENXIO]       The named file is a FIFO, the O_NONBLOCK and O_WRONLY flags
                   are set, and no process has the file open for reading.

     [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.

                   O_NONBLOCK and one of O_SHLOCK or O_EXLOCK is specified and
                   the file is already locked.

     [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 filesystem 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 file is being
                   placed cannot be extended because the user's quota of disk
                   blocks on the filesystem containing the directory has been

     [EDQUOT]      O_CREAT is specified, the file does not exist, and the
                   user's quota of inodes on the filesystem 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-


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


     The open() function conforms to IEEE Std 1003.1-1990 ("POSIX.1") and
     X/Open Portability Guide Issue 4, Version 2 ("XPG4.2").

     POSIX specifies three different flavors for synchronous I/O: O_SYNC,
     O_DSYNC, and O_RSYNC. In OpenBSD, these are all equivalent.

     The O_SHLOCK, O_EXLOCK, and O_NOFOLLOW flags are non-standard extensions
     and should not be used if portability is of concern.


     An open() function call appeared in Version 2 AT&T UNIX.


     The O_TRUNC flag requires that one of O_RDWR or O_WRONLY also be speci-
     fied, else EINVAL is returned.

MirBSD #10-current              August 7, 2017                               3

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