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OPEN(2) FreeBSD System Calls Manual OPEN(2)
NAME
open -- open or create a file for reading or writing
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include
int
open(const char *path, int flags, ...);
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 this case
open() requires a third argument mode_t mode, and 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
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
O_DIRECT eliminate or reduce cache effects
O_FSYNC synchronous writes
O_NOFOLLOW do not follow symlinks
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 and the
file already exists, open() returns an error. This may be used to imple-
ment 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 and the open() system call would result in
the process being blocked for some reason (e.g., waiting for carrier on a
dialup line), open() returns immediately. The descriptor remains in non-
blocking mode for subsequent operations.
If O_FSYNC is used in the mask, all writes will immediately be written to
disk, the kernel will not cache written data and all writes on the
descriptor will not return until the data to be written completes.
If O_NOFOLLOW is used in the mask and the target file passed to open() is
a symbolic link then the open() will fail.
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 file system supports locking).
O_DIRECT may be used to minimize or eliminate the cache effects of read-
ing and writing. The system will attempt to avoid caching the data you
read or write. If it cannot avoid caching the data, it will minimize the
impact the data has on the cache. Use of this flag can drastically
reduce performance if not used with care.
If successful, open() returns a non-negative integer, termed a file
descriptor. 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. The getdtablesize(2) system call returns the
current system limit.
RETURN VALUES
If successful, open() returns a non-negative integer, termed a file
descriptor. It returns -1 on failure, and sets errno to indicate the
error.
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 255 characters, or
an entire path name exceeded 1023 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
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 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
writing.
[ELOOP] Too many symbolic links were encountered in translat-
ing the pathname.
[EISDIR] The named file is a directory, and the arguments spec-
ify 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 descriptors.
[ENFILE] The system file table is full.
[EMLINK] O_NOFOLLOW was specified and the target is a symbolic
link.
[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, no process has it open for
reading, and the arguments specify it is to be opened
for writing.
[EINTR] The open() operation was interrupted by a signal.
[EOPNOTSUPP] O_SHLOCK or O_EXLOCK is specified but the underlying
file system does not support locking.
[EOPNOTSUPP] The named file is a special file mounted through a
file system that does not support access to it (e.g.
NFS).
[EWOULDBLOCK] O_NONBLOCK and one of O_SHLOCK or O_EXLOCK is speci-
fied and the file is 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 file system 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 file system containing the direc-
tory 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.
[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() system call requests
write access.
[EFAULT] The path argument points outside the process's allo-
cated 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).
[EINVAL] An attempt was made to open a descriptor with an ille-
gal combination of O_RDONLY, O_WRONLY, and O_RDWR.
SEE ALSO
chmod(2), close(2), dup(2), getdtablesize(2), lseek(2), read(2),
umask(2), write(2), fopen(3)
HISTORY
The open() function appeared in Version 6 AT&T UNIX.
FreeBSD 6.1 November 16, 1993 FreeBSD 6.1
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