OPEN(S) UNIX System V OPEN(S)
Name
open - open for reading or writing
Syntax
#include <fcntl.h>
int open (path, oflag [, mode] )
char *path;
int oflag, mode;
Description
path points to a path name naming a file. The open system
call opens a file descriptor for the named file and sets the
file status flags according to the value of oflag. For
non-STREAMS (see intro(S)) files, oflag values are
constructed by OR-ing flags from the following list (only
one of the first three flags below may be used):
O_RDONLY Open for reading only.
O_WRONLY Open for writing only.
O_RDWR Open for reading and writing.
O_NDELAY This flag may affect subsequent reads and writes
(see read(S) and write(S)).
When opening a FIFO with O_RDONLY or O_WRONLY set:
If O_NDELAY is set:
An open for reading-only will return without
delay. An open for writing-only will return
an error if no process currently has the file
open for reading.
If O_NDELAY is clear:
An open for reading-only will block until a
process opens the file for writing. An open
for writing-only will block until a process
opens the file for reading.
When opening a file associated with a communication
line:
If O_NDELAY is set:
The open will return without waiting for
carrier.
If O_NDELAY is clear:
The open will block until carrier is present.
O_APPEND If set, the file pointer will be set to the end of
the file prior to each write.
O_SYNC When opening a regular file, this flag affects
subsequent writes. If set, each write(S) will wait
for both the file data and file status to be
physically updated.
O_CREAT If the file exists, this flag has no effect.
Otherwise, the owner ID of the file is set to the
effective user ID of the process; the group ID of
the file is set to the effective group ID of the
process; and the low-order 12 bits of the file mode
are set to the value of mode, modified as follows
[see creat(S)):
All bits set in the file mode creation mask of
the process are cleared (see umask(S)).
The ``save text image after execution bit'' of
the mode is cleared (see chmod(S)).
O_TRUNC If the file exists, its length is truncated to 0
and the mode and owner are unchanged.
O_EXCL If O_EXCL and O_CREAT are set, open will fail if
the file exists.
When opening a STREAMS file, oflag may be constructed from
O_NDELAY or-ed with either O_RDONLY, O_WRONLY or O_RDWR.
Other flag values are not applicable to STREAMS devices and
have no effect on them. The value of O_NDELAY affects the
operation of STREAMS drivers and certain system calls (see
read(S), getmsg(S), putmsg(S), and write(S)). For drivers,
the implementation of O_NDELAY is device-specific. Each
STREAMS device driver may treat this option differently.
Certain flag values can be set following open as described
in fcntl(S).
The file pointer used to mark the current position within
the file is set to the beginning of the file.
The new file descriptor is set to remain open across exec
system calls (see fcntl(S)).
The named file is opened unless one or more of the following
is true:
[EACCES] A component of the path prefix denies search
permission.
[EACCES] oflag permission is denied for the named
file.
[EAGAIN] The file exists, mandatory file/record
locking is set, and there are outstanding
record locks on the file (see chmod (S)).
[EEXIST] O_CREAT and O_EXCL are set, and the named
file exists.
[EFAULT] path points outside the allocated address
space of the process.
[EINTR] A signal was caught during the open system
call.
[EIO] A hangup or error occurred during a STREAMS
open.
[EISDIR] The named file is a directory and oflag is
write or read/write.
[EMFILE] No file descriptors are currently open.
[EMULTIHOP] Components of path require hopping to
multiple remote machines.
[ENFILE] The system file table is full.
[ENOENT] O_CREAT is not set and the named file does
not exist.
[ENOLINK] path points to a remote machine, and the link
to that machine is no longer active.
[ENOMEM] The system is unable to allocate a send
descriptor.
[ENOSPC] O_CREAT and O_EXCL are set, and the file
system is out of inodes.
[ENOSR] Unable to allocate a stream.
[ENOTDIR] A component of the path prefix is not a
directory.
[ENXIO] The named file is a character special or
block special file, and the device associated
with this special file does not exist.
[ENXIO] O_NDELAY is set, the named file is a FIFO,
O_WRONLY is set, and no process has the file
open for reading.
[ENXIO] A STREAMS module or driver open routine
failed.
[EROFS] The named file resides on a read-only file
system and oflag is write or read/write.
[ETXTBSY] The file is a pure procedure (shared text)
file that is being executed and oflag is
write or read/write.
See Also
chmod(S), close(S), creat(S), dup(S), fcntl(S), intro(S),
lseek(S), read(S), getmsg(S), putmsg(S), umask(S), write(S)
Diagnostics
Upon successful completion, the file descriptor is returned.
Otherwise, a value of -1 is returned, and errno is set to
indicate the error.
Standards Conformance
open is conformant with:
AT&T SVID Issue 2, Select Code 307-127;
The X/Open Portability Guide II of January 1987;
IEEE POSIX Std 1003.1-1988 with C Standard Language-
Dependent System Support;
and NIST FIPS 151-1.
(printed 6/20/89)