 |
» |
|
|
|
NAMEopen() — open file for reading or writing SYNOPSIS#include <fcntl.h> int open(const char *path, int oflag, ... /* [mode_t mode] */ ); RemarksThe ANSI C ", ..." construct specifies a variable length argument list
whose optional member is given in the associated comment
(/* */). DESCRIPTIONThe
open()
system call opens a file descriptor for the named file
and sets the file status flags according to the value of
oflag. The
path
argument points to a path name naming a file, and must not exceed
PATH_MAX
bytes in length. The
oflag
argument is a value that is the bitwise inclusive OR of
flags listed in "Read-Write Flags," "General Flags,"
"Synchronized I/O Flags," and "Direct I/O Flags" below. The optional
mode
argument is only effective when the
O_CREAT
flag is specified. 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(2). Read-Write FlagsExactly one of the
O_RDONLY,
O_WRONLY,
or
O_RDWR
flags must be used in composing the value of
oflag.
If none or more than one is used, the behavior is undefined.
- O_RDONLY
Open for reading only. - O_WRONLY
Open for writing only. - O_RDWR
Open for reading and writing.
General FlagsSeveral of the flags listed below can be changed with the
fcntl()
system call while the file is open.
See
fcntl(2)
and
fcntl(5)
for details.
- O_APPEND
If set, the file offset is set to the end of the file prior
to each write. - O_CREAT
If the file exists, this flag has no effect, except as
noted under
O_EXCL
below.
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
if the set-group-ID bit of the parent directory is not set,
or to the group ID of the parent directory
if the set-group-ID bit of the parent directory is set. The file access permission bits of the new file mode are
set to the value of
mode,
modified as follows (see
creat(2)):
For each bit set in the file mode creation mask of the process,
the corresponding bit in the new file mode is cleared (see
umask(2)). The "save text image after execution" bit of the new file
mode is cleared.
See
chmod(2). On HFS file systems with access control lists,
three base ACL entries are created
corresponding to the file access permissions (see
acl(5)). On JFS file systems that support access control lists,
optional ACL entries are created
corresponding to the parent directory's default ACL entries
(see
aclv(5)).
- O_EXCL
If
O_EXCL
and
O_CREAT
are set and the file exists,
open()
fails. - O_LARGEFILE
This is a non-standard flag which may be used by
32-bit applications to access files larger than 2 GB.
See
creat64(2). - O_NDELAY
This flag might affect subsequent reads and writes.
See
read(2)
and
write(2). When opening a FIFO with
O_RDONLY
or
O_WRONLY
set:
If
O_NDELAY
is set:
A read-only
open()
returns without delay. A write-only
open()
returns an error if no process
currently has the file open for reading.
If
O_NDELAY
is clear:
A read-only
open()
does not return until a process opens the file for writing. A write-only
open()
does not return until a process opens the file for reading.
When opening a file associated with a communication line:
If
O_NDELAY
is set:
The
open()
returns without waiting for carrier.
If
O_NDELAY
is clear:
The
open()
does not return until carrier is present.
- O_NOCTTY
If set, and
path
identifies a terminal device,
open()
does not cause the terminal to become the controlling terminal
for the process. - O_NOFOLLOW
If the last component of
path
refers to a symbolic link, the
open()
will fail. This is true regardless of whether the target of the link exists
or not. NOTE: If the symbolic link in
path
is followed by a trailing '/', the link will be traversed, regardless of
the setting of
O_NOFOLLOW. - O_NONBLOCK
Same effect as
O_NDELAY
for
open(2),
but slightly different effect in
read(2)
and
write(2).
If both
O_NONBLOCK
and
O_NDELAY
are specified,
O_NONBLOCK
takes precedence. - O_TRUNC
If the file exists, its length is truncated to 0
and the mode and owner are unchanged.
Synchronized I/O FlagsTogether, the
O_DSYNC,
O_RSYNC,
and
O_SYNC
flags constitute support for Synchronized I/O.
These flags are ignored for files
other than ordinary files and block special files
on those systems that permit I/O to block special devices (see
pathconf(2)).
If both the
O_DSYNC
and
O_SYNC
flags are set,
the effect is as if only the
O_SYNC
flag was set.
The
O_RSYNC
flag is ignored if it is not set along with the
O_DSYNC
or
O_SYNC
flag.
- O_DSYNC
If a file is opened with
O_DSYNC
or that flag is set with the
F_SETFL
option of
fcntl(),
writes to that file by the process block
until the data specified in the write request
and all file attributes required to retrieve the data
are written to the disk.
File attributes that are not necessary for data retrieval
(access time, modification time, status change time)
are not necessarily written to the disk
prior to returning to the calling process. - O_SYNC
Identical to
O_DSYNC,
with the addition that all file attributes changed by the write operation
(including access time, modification time, and status change time)
are also written to the disk prior to returning to the calling process. - O_RSYNC|O_DSYNC (specified together)
Identical to
O_DSYNC
for file system writes. For file system reads,
the calling process blocks until the data being read
and all file attributes required to retrieve the data
are the same as their image on disk.
Writes pending on the data to be read are executed
prior to returning to the calling process. - O_RSYNC|O_SYNC (specified together)
Identical to
O_SYNC
for file system writes. Identical to
O_RSYNC|O_DSYNC
for file system reads,
with the addition that all file attributes changed by the read operation
(including access time, modification time, and status change time)
too are the same as their image on disk.
RETURN VALUEopen()
returns the following values:
- n
Successful completion.
n
is a file descriptor for the opened file. - -1
Failure.
errno
is set to indicate the error.
ERRORSIf
open()
fails,
errno
is set to one of the following values.
- [EACCES]
oflag
permission is denied for the named file. - [EACCES]
A component of the path prefix denies search permission. - [EACCES]
The file does not exist and the directory
in which the file is to be created does not permit writing. - [EACCES]
O_TRUNC
is specified and write permission is denied. - [EAGAIN]
The file exists, enforcement mode file/record locking is set (see
chmod(2)),
there are outstanding record locks on the file with the
flock(),
lockf(),
or
fcntl()
system calls, and
O_TRUNC
is set. - [EDQUOT]
User's disk quota block or inode limit
has been reached for this file system. - [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, and the system call was not restarted (see
signal(5)). - [EINVAL]
oflag
specifies both
O_WRONLY
and
O_RDWR. - [EISDIR]
The named file is a directory and
oflag
is write or read/write. - [ELOOP]
Too many symbolic links are encountered in translating the path name. - [EMFILE]
The maximum number of file descriptors allowed are currently open. - [ENAMETOOLONG]
The length of the specified path name exceeds
PATH_MAX
bytes, or the length of a component of the path name exceeds
NAME_MAX
bytes while
_POSIX_NO_TRUNC
is in effect. - [ENFILE]
The system file table is full. - [ENODEV]
The named file is a character special or block special file,
and the driver associated with this special file has not
been configured into the kernel or the DLKM driver associated
with this special file failed to load. - [ENOENT]
The named file does not exist (for example,
path
is null or a component of
path
does not exist, or the file itself does not exist and
O_CREAT
is not set). - [ENOTDIR]
A component of the path prefix is not a directory. - [ENOTSUP]
The last component of
path
is a sybolic link and
O_NOFOLLOW
was set. - [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]
The named file is a character special or block special file,
and the device associated with this special file does not exist. - [ENOSPC]
O_CREAT
is specified,
the file does not already exist,
and the directory that would contain the file cannot be extended. - [EOVERFLOW]
The named file is a regular file and the size of the file cannot
be represented correctly in an object of size
off_t. - [EROFS]
The named file resides on a read-only file system and
oflag
is write or read/write. - [ETXTBSY]
The file is open for execution and
oflag
is write or read/write.
Normal executable files are only open for a short time
when they start execution.
Other executable file types can be kept open for a long time,
or indefinitely under some circumstances.
EXAMPLESThe following call to
open()
opens file
inputfile
for reading only and returns a file descriptor for
inputfile.
For an example of reading from file
inputfile,
see the
read(2)
manual entry. int infd;
infd = open ("inputfile", O_RDONLY); The following call to
open()
opens file
outputfile
for writing and returns a file descriptor for
outputfile.
For an example of preallocating disk space for
outputfile,
see the
prealloc(2)
manual entry.
For an example of writing to
outputfile,
see the
write(2)
manual entry. int outfd;
outfd = open ("outputfile", O_WRONLY); The following call opens file
iofile
for synchronized I/O file integrity for reads and writes. int iofd;
iofd = open ("iofile", O_RDWR|O_SYNC|O_RSYNC); AUTHORopen()
was developed by HP, AT&T, and the University of California, Berkeley. SEE ALSOacl(2),
chmod(2),
close(2),
creat(2),
dup(2),
fcntl(2),
flock(2),
lockf(2),
lseek(2),
creat64(2),
pathconf(2),
read(2),
select(2),
umask(2),
write(2),
setacl(2),
acl(5),
aclv(5),
fcntl(5),
privileges(5),
signal(5),
thread_safety(5),
unistd(5). STANDARDS CONFORMANCEopen(): AES, SVID2, SVID3, XPG2, XPG3, XPG4, FIPS 151-2, POSIX.1, POSIX.4 |
Printable version
