Originální popis anglicky:
open - open a file
Návod, kniha: POSIX Programmer's Manual
#include <sys/stat.h>
#include <fcntl.h>
int open(const char *
path, int
oflag, ... );
The
open() function shall establish the connection between a file and a
file descriptor. It shall create an open file description that refers to a
file and a file descriptor that refers to that open file description. The file
descriptor is used by other I/O functions to refer to that file. The
path argument points to a pathname naming the file.
The
open() function shall return a file descriptor for the named file
that is the lowest file descriptor not currently open for that process. The
open file description is new, and therefore the file descriptor shall not
share it with any other process in the system. The FD_CLOEXEC file descriptor
flag associated with the new file descriptor shall be cleared.
The file offset used to mark the current position within the file shall be set
to the beginning of the file.
The file status flags and file access modes of the open file description shall
be set according to the value of
oflag.
Values for
oflag are constructed by a bitwise-inclusive OR of flags from
the following list, defined in
<fcntl.h>. Applications shall
specify exactly one of the first three values (file access modes) below in the
value of
oflag:
- O_RDONLY
- Open for reading only.
- O_WRONLY
- Open for writing only.
- O_RDWR
- Open for reading and writing. The result is undefined if
this flag is applied to a FIFO.
Any combination of the following may be used:
- O_APPEND
- If set, the file offset shall be 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 file shall be created; the user ID of
the file shall be set to the effective user ID of the process; the group
ID of the file shall be set to the group ID of the file's parent directory
or to the effective group ID of the process; and the access permission
bits (see <sys/stat.h>) of the file mode shall be set to the
value of the third argument taken as type mode_t modified as
follows: a bitwise AND is performed on the file-mode bits and the
corresponding bits in the complement of the process' file mode creation
mask. Thus, all bits in the file mode whose corresponding bit in the file
mode creation mask is set are cleared. When bits other than the file
permission bits are set, the effect is unspecified. The third argument
does not affect whether the file is open for reading, writing, or for
both. Implementations shall provide a way to initialize the file's group
ID to the group ID of the parent directory. Implementations may, but need
not, provide an implementation-defined way to initialize the file's group
ID to the effective group ID of the calling process.
- O_DSYNC
- Write I/O operations on the file descriptor shall complete
as defined by synchronized I/O data integrity completion.
- O_EXCL
- If O_CREAT and O_EXCL are set, open() shall fail if
the file exists. The check for the existence of the file and the creation
of the file if it does not exist shall be atomic with respect to other
threads executing open() naming the same filename in the same
directory with O_EXCL and O_CREAT set. If O_EXCL and O_CREAT are set, and
path names a symbolic link, open() shall fail and set
errno to [EEXIST], regardless of the contents of the symbolic link.
If O_EXCL is set and O_CREAT is not set, the result is undefined.
- O_NOCTTY
- If set and path identifies a terminal device,
open() shall not cause the terminal device to become the
controlling terminal for the process.
- O_NONBLOCK
- When opening a FIFO with O_RDONLY or O_WRONLY set:
- *
- If O_NONBLOCK is set, an open() for reading-only
shall return without delay. An open() for writing-only shall return
an error if no process currently has the file open for reading.
- *
- If O_NONBLOCK is clear, an open() for reading-only
shall block the calling thread until a thread opens the file for writing.
An open() for writing-only shall block the calling thread until a
thread opens the file for reading.
When opening a block special or character special file that supports
non-blocking opens:
- *
- If O_NONBLOCK is set, the open() function shall
return without blocking for the device to be ready or available.
Subsequent behavior of the device is device-specific.
- *
- If O_NONBLOCK is clear, the open() function shall
block the calling thread until the device is ready or available before
returning.
Otherwise, the behavior of O_NONBLOCK is unspecified.
- O_RSYNC
- Read I/O operations on the file descriptor shall complete
at the same level of integrity as specified by the O_DSYNC and O_SYNC
flags. If both O_DSYNC and O_RSYNC are set in oflag, all I/O
operations on the file descriptor shall complete as defined by
synchronized I/O data integrity completion. If both O_SYNC and O_RSYNC are
set in flags, all I/O operations on the file descriptor shall complete as
defined by synchronized I/O file integrity completion.
- O_SYNC
- Write I/O operations on the file descriptor shall complete
as defined by synchronized I/O file integrity completion.
- O_TRUNC
- If the file exists and is a regular file, and the file is
successfully opened O_RDWR or O_WRONLY, its length shall be truncated to
0, and the mode and owner shall be unchanged. It shall have no effect on
FIFO special files or terminal device files. Its effect on other file
types is implementation-defined. The result of using O_TRUNC with O_RDONLY
is undefined.
If O_CREAT is set and the file did not previously exist, upon successful
completion,
open() shall mark for update the
st_atime,
st_ctime, and
st_mtime fields of the file and the
st_ctime and
st_mtime fields of the parent directory.
If O_TRUNC is set and the file did previously exist, upon successful completion,
open() shall mark for update the
st_ctime and
st_mtime
fields of the file.
If both the O_SYNC and O_DSYNC flags are set, the effect is as if only the
O_SYNC flag was set.
If
path refers to a STREAMS file,
oflag may be constructed from
O_NONBLOCK OR'ed with either O_RDONLY, O_WRONLY, or O_RDWR. Other flag values
are not applicable to STREAMS devices and shall have no effect on them. The
value O_NONBLOCK affects the operation of STREAMS drivers and certain
functions applied to file descriptors associated with STREAMS files. For
STREAMS drivers, the implementation of O_NONBLOCK is device-specific.
If
path names the master side of a pseudo-terminal device, then it is
unspecified whether
open() locks the slave side so that it cannot be
opened. Conforming applications shall call
unlockpt() before opening
the slave side.
The largest value that can be represented correctly in an object of type
off_t shall be established as the offset maximum in the open file
description.
Upon successful completion, the function shall open the file and return a
non-negative integer representing the lowest numbered unused file descriptor.
Otherwise, -1 shall be returned and
errno set to indicate the error. No
files shall be created or modified if the function returns -1.
The
open() function shall fail if:
- EACCES
- Search permission is denied on a component of the path
prefix, or the file exists and the permissions specified by oflag
are denied, or the file does not exist and write permission is denied for
the parent directory of the file to be created, or O_TRUNC is specified
and write permission is denied.
- EEXIST
- O_CREAT and O_EXCL are set, and the named file exists.
- EINTR
- A signal was caught during open().
- EINVAL
- The implementation does not support synchronized I/O for
this file.
- EIO
- The path argument names a STREAMS file and a hangup
or error occurred during the open().
- EISDIR
- The named file is a directory and oflag includes
O_WRONLY or O_RDWR.
- ELOOP
- A loop exists in symbolic links encountered during
resolution of the path argument.
- EMFILE
- {OPEN_MAX} file descriptors are currently open in the
calling process.
- ENAMETOOLONG
- The length of the path argument exceeds {PATH_MAX}
or a pathname component is longer than {NAME_MAX}.
- ENFILE
- The maximum allowable number of files is currently open in
the system.
- ENOENT
- O_CREAT is not set and the named file does not exist; or
O_CREAT is set and either the path prefix does not exist or the
path argument points to an empty string.
- ENOSR
- The path argument names a STREAMS-based file and the
system is unable to allocate a STREAM.
- ENOSPC
- The directory or file system that would contain the new
file cannot be expanded, the file does not exist, and O_CREAT is
specified.
- ENOTDIR
- A component of the path prefix is not a directory.
- ENXIO
- O_NONBLOCK 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.
- EOVERFLOW
- The named file is a regular file and the size of the file
cannot be represented correctly in an object of type off_t.
- EROFS
- The named file resides on a read-only file system and
either O_WRONLY, O_RDWR, O_CREAT (if the file does not exist), or O_TRUNC
is set in the oflag argument.
The
open() function may fail if:
- EAGAIN
- The path argument names the slave side of a
pseudo-terminal device that is locked.
- EINVAL
- The value of the oflag argument is not valid.
- ELOOP
- More than {SYMLOOP_MAX} symbolic links were encountered
during resolution of the path argument.
- ENAMETOOLONG
- As a result of encountering a symbolic link in resolution
of the path argument, the length of the substituted pathname string
exceeded {PATH_MAX}.
- ENOMEM
- The path argument names a STREAMS file and the
system is unable to allocate resources.
- ETXTBSY
- The file is a pure procedure (shared text) file that is
being executed and oflag is O_WRONLY or O_RDWR.
The following sections are informative.
The following example opens the file
/tmp/file, either by creating it (if
it does not already exist), or by truncating its length to 0 (if it does
exist). In the former case, if the call creates a new file, the access
permission bits in the file mode of the file are set to permit reading and
writing by the owner, and to permit reading only by group members and others.
If the call to
open() is successful, the file is opened for writing.
#include <fcntl.h>
...
int fd;
mode_t mode = S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH;
char *filename = "/tmp/file";
...
fd = open(filename, O_WRONLY | O_CREAT | O_TRUNC, mode);
...
The following example uses the
open() function to try to create the
LOCKFILE file and open it for writing. Since the
open() function
specifies the O_EXCL flag, the call fails if the file already exists. In that
case, the program assumes that someone else is updating the password file and
exits.
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#define LOCKFILE "/etc/ptmp"
...
int pfd; /* Integer for file descriptor returned by open() call. */
...
if ((pfd = open(LOCKFILE, O_WRONLY | O_CREAT | O_EXCL,
S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH)) == -1)
{
fprintf(stderr, "Cannot open /etc/ptmp. Try again later.\n");
exit(1);
}
...
The following example opens a file for writing, creating the file if it does not
already exist. If the file does exist, the system truncates the file to zero
bytes.
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#define LOCKFILE "/etc/ptmp"
...
int pfd;
char filename[PATH_MAX+1];
...
if ((pfd = open(filename, O_WRONLY | O_CREAT | O_TRUNC,
S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH)) == -1)
{
perror("Cannot open output file\n"); exit(1);
}
...
None.
Except as specified in this volume of IEEE Std 1003.1-2001, the
flags allowed in
oflag are not mutually-exclusive and any number of
them may be used simultaneously.
Some implementations permit opening FIFOs with O_RDWR. Since FIFOs could be
implemented in other ways, and since two file descriptors can be used to the
same effect, this possibility is left as undefined.
See
getgroups() about the group of a newly created file.
The use of
open() to create a regular file is preferable to the use of
creat(), because the latter is redundant and included only for
historical reasons.
The use of the O_TRUNC flag on FIFOs and directories (pipes cannot be
open()-ed) must be permissible without unexpected side effects (for
example,
creat() on a FIFO must not remove data). Since terminal
special files might have type-ahead data stored in the buffer, O_TRUNC should
not affect their content, particularly if a program that normally opens a
regular file should open the current controlling terminal instead. Other file
types, particularly implementation-defined ones, are left
implementation-defined.
IEEE Std 1003.1-2001 permits [EACCES] to be returned for
conditions other than those explicitly listed.
The O_NOCTTY flag was added to allow applications to avoid unintentionally
acquiring a controlling terminal as a side effect of opening a terminal file.
This volume of IEEE Std 1003.1-2001 does not specify how a
controlling terminal is acquired, but it allows an implementation to provide
this on
open() if the O_NOCTTY flag is not set and other conditions
specified in the Base Definitions volume of
IEEE Std 1003.1-2001, Chapter 11, General Terminal Interface are
met. The O_NOCTTY flag is an effective no-op if the file being opened is not a
terminal device.
In historical implementations the value of O_RDONLY is zero. Because of that, it
is not possible to detect the presence of O_RDONLY and another option. Future
implementations should encode O_RDONLY and O_WRONLY as bit flags so that:
O_RDONLY | O_WRONLY == O_RDWR
In general, the
open() function follows the symbolic link if
path
names a symbolic link. However, the
open() function, when called with
O_CREAT and O_EXCL, is required to fail with [EEXIST] if
path names an
existing symbolic link, even if the symbolic link refers to a nonexistent
file. This behavior is required so that privileged applications can create a
new file in a known location without the possibility that a symbolic link
might cause the file to be created in a different location.
For example, a privileged application that must create a file with a predictable
name in a user-writable directory, such as the user's home directory, could be
compromised if the user creates a symbolic link with that name that refers to
a nonexistent file in a system directory. If the user can influence the
contents of a file, the user could compromise the system by creating a new
system configuration or spool file that would then be interpreted by the
system. The test for a symbolic link which refers to a nonexisting file must
be atomic with the creation of a new file.
The POSIX.1-1990 standard required that the group ID of a newly created file be
set to the group ID of its parent directory or to the effective group ID of
the creating process. FIPS 151-2 required that implementations provide a way
to have the group ID be set to the group ID of the containing directory, but
did not prohibit implementations also supporting a way to set the group ID to
the effective group ID of the creating process. Conforming applications should
not assume which group ID will be used. If it matters, an application can use
chown() to set the group ID after the file is created, or determine
under what conditions the implementation will set the desired group ID.
None.
chmod() ,
close() ,
creat() ,
dup() ,
fcntl()
,
lseek() ,
read() ,
umask() ,
unlockpt() ,
write() , the Base Definitions volume of
IEEE Std 1003.1-2001,
<fcntl.h>,
<sys/stat.h>,
<sys/types.h>
Portions of this text are reprinted and reproduced in electronic form from IEEE
Std 1003.1, 2003 Edition, Standard for Information Technology -- Portable
Operating System Interface (POSIX), The Open Group Base Specifications Issue
6, Copyright (C) 2001-2003 by the Institute of Electrical and Electronics
Engineers, Inc and The Open Group. In the event of any discrepancy between
this version and the original IEEE and The Open Group Standard, the original
IEEE and The Open Group Standard is the referee document. The original
Standard can be obtained online at http://www.opengroup.org/unix/online.html
.
