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NAME
open - open a file
SYNOPSIS
#include <sys/stat.h>
#include <fcntl.h>
int open(const char *path, int oflag, ... );
DESCRIPTION
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.
RETURN VALUE
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.
ERRORS
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.
EXAMPLES
Opening a File for Writing by the Owner
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);
...
Opening a File Using an Existence Check
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);
}
...
Opening a File for Writing
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);
}
...
APPLICATION USAGE
None.
RATIONALE
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.
FUTURE DIRECTIONS
None.
SEE ALSO
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>
COPYRIGHT
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 .