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PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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OPEN(3P) POSIX Programmer's Manual OPEN(3P)
This manual page is part of the POSIX Programmer's Manual. The
Linux implementation of this interface may differ (consult the
corresponding Linux manual page for details of Linux behavior),
or the interface may not be implemented on Linux.
open, openat — open file
#include <sys/stat.h>
#include <fcntl.h>
int open(const char *path, int oflag, ...);
int openat(int fd, 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, allocated as described in Section 2.14, File Descriptor
Allocation. 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 unless the O_CLOEXEC flag is
set in oflag.
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 five values
(file access modes) below in the value of oflag:
O_EXEC Open for execute only (non-directory files). The
result is unspecified if this flag is applied to a
directory.
O_RDONLY Open for reading only.
O_RDWR Open for reading and writing. The result is
undefined if this flag is applied to a FIFO.
O_SEARCH Open directory for search only. The result is
unspecified if this flag is applied to a non-
directory file.
O_WRONLY Open for writing only.
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_CLOEXEC If set, the FD_CLOEXEC flag for the new file
descriptor shall be set.
O_CREAT If the file exists, this flag has no effect except
as noted under O_EXCL below. Otherwise, if
O_DIRECTORY is not set the file shall be created as
a regular file; 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 argument following the
oflag 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
argument following the oflag 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_DIRECTORY If path resolves to a non-directory file, fail and
set errno to [ENOTDIR].
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. If
path does not identify a terminal device, O_NOCTTY
shall be ignored.
O_NOFOLLOW If path names a symbolic link, fail and set errno
to [ELOOP].
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 O_NONBLOCK flag shall not cause an
error, but it is unspecified whether the file
status flags will include the O_NONBLOCK flag.
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.
The O_SYNC flag shall be supported for regular
files, even if the Synchronized Input and Output
option is not supported.
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 without either
O_RDWR or O_WRONLY is undefined.
O_TTY_INIT If path identifies a terminal device other than a
pseudo-terminal, the device is not already open in
any process, and either O_TTY_INIT is set in oflag
or O_TTY_INIT has the value zero, open() shall set
any non-standard termios structure terminal
parameters to a state that provides conforming
behavior; see the Base Definitions volume of
POSIX.1‐2017, Section 11.2, Parameters that Can be
Set. It is unspecified whether O_TTY_INIT has any
effect if the device is already open in any
process. If path identifies the slave side of a
pseudo-terminal that is not already open in any
process, open() shall set any non-standard termios
structure terminal parameters to a state that
provides conforming behavior, regardless of whether
O_TTY_INIT is set. If path does not identify a
terminal device, O_TTY_INIT shall be ignored.
If O_CREAT and O_DIRECTORY are set and the requested access mode
is neither O_WRONLY nor O_RDWR, the result is unspecified.
If O_CREAT is set and the file did not previously exist, upon
successful completion, open() shall mark for update the last data
access, last data modification, and last file status change
timestamps of the file and the last data modification and last
file status change timestamps of the parent directory.
If O_TRUNC is set and the file did previously exist, upon
successful completion, open() shall mark for update the last data
modification and last file status change timestamps 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.
The application shall ensure that it specifies the O_TTY_INIT
flag on the first open of a terminal device since system boot or
since the device was closed by the process that last had it open.
The application need not specify the O_TTY_INIT flag when opening
pseudo-terminals. 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.
The openat() function shall be equivalent to the open() function
except in the case where path specifies a relative path. In this
case the file to be opened is determined relative to the
directory associated with the file descriptor fd instead of the
current working directory. If the access mode of the open file
description associated with the file descriptor is not O_SEARCH,
the function shall check whether directory searches are permitted
using the current permissions of the directory underlying the
file descriptor. If the access mode is O_SEARCH, the function
shall not perform the check.
The oflag parameter and the optional fourth parameter correspond
exactly to the parameters of open().
If openat() is passed the special value AT_FDCWD in the fd
parameter, the current working directory shall be used and the
behavior shall be identical to a call to open().
Upon successful completion, these functions shall open the file
and return a non-negative integer representing the file
descriptor. Otherwise, these functions shall return -1 and set
errno to indicate the error. If -1 is returned, no files shall be
created or modified.
These functions 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, or includes O_CREAT without O_DIRECTORY.
ELOOP A loop exists in symbolic links encountered during
resolution of the path argument, or O_NOFOLLOW was
specified and the path argument names a symbolic link.
EMFILE All file descriptors available to the process are
currently open.
ENAMETOOLONG
The length of a component of a pathname 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 a component of path does not name
an existing file, or O_CREAT is set and a component of the
path prefix of path does not name an existing file, or
path points to an empty string.
ENOENT or ENOTDIR
O_CREAT is set, and the path argument contains at least
one non-<slash> character and ends with one or more
trailing <slash> characters. If path without the trailing
<slash> characters would name an existing file, an
[ENOENT] error shall not occur.
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 names an existing file that
is neither a directory nor a symbolic link to a directory;
or O_CREAT and O_EXCL are not specified, the path argument
contains at least one non-<slash> character and ends with
one or more trailing <slash> characters, and the last
pathname component names an existing file that is neither
a directory nor a symbolic link to a directory; or
O_DIRECTORY was specified and the path argument resolves
to a non-directory file.
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 openat() function shall fail if:
EACCES The access mode of the open file description associated
with fd is not O_SEARCH and the permissions of the
directory underlying fd do not permit directory searches.
EBADF The path argument does not specify an absolute path and
the fd argument is neither AT_FDCWD nor a valid file
descriptor open for reading or searching.
ENOTDIR
The path argument is not an absolute path and fd is a file
descriptor associated with a non-directory file.
These functions 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
The length of a pathname exceeds {PATH_MAX}, or pathname
resolution of a symbolic link produced an intermediate
result with a length that exceeds {PATH_MAX}.
ENOMEM The path argument names a STREAMS file and the system is
unable to allocate resources.
EOPNOTSUPP
The path argument names a socket.
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.
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 *pathname = "/tmp/file";
...
fd = open(pathname, 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 pathname[PATH_MAX+1];
...
if ((pfd = open(pathname, O_WRONLY | O_CREAT | O_TRUNC,
S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH)) == -1)
{
perror("Cannot open output file\n"); exit(1);
}
...
POSIX.1‐2008 does not require that terminal parameters be
automatically set to any state on first open, nor that they be
reset after the last close. It is possible for a non-conforming
application to leave a terminal device in a state where the next
process to use that device finds it in a non-conforming state,
but has no way of determining this. To ensure that the device is
set to a conforming initial state, applications which perform a
first open of a terminal (other than a pseudo-terminal) should do
so using the O_TTY_INIT flag to set the parameters associated
with the terminal to a conforming state.
Except as specified in this volume of POSIX.1‐2017, the flags
allowed in oflag are not mutually-exclusive and any number of
them may be used simultaneously. Not all combinations of flags
make sense. For example, using O_SEARCH | O_CREAT will
successfully open a pre-existing directory for searching, but if
there is no existing file by that name, then it is unspecified
whether a regular file will be created. Likewise, if a non-
directory file descriptor is successfully returned, it is
unspecified whether that descriptor will have execute permissions
as if by O_EXEC (note that it is unspecified whether O_EXEC and
O_SEARCH have the same value).
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(3p) 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.
POSIX.1‐2008 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 POSIX.1‐2017 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 POSIX.1‐2017, Chapter 11, General Terminal Interface
are met.
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
O_EXEC and O_SEARCH are specified as two of the five file access
modes. Since O_EXEC does not apply to directories, and O_SEARCH
only applies to directories, their values need not be distinct.
Since O_RDONLY has historically had the value zero,
implementations are not able to distinguish between O_SEARCH and
O_SEARCH | O_RDONLY, and similarly for O_EXEC.
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.
In addition, the open() function refuses to open non-directories
if the O_DIRECTORY flag is set. This avoids race conditions
whereby a user might compromise the system by substituting a hard
link to a sensitive file (e.g., a device or a FIFO) while a
privileged application is running, where opening a file even for
read access might have undesirable side-effects.
In addition, the open() function does not follow symbolic links
if the O_NOFOLLOW flag is set. This avoids race conditions
whereby a user might compromise the system by substituting a
symbolic link to a sensitive file (e.g., a device) while a
privileged application is running, where opening a file even for
read access might have undesirable side-effects.
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.
The purpose of the openat() function is to enable opening files
in directories other than the current working directory without
exposure to race conditions. Any part of the path of a file could
be changed in parallel to a call to open(), resulting in
unspecified behavior. By opening a file descriptor for the target
directory and using the openat() function it can be guaranteed
that the opened file is located relative to the desired
directory. Some implementations use the openat() function for
other purposes as well. In some cases, if the oflag parameter has
the O_XATTR bit set, the returned file descriptor provides access
to extended attributes. This functionality is not standardized
here.
None.
chmod(3p), close(3p), creat(3p), dirfd(3p), dup(3p), exec(1p),
fcntl(3p), fdopendir(3p), link(3p), lseek(3p), mkdtemp(3p),
mknod(3p), read(3p), symlink(3p), umask(3p), unlockpt(3p),
write(3p)
The Base Definitions volume of POSIX.1‐2017, Chapter 11, General
Terminal Interface, fcntl.h(0p), sys_stat.h(0p), sys_types.h(0p)
Portions of this text are reprinted and reproduced in electronic
form from IEEE Std 1003.1-2017, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The
Open Group Base Specifications Issue 7, 2018 Edition, Copyright
(C) 2018 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 .
Any typographical or formatting errors that appear in this page
are most likely to have been introduced during the conversion of
the source files to man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2017 OPEN(3P)
Pages that refer to this page: fcntl.h(0p), stropts.h(0p), cp(1p), sh(1p), aio_fsync(3p), chmod(3p), close(3p), creat(3p), dbm_clearerr(3p), dirfd(3p), dup(3p), exec(3p), fchmod(3p), fcntl(3p), fdatasync(3p), fdopen(3p), fdopendir(3p), freopen(3p), fstatvfs(3p), ftruncate(3p), getrlimit(3p), grantpt(3p), ioctl(3p), lockf(3p), lseek(3p), mkdtemp(3p), mknod(3p), openat(3p), posix_fallocate(3p), posix_openpt(3p), posix_spawn(3p), posix_spawn_file_actions_addclose(3p), ptsname(3p), read(3p), symlink(3p), tempnam(3p), tmpnam(3p), truncate(3p), umask(3p), unlockpt(3p), write(3p)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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